Adsorption and Dehydration Reaction of Ethanol to Ethylene on Isomorphous B, Al, and Ga Substitution of H-ZSM-5 Zeolite: An Embedded ONIOM Study


 Dehydration reactions are important in the petroleum and petrochemical industries, especially for the feedstock production. In this work, the catalytic activity of zeolites with different acidities for the dehydration of ethanol to ethylene was investigated by calculations on cluster models of three isomorphous B, Al, and Ga substitution of H-ZSM-5 zeolites. Detailed reaction profiles for the dehydration reaction, assuming either a stepwise or a concerted mechanism, were calculated by using the ONIOM(MP2:M06-2X) + SCREEP method. The adsorption energies of ethanol are -21.6, -28.1 and -27.7 kcal/mol on H-[B]-ZSM-5, H-[Al]-ZSM-5, H-[Ga]-ZSM-5 zeolites, respectively. The stepwise mechanism was preferred on all isomorphous zeolites. The activation energies for the ethoxy formation as the rate-determining step are in range of 40.0 to 42.3 kcal/mol. The results indicated that the order of catalytic activity were H-[Al]-ZSM-5 > H-[Ga]-ZSM-5 > H-[B]-ZSM-5 for catalyzing the dehydration of ethanol to ethylene. Besides the acid strength, the zeolite framework affected the reaction by stabilizing the reaction intermediates leading to more stable adsorption complexes and lower activation barriers.


Introduction
Zeolites are used as catalysts in the petrochemical industries due to their remarkable properties such as selectivity control, thermal stability, and tunable acid strength. [1][2][3][4] Among a large number of reactions catalyzed by zeolites, dehydration reactions are especially important. Dehydration is widely accepted as the key reaction involved in the conversion of biomass into value-added chemicals. For example, bioethanol from fermentation processes can be changed to ethylene, an important feedstock for polymer production. [5][6][7][8] Several zeolites have been reported to use for an environmentally friendly biomass conversion. [3,9] Several techniques such as nuclear magnetic resonance (NMR) and Infrared (IR) spectroscopy are commonly used to observe the reaction species. [10][11][12] There were several experimental studies on the dehydration of ethanol to ethylene with H-[Al]-ZSM-5 zeolite. [13][14][15] The dehydration reaction of ethanol were also studied over Sn-Beta zeolite [16], Ni-ZSM-5 [17]. The ethoxy group was considered to be an intermediate for the dehydration of ethanol to ethylene. [18,19] The details of adsorption and reaction of the dehydration of ethanol on acidic zeolite have been previously studied by several computational methods. The ethanol dehydration over H-ZSM-5, H-BEA and H-AEL were studied with ONIOM calculation. [20]The ONIOM (DFT:UFF) method was used to study the adsorption of ethanol, 1-propanol, and 1-butanol on H-ZSM-5 zeolite. [21] The comparison of the adsorption heats of ethanol on H-ZSM-5 zeolite and on silicalite revealed the con nement effect of the framework on the adsorption properties. [22] Several density functional theory (DFT) studied on the dehydration reaction of ethanol have been performed to determine the reaction mechanism and the effects of acid strength and zeolite framework on it. [18,[23][24][25][26][27] Recently, more evidence was reported from experimental study as the oxonium ion was observed during the ethanol dehydration reaction. [28] Zeolites isomorphously substituted with several metals have been examined to obtain guideline for designing new enhanced catalysts. Isomorphous substitutions of B, Fe, Ga and Al into the ZSM-5 zeolite framework were studied by Fourier transform infrared spectroscopy and temperature-programmed ammonia desorption techniques. [29] The relative Brønsted acidity (OH) order of modi ed zeolites was found to be Si < B < < Fe < Ga < Al. Density functional calculations were used to determine the acid strength of the substituted zeolites. [30][31][32][33] Deka et al. studied the effect of substitutions with Al, B and Ga by using a cluster model and suggested that the acidity trend of the catalysts can be derived from their relative electrophilicities. [31] The order of the acidities of isomorphously metal substituted ZSM-5 determined from adsorption of ammonia, was B < Fe < Ga < Al, which was in agreement with experimental result. [32] The same acidity trend was found for larger pore size H-MCM-22. [34,35] The aromatization of furans by dehydration on Al, Fe, Ga and B in BEA zeolite was studied by a combination of calculations and experiment. [36] The H-[B]-BEA zeolite was determined to be the weakest acid with H-[Al]-BEA being the strongest one. Recently, catalytic properties of Ga-modi ed hierarchical H-ZSM-5 nanosheets for the aromatization of C5 hydrocarbons was also demonstrated. [37] The con nement effect was suggested to be crucial for the interaction between the zeolite framework and adsorbed molecules since con nement can alter physisorption and activation energies. [38] These effects on unsaturated aliphatic, aromatic and heterocyclic compounds were theoretically examined by a combination of second order Møller-Plesset perturbation theory (MP2) and hybrid density functional M06-2X. [39] The M06 functionals [40] developed by the Truhlar group had also been successfully used for studying adsorption and reactions on acidic Zeolites [39,41,42], on Lewis-acid zeolites [43,44], on metallic catalysts [45][46][47][48][49], and on organic molecules [50,51]. Recently they have been applied to determine P-NMR chemical shifts of phosphorus-modi ed CHA zeolite. [52] These higher adsorption energy and lower activation barrier caused by the con nement effect, determined from hybrid functional DFT calculation, were in good agreement with experimental results. [39,53] While reaction mechanisms of the dehydration of the ethanol to ethylene on several zeolite catalysts have been investigated, [23,36] no systematic investigation of zeolites with various acid strengths catalyzing the dehydration of ethanol has been reported.
In this work, the ethanol adsorption and its dehydration reaction on isomorphously metal (B, Al or Ga) substituted H-ZSM-5 were investigated by means of the ONIOM method. ONIOM is a methodology that allows to combine highly accurate calculations of reaction centers with lower accuracy calculations of the environment. [54] The inner layer 5T cluster containing the Brønsted acid was treated with the highly accurate MP2 method while the framework 34T cluster represented the zeolite framework was treated with the M06-2X DFT. All atoms of the rst two layers were embedded into point charges to represent the zeolitic Madelung potential from an in nite lattice generated by the SCREEP method. [39] The objective of this study was to determine the effect of the acidity of the substituted zeolite on the adsorption and dehydration reaction.

Methodology
The adsorption and dehydration reaction of ethanol on H-ZSM-5 zeolites have been theoretically studied with a combination of MP2 and DFT calculations. Brønsted acid strengths of isomorphously metalsubstituted ZSM-5 zeolites are calculated and discussed. The 34T (T: tetrahedra of Si, B, Al and Ga atoms) quantum clusters, as shown in Figure S1 in the supporting information, were tailored from the crystallographic coordinates to represent the active region and the nearby zigzag channel of zeolite. The metal modi ed zeolites were prepared by substituting the silicon atom at T12 position of ZSM-5 cluster with either B, Al or Ga atoms, denoted as H-[B]-ZSM-5, H-[Al]-ZSM-5 and H-[Ga]-ZSM-5 zeolites, respectively. The ONIOM [54] scheme was applied on the 34T clusters to represent both the active Brønsted acid site and the con nement effect as well as the extended framework in a feasible way. [55] Geometry optimization was performed using the ONIOM(MP2:M06-2X) model. The inner 5T quantum cluster covering the Brønsted acid and all adsorbed species was optimized at the MP2 level and the outer 34T quantum cluster was treated with the DFT/M06-2X functional. To reduce computational cost, only the inner 5T layer and the adsorbed species were relaxed while the outer 34T layer were kept xed at its crystallographic position. The 6-31G(d,p) basis set was used for structure optimization. Then, single point calculations with the 6-311 + G(2df,2p) basis set were performed on top of these. Several adsorption modes of ethanol on the zeolite surface were studied.
The consequence of the long-range electrostatic potential from the in nite zeolite lattice, represented by embedded point charges [56,57], has often been stressed. The long range effects represented by the embedded-ONIOM (e-ONIOM) method was found to be crucial for the adsorption and reaction on zeolites. [39,58,59] The 5T:34T model of H-[Al]-ZSM-5 with embedded ONIOM(MP2/6-311 + G(2df,2p):M06-2X/6-311 + G(2df,2p)) on the optimized structure at ONIOM(MP2/6-31G(d,p):M06-2X/6-31G(d,p))) level of theory is shown in Fig. 1. Two possible mechanisms proposed for the ethanol dehydration reaction are shown in scheme 1. The transition states were determined by the Berny algorithm and were con rmed by a single negative-normal mode corresponding to their reaction pathway. All calculations were performed with the Gaussian 09 program [60].

Results And Discussion
The optimized structural parameters of all H-ZSM-5 zeolite determined from ONIOM(MP2:M06-2X) hierarchical model with embedded point charge are shown in Table S1 in the supporting information. This AlŸŸŸHz distance was similar to experimental data (2.40 Å).  Figure S8 and selected geometrical parameters are tabulated in Table S14 in the supporting information. The energy pro les of both mechanisms were given in Fig. 3 The other possible mechanism was the concerted one as shown in Fig. 4. The optimized structures for the concerted pathway are shown in Figure S9 and Table S16 in the supporting information. The adsorption AD-Al-1A was observed to correspond the concerted reaction pathway. The transition state (TSc-Al) with one imaginary frequency at 852.3i cm − 1 led to the breaking of the O-H bond of zeolite and the C-H bond of ethanol. The products from this step were ethylene and water (PRc-Al) with a relative energy of 42.6 kcal/mol. A previous study with ONIOM(M062X:PM6) level of calculation reported the adsorption energies in the range of 43.1-48.1 kcal/mol. [23] The lower activation barrier of TS1-Al compared to TSc-Al indicated that the concerted mechanism was thermodynamically inferior to the stepwise one even though the differences in the relative energies of the transition state were not large.
The framework effects are illustrated in table S15 and S17 of the supporting information. The high-level calculation (5T) represented the effect from Brønsted acid energy and provided the activation barrier of 40.0, 30.7 and 42.6 kcal/mol for the TS1-Al, TS2-Al and TSc-Al, respectively. The apparent activation barriers are 11.9, 28.5 and 19.3 kcal/mol, respectively. For H-[Al]-ZSM-5, the effect of the framework reduced the total activation energy by 7-12% for TS1-Al and TS2-Al of the stepwise mechanism. However, the framework had no signi cantly in uence on TSc-Al of the concerted mechanism.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. 210517SupportinginformationZSM5.docx