H-FER-Catalysed Conversion of Methanol to Ethanol and Dimethyl Ether: A First Principles DFT Study

Methanol adsorption and dehydration reactions within zeolites represent important steps in the catalytic conversion process to form long-chain hydrocarbons. Herein, first-principle density functional theory (DFT) is employed in the determination of methanol adsorption and conversion in ferrierite (FER), where we predict the fundamental adsorption geometries and energetics of methanol adsorption. The methanol molecule is shown to physisorb at all explored binding sites, stabilized through hydrogen-bonded interactions with the acid site at O meth---Hfram bond distances ranging from 1.33-1.51 Å. We demonstrate that zeolites’ adsorption capability is affected by the Silicon/Aluminium ratio, with stronger adsorptions predicted in the material with silicon to aluminium fractions of 5 than 8. The adsorption strength is also found to vary depending on the tetrahedral binding site, with the T1O2 site yielding the most stable methanol adsorption structure in the Si/Al ratio = 5 (Eads = -22.5 kcal/mol ) whereas the T1O1 site yields the most stable adsorption geometry ( Eads= -19.2 kcal/mol) in the Si/Al ratio = 8. Upon translational and rotational motion, methanol is protonated resulting in the breaking of its C-O bond to form a methoxy species bound to the framework oxygen (O –CH3 distance of 1.37 Å), whereas the water molecule is stabilized at the acid site through H-bonding (O wat-H =2.0 Å). Further reaction between the methoxy species and a second methanol molecule results in the formation of ethanol and protonated dimethyl ether, w ith adsorption energies of -42 and -25 kcal/mol, respectively. The results in this study provide atomistic insight into the effect of acidity of the FER zeolite on the adsorption and conversion of methanol.


Introduction
Methanol is an attractive energy carrier and an abundant resource for the synthesis of important liquid fuels and hydrocarbon products. 1, 2 The extensively studied methanol to hydrocarbons process (MTH) is an important step in the promising route to obtaining products that are relevant to the petrochemical industry, [3][4][5] which is crucial for the "Methanol Economy" concept. The olefinand aromatic-cycles are proposed as the central mechanism of methanol conversion, which consists of two catalytic cycles 6 interconverting a range of surface species (hydrocarbon pool).
The hydrocarbon pool mechanism can be categorized into two main parts: the olefin cycle which involves the methylation and subsequent cracking of alkenes (both small and large) and the aromatic cycle which is governed by methylation of aromatic compounds with cracking of side chains. The local concentrations of hydrocarbon species within the zeolite dictates the contribution of each cycle. 10 Earlier reports have shown that platinum-based catalyst Pt-Re/Al2O3, shows great selectivity in the alcohol conversion process with products within the range C4-C12. 1 Even with a varying yield of 20-50 wt.%, the general implementation in renewable systems is severely limited by the high cost of precious metal catalysts. 7 This has caused peaked interest in the development of more earthabundant materials as substitutes for precious metal catalysts. Zeolites, also called molecular sieves, are attractive candidates for catalytic applications. 7,8 The three-dimensional (3D) frameworks of zeolites with distinctive molecular scale features, such as pores, channels, and cavities, make them very attractive candidates for methanol conversion catalysis. The channels and cages within zeolites aid distinguishing of molecules of different geometries and sizes. 9 Because of their excellent catalytic activity and high hydrothermal stability under a broad scope of environmental conditions, these aluminosilicate crystals have been utilized in the refining of petrochemical products through ion exchange and adsorption/separation processes. [10][11][12] The reaction mechanism and product selectivity in zeolites are significantly influenced by the zeolite structure. 13,14 Intermediate formation and hydrocarbon production are shown to be greatly influenced by the acidity of the zeolite. 4,15 Reduced selectivity for light olefin products through coking is promoted by high Bronsted acid concentrations. [16][17][18] . Cleavage of the C-O bond is considered to be the rate-determining step of the overall reaction with some theoretical studies determining its activation barrier to be 72 kcal/mol. 19 Methanol conversion to hydrocarbons requires the cleavage of the C-O bond and subsequent formation of C-C bonds, hence the determination of the thermodynamic stabilities of methanol and its dissociated products is of great relevance 6,20 . The activation energy barrier (54 kcal/mol) for the surface methoxy species formation in FER can be reduced by 10 kcal/mol when the C-O cleavage occurs near an additional methanol molecule. However, the data is limited to frameworks with a Si/Al ratio of 35 and there is barely any mention of the effect of increased acidity. 21 Herein, we investigate the effects of Silicon/Aluminium ratios (5and 8) on the methanol adsorption using first-principles Density Functional Theory. This is to elucidate the possible reaction pathways for the methanol C-O bond breaking and C-C bond formation proposed in previous studies. The results obtained give insights, on a molecular level, into the stable adsorption configuration with thermochemical data associated with the dehydrated process when methanol is converted in zeolite H-FER to possible precursors of short-chain hydrocarbons.

Computational Details
The optimized structures and energetics were determined from density functional theory calculations as implemented in the Quantum Espresso package. 22,23 The generalized gradient approximation (GGA) with the Perdew-Burke and Erzenhof (PBE) exchange-correlation functional was used for geometry optimizations. 24 The kinetic-energy cut-off of the plane-wave was set to 40 Ry and the charge density cut-off to 480 Ry. This ensures the convergence of the total energy is within 10 −6 eV and the residual Hellmann-Feynman forces on all relaxed atoms reach 0.01 eV Å −1 . 25,26 Due to the very large unit cell of FER (a=19.0 Å, b=14.3 Å, c= 7.5 Å), 27 a 1x1x1 Monkhost-Pack k-point mesh was used for the integration over the Brillouin zone, which was found to be statistically adequate in describing the structural parameters of the zeolite. The lowest-energy adsorption structures and energetics of methanol were determined by adsorbing it at different sites and in different adsorption configurations. The adsorption energy (Eads), which characterized the strength and stability of the adsorbate species in the zeolite framework was calculated using the relation: where Ezeolite+adsorbate, Ezeo, and Eads are the total adsorption energy of the zeolite with the adsorbate, isolated zeolite framework and of the free adsorbate molecule. Based on this definition, negative or positive adsorption energy denotes an exothermic (favorable) or endothermic (unfavorable) process. The visualizations and graphical representation of all structures in this work were obtained using XCRYSDEN 28 and VESTA software 29 .

Characterization of Ferrierite
All-silica FER was modeled with space group Immm, No. 71 with an orthorhombic structure. 30 The initial coordinates (lattice parameters and atomic positions) obtained from the International Zeolite Association (IZA) database was subjected to full geometry optimization. This was done to attain the most stable configuration for the structure for lattice parameters such as bond length and angles based on the level of theory. Silicon atoms within the fully optimized FER framework were then substituted for Aluminium atoms at the various tetrahedral sites to suit the desired Si/Al ratios  Table 1 are the optimized structural parameters including the lattice parameters, interatomic bond distances and angles, which are all in good agreement with known experimental data 33,34 and previous DFT calculations. [35][36][37]

Methanol adsorption in ferrierite
The adsorption and dehydration of a methanol molecule in the zeolite framework is an important starting step in its conversion to safer and more useful renewable fuels. We therefore first determined the lowest-energy adsorption configuration of methanol in the FER framework with Si/Al ratios of 8 and 5 and characterized the extent of C−O bond activation. The preferred methanol adsorption sites within the framework were determined by exploring the T1, T2, and T3 sites in the 10MR channel of the H-FER zeolite (Figure 1). The lowest-energy adsorption structures of methanol in the FER framework with ratios of 8 and 5 are shown in Figures 2 and 3, respectively.
The methanol molecule is physisorbed at all explored binding sites where it is stabilized through H-bonding with the acid site at Ometh---Hfram bond distances ranging from 1.33-1.51 Å. As shown in Table 2, the adsorption energies are found to be generally more stable in the Si/Al ratio of 5 than 8, which can be linked to the high concentration of acid sites within the zeolite ratio of 5 that permits the formation of more hydrogen-bonded interactions compared to ratio of 8. The adsorption strength is found to vary depending on the tetrahedral binding site, with the T1O2 and T1O1 sites yielding the most stable methanol adsorption structure in ratio 5 (Eads = -22.5 kcal/mol) and 8 (Eads= -19.2 kcal/mol), respectively. The most stable adsorption structures are characterized by shorter Ometh---Hfram bond distances as shown in Table 2. The relative energies obtained are in good agreement with previously reported values of 15-27 kcal/mol in literature. 38,20 From the differential charge density isosurface analysis (Figure 4), we observed electron density accumulation in the Ometh---Hfram regions, which is consistent with H-bonded interactions.

Post dehydration reactions (Ethanol formation)
The thermodynamic stability of the products formed when a second methanol molecule reacts with the framework methoxy species was also investigated. The incoming methanol molecule can attach to the methoxy species via two possible modes: either through its carbon end (forming a C-C bond) or the hydroxyl oxygen (forming an O-C bond). The C-C mode of attachment resulted in the formation of ethanol, which released an energy of 42 kcal/mol, as clearly illustrated in Figure   6a.  (Figure 6b). This result can be attributed to the highly acidic nature of the zeolite and the existence of a water molecule as a product of dehydration in the zeolite. Water molecules are not only speculated to pose as delocalizing agents of protons in the framework but also facilitate the thermodynamic stability of the products that are formed through H-bonding. It was also observed that the FER pore undergoes a distortion where the T-O-Si bond angle changes by 28˚ with a corresponding reduction in the pore diameter from 8.9 to 8.3 Å. Since the pore is charge-saturated, the framework oxygens are more likely to cause an elliptical distortion of the channel to better accommodate the ethoxonium, which has been reported previously. [44][45][46] The adsorbed ethanol molecule could undergo deprotonation and dehydration reactions to produce ethylene, based on the proposed scheme shown in Figure 8.