RESEARCH ARTICLE

 

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

 

 

Cecil H. Botchway^I,*; Richard Tia^I; Evans Adei^I; Nelson Y. Dzade^II,*; Nora H. de Leeuw^{II, III,*}

^IDepartment of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
^IISchool of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom
^IIISchool of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom

 

 

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ABSTRACT

Methanol adsorption and dehydration reactions within zeolites represent important steps in the catalytic conversion process to form long-chain hydrocarbons. Herein, first-principles 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 atO_meth-H_frambond distances ranging from 1.33-1.51 A. We demonstrate that the 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(E_ads = -22.5 kcal mol^-1), whereas the T1O1 site yields the most stable adsorption geometry (E_ads = -19.2 kcal mol^-1) 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 A), whereas the water molecule is stabilized at the acid site through H-bonding (O_wat-H = 2.0 A). Further reaction between the methoxy species and a second methanol molecule results in the formation of ethanol and protonated dimethyl ether, with adsorption energies of -42 and -25 kcal mol^-1, 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.

Keywords: Zeolites, ferrierite, methanol adsorption, acid sites, density functional theory (DFT).

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1. 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,^3-5 which is crucial for the 'Methanol Economy' concept. The olefin- and aromatic-cycles are proposed as the central mechanism of methanol conversion, which consists of two catalytic cycles⁶ 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 dictate the contribution of each cycle.¹⁰

Earlier reports have shown that the 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 interest in the development of more earth-abundant 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.^10-12 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 For instance, reduced selectivity for light olefin products through coking is promoted by high Bransted acid concentrations.^16-18 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^-1.¹⁹

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^-1) for the surface methoxy species formation in FER can be reduced by 10 kcal mol^-1 when the C-O cleavage occurs near an additional methanol molecule. However, the data are 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 (5 and 8) on the methanol adsorption, using first-principles density functional theory (DFT) 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.

 

2. Computational Details

The optimized structures and energetics were determined from DFT calculations as implemented in the Quantum Espresso package.^22,23 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (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 that 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 Ã-¹.^25,26 Due to the very large unit cell of FER (a = 19.0 Ã, b = 14.3 Â, c = 7.5Ã),²⁷a1x1x1Monkhost-Packk-pointmesh 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 (E_ads), which characterized the strength and stability of the adsorbate species in the zeolite framework, was calculated using the relation:

where E_zeo+_ads, E_zeo, and E_ads are the total adsorption energy of the zeolite with the adsorbate, isolated zeolite framework, and the free adsorbate molecule, respectively. Based on this definition, a negative or positive adsorption energy denotes an exothermic (favourable) or endothermic (unfavourable) process. The visualizations and graphical representation of all structures in this work were obtained using XCRYSDEN²⁸ and VESTA software²⁹.

 

3. Results and Discussion

3.1. Characterization of Ferrierite

All-silica FER was modelled 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 were subjected to full geometry optimization to attain the most stable configuration for the structure and 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 of 5 and 8. The distribution of the substituted Al atoms obeyed the Löwenstein's rule,³¹ prohibiting Al-O-Al connections and also the Dempsey's rule permitting the maximum allocation of negative charges within the frame-work.³²Figure 1 shows the fully optimized all-silica and Al-substituted H-FER. Summarized in 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.^35-37

 

 

 

 

The charge deficiency created in the Al substituted framework was compensated with H protons at neighbouring oxygen atoms, thereby forming Bronsted acid sites. Two different Si/Al ratios were considered (5 and 8) and the structural parameters in each composition were determined as reported in Table 1. We observed no significant changes in the structural parameters of the Si/Al ratio composition compared to the all-silica FER.

3.2. 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 10 MR channel of the H-FER zeolite (Fig. 1). The lowest-energy adsorption structures of methanol in the FER framework with ratios of 8 and 5 are shown in Figs. 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 O_meth-H_fram bond distances ranging from 1.33-1.51 Â. 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 a 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(E_ads = -22.5 kcal mol^-1) and 8 (E_ads = -19.2 kcal mol^-1), respectively. The most stable adsorption structures are characterized by shorter O_meth-H_fram bond distances as shown in Table 2. The relative energies obtained are in good agreement with previously reported values of 15-27 kcal mol^-1 in the literature.^38,20

 

 

 

 

From the differential charge density isosurface analysis (Fig. 4), we observed electron density accumulation in the O_meth-H_fram regions, which is consistent with H-bonded interactions.

 

 

3.3. Methanol Dehydration to form Methoxy Species in FER

Protonation of short-chain alcohols in zeolites occurs through geometrical changes owing to a high concentration of charge. The protonation step has been described as a concerted reaction between the O-H bond of the framework and the O-H of methanol, which results in the C-O bond breaking. The protonation process is generally characterized by various translational and rotational motions³⁹ that leads to the cleavage of the C-O in dehydration. Shown in Fig. 5 is the schematic of the dehydration process of methanol within the FER framework with Si/Al = 5, where the physisorbed methanol attracts a proton at the acid site, reorients such that the -CH₃ end binds at an O-site, followed by the final spontaneous dehydration step. It is worth noting that stable protonated methanol (as observed in Fig. 5b), which was not observed in earlier studies, was obtained after geometry optimization.^40,21 This structure, leading to an adsorption energy of -15 kcal mol^-1, is stabilized because of the orientation of the methanol molecule between two close and electro-negatively equivalent framework O2 oxygens (O2 are 2.63 A apart). The methoxy species is bound at an O-CH₃ distance of 1.37 A, whereas the water molecule is stabilized at the acid site through H-bonding (O_wat-H = 2.0 A). The co-adsorption of the methoxy species and water molecule releases an energy of 11 kcal mol^-1, which compared to the physisorbed methanol molecule, indicates an endothermic reaction. The understand-ingbehind the endothermicity of the methoxy species formation is that breaking a very strong methanol HO-CH₃ bond requires a greater amount of energy compared to forming the weaker framework O-CH₃ and H-bonded H₂O to the framework.

 

 

3.4. 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^-1, as clearly illustrated in Fig. 6a. The O-C mode of attachment, on the other hand, resulted in the formation of a stable protonated dimethyl ether species (Fig. 6b), which released an energy of 25 kcal mol^-1. The increased stability of the ethanol molecule is a direct result of strong H-bond interaction between the O-H end of the ethanol and the proton of the Br0nsted acid sites (O-H = 1.36 A), which is consistent with the observed electron density accumulation in the interaction regions, as revealed by the differential charge densityisosurface contour plot shown in Fig. 7a. A similar observation was made in a previous study, where the product was referred to as an ethoxonium ion.⁴¹ The formation of the ethanol molecule is made possible by hydrogen transfer from the carbon end of the incoming methanol molecule to the hydroxyl oxygen end, which finally detaches from the framework O site. Although the protonated dimethyl ether species is lower in thermodynamic stability compared to the formed ethanol, it is known to be the predominant species that is formed in the methanol to gasoline (MTG) process.^42,43 The protonated dimethyl ether is observed in some experiments to form at lower temperatures, but at higher temperatures, it degenerates back to the methoxy species.³³ H-bonding between the proton attached to the dimethyl ether and the water molecule in the channel at 1.2 A enhances the stabilization of the ion formed (Fig. 7b). An O-H bond distance of 1.18 A and C-O-C bond angle of 112 ° for dimethyl ether was observed, as canbe seen in the optimized structure (Fig. 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 A. 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.^44-46 The adsorbed ethanol molecule could undergo deprotonation and dehydration reactions to produce ethylene, based on the proposed scheme shown in Fig. 8.

 

 

 

 

 

 

4. Conclusion

The adsorption and conversion reactions of methanol in the FER framework with different Si/Al ratios of 5 and 8, have been studied employing first-principles DFT calculations. Based on predicted adsorption geometries and energetics, it was demonstrated that the methanol molecule is physisorbed at all explored binding sites, where it is stabilized through hydrogen-bonded interactions with the acid site at O_meth-H_fram bond distances ranging from 1.33-1.51 A. Stronger adsorption energies were predicted for the FER with Si/Al ratio of 5 than 8, with the most stable adsorption geometries releasing energies of -22.5 and -19.2 kcal mol^-1, which suggests that the adsorption strength of methanol is affected by the Si/Al ratio. Protonation of the adsorbed methanol molecule results in translational and rotational motions leading to the breaking of the C-O bond to form methoxy species bound to the framework oxygen (O-CH₃ distance of 1.37 A), whereas a water molecule is stabilized at the acid site through hydrogen-bonded interactions (O_wat-H = 2.0 A). The formation of stable physisorbed ethanol and protonated dimethyl ether species is demonstrated from further reaction with a second methanol molecule. These results provide atomistic insight into the adsorption geometries and energetics of methanol and its reaction products in the FER zeolite.

 

Acknowledgements

The authors acknowledge the Royal Society and the UK Department for International Development, for funding under the Africa Capacity Building Initiative (ACBI), which has supported this research, and we thank Prof. Richard Catlow for valuable consultations. We also acknowledge funding from the UK Engineering and Physical Sciences Research Council (NHdL: grant no. EP/K009567; NYD: grant no. EP/S001395/1) and UK Natural Environment Research Council (NH.d.L.: grant no. NE/R009376). Computing resources of the Centre for High-Performance Computing (CHPC), South Africa, and the Advanced Research Computing ©Cardiff (ARCCA) Division were used for the calculations in this work.

 

Supplementary Material

The supplementary information contains the optimized atomic coordinates for acidic FER with and without adsorbing species.

 

ORCID iDs

C.H. Botchway: orcid.org/0000-0001-7733-9473

N.Y.Dzade: orcid.org/0000-0003-1583-588X

N.H. de Leeuw: orcid.org/0000-0002-8271-0545

 

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Received 10 February 2020
Revised 6 August 2020
Accepted 9 September 2020

 

 

* To whom correspondence should be addressed. E-mail: C.H.B., cecllhbotchway@outlook.com / N.Y.D., dzadeny@cardlff.ac.uk / N.H.d.L., n.h.deleeuw@leeds.ac.uk

 

 

Supplementary Data

The supplementary data is available in pdf: [Supplementary data]