A First-Principles Investigation of Gas-Phase Ring-Opening Reaction

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Kinetics, Catalysis, and Reaction Engineering

A first-principles investigation of gas phase ring opening reaction of furan over HZSM-5 and Ga-substituted ZSM-5 Mingxia Zhou, Lei Cheng, Bin Liu, Larry A Curtiss, and Rajeev S. Assary Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01969 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Industrial & Engineering Chemistry Research

A first-principles investigation of gas phase ring opening reaction of furan over HZSM-5 and Ga-substituted ZSM-5 Mingxia Zhou, † Lei Cheng, † Bin Liu, ‡ Larry A. Curtiss †, and Rajeev S. Assary †* † Materials

Science Division, Argonne National Laboratories, 9700 S. Cass Avenue, Argonne, IL,

60439, USA ‡Tim

Taylor Department of Chemical Engineering, Kansas State University, 1005 Durland Hall,

Manhattan, KS 66505, United States

*[email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The furan ring opening reaction using three catalytic models, HZSM-5, the extraframework sites [GaO]/ZSM-5, and [Ga(OH)2]/ZSM-5 was investigated using periodic density functional theory as a model reaction for mechanistic understanding of catalytic vapor phase biomass upgrading. The formation of formyl allene from the furan was investigated in detail by computing the energy profiles and reaction barriers. Based on the computed free energy profiles, the HZSM-5 and Ga(OH)2/ZSM-5 are active catalysts while GaO/ZSM-5 is not active for the furan ring opening reaction. In HZSM-5, the likely rate controlling steps is furan CH2-O bond cleavage upon protonation (1.70 eV). The computed energy difference between the highest and lowest points in the free energy profile is 1.89 eV. The [Ga(OH)2]+ is likely the catalytic center for the gallium incorporated ZSM-5 catalyst. The rate-controlling step is a hydrogen transfer reaction, which requires a reaction barrier of 2.48 eV. The computed energy difference between the highest and lowest points in the Ga(OH)2 catalyzed potential energy surface is 2.61 eV. These computational studies provide new mechanistic understanding of the role of catalytic extra-framework sites in the vapor phase upgrading reactions. .


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1

Introduction Non-edible lignocellulosic biomass has the potential to become a sustainable

precursor to industrial chemicals and transportation fuels, hence reducing the dependency on fossil fuels and petroleum derivatives.1-3 There has been increasing research and development efforts in demonstrating efficient and low-cost biomass conversion routes. Among various strategies for biomass conversion, catalytic fast pyrolysis (CFP) is a promising process for converting solid biomass directly into fuels and chemicals such as benzene, toluene, and xylenes (BTX).4-10 In typical pyrolysis, biomass is treated rapidly at high temperatures (773 K- 973K) in an oxygen free environment. The heat breaks down biomass into pyrolysis vapors, gas, and char. Upon removing char, the vapors are cooled and condensed to bio-oil that can be upgraded to hydrocarbon blends. One of the challenges for bio-oil upgrading in this route is the complexity and the reactivity of biomass oxygenates. Recent research efforts suggest that the quality of bio-oil can be further improved by catalytic treatment to reduce the oxygen content prior to the condensation of bio-oil.11-15 Such catalytic treatment results in the production of upgraded bio-oil that could be integrated into conventional petrochemical refineries. Catalysts such as zeolites are commonly used and they play crucial role in the selectivity of desired pyrolysis products and their distributions during the CFP of biomass. Commonly used zeolite frameworks for CFP include ZSM-5, mordenite, beta zeolite, and Y zeolite.16-17 Among the zeolite catalysts, ZSM-5 has the optimal structure and produces the highest yield of aromatics and the least amount of coke during the conversion of various biomass components (e.g. cellulose, wood, lignin).5, 16, 18 Detailed studies by French et al. 19

have evaluated a set of 40 selected commercial and laboratory-synthesized zeolite

catalysts for upgrading of biomass and found that ZSM-5 performed better than other zeolite catalysts and a relatively high yield of hydrocarbons were achieved by using nickel, cobalt, iron and gallium-substituted ZSM-5. In particular, the introduction of metal ions such as gallium (Ga) to the ZSM-5 catalyst has been shown to enhance the decarbonylation and aromatics formation rate.20-26 Cheng et al.21 have investigated the production of renewable compounds from furan using Ga/ZSM-5 catalysts and proposed that the catalysts prepared by the ion-exchange and incipient-wetness method have shown the best aromatic selectivity. Ono et al. 3 ACS Paragon Plus Environment

20

have


observed that the introduction of Ga cations to ZSM-5 give an increased yield of aromatic hydrocarbons (40.3% without Ga  to 67.4 % with Ga) during the conversion of methanol. Qiu et al.22 have compared the catalytic activity of HZSM-5 and Ga/ZSM-5 for aromatization of dilute ethylene stream and found that 93% ethylene conversion with aromatics selectivity of 81% was obtained over a 5 wt.% Ga/ZSM-5 catalyst at 520 °C, while a large amount of coke formation has been observed with the HZSM-5 catalyst. Additionally, experimental study27 has indicated that the presence of extra-framework ‘Ga’ sites are responsible for the catalytic improvement compared to any framework zeolite (Ga3+ replaces typical Al3+) sites in the zeolite, however, the exact nature of the active site is not known.

Scheme 1. Furan decarbonylation to C3 species and subsequent aromatization chemistry in the presence of Ga/ZSM-5 catalyst. Mechanistic details of biomass vapor phase upgrading using zeolite catalysts are limited due to the complexity of biomass feed and often model compounds are used to understand a selected class of reactions and their outcomes for a particular group of molecules. Furan is a normal model compound for aromatic oxygenates produced from pyrolysis vapors and this species can be used to identify catalytic and pyrolysis reactions occurring at the catalyst site or inside a confinement.28-31 Furan can also be produced from lower molecular weight carbohydrates derived from the pyrolysis of biomass in the presence of zeolite.32 Furan may be utilized via decarbonylation to form aromatics, as shown in Scheme 1. Alternative synthetic strategy to convert furan to aromatics can be alkylation/Diels-Alder cycloaddition reaction using ethanol.33 Due to its aromaticity, furan molecule is extremely stable in the gas phase, suggesting that the chemical transformation of furan molecule could be kinetically challenging and require a combination of higher reaction temperatures and catalysts. In terms of thermal decomposition of furan, Sendt et al. 34 reported a kinetic modeling study of gas phase pyrolysis. Mechanistic understanding


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of furan ring opening in conventional zeolites or metal-modified zeolites is lacking, and quantum chemical modeling could add insight into this aspect of vapor phase upgrading. Gallyl (GaO+) cation is a normal cationic form for Ga presented in ZSM-5.35 In addition, the GaO+ can be stabilized by the addition of steam to form [Ga(OH)2]+,27, 36-37 which could act as an active catalytic site. The X-ray Absorption Near Edge Spectroscopy measurements indicate the predominant of [Ga(OH)2+] cation in Ga-ZSM5.27, 37 Herein, [GaO]/ZSM-5 and [Ga(OH)2]/ZSM-5 are used as the catalyst models, representing Gamodified zeolite. In this paper, we provided a detailed comparison of the energetics of the gas phase ring opening reaction of furan molecule catalyzed by HZSM-5 and Ga/ZSM-5 model catalysts using periodic density functional theory with the goal of elucidating the catalytic mechanisms. The details of the theoretical methods used in this study are described in Section 2. The comparison of energetics and kinetics for furan ring opening reaction in HZSM-5, [GaO]/ZSM-5, and [Ga(OH)2]/ZSM-5 is discussed in Section 3 based on periodic density functional theory (DFT) calculations.

2

Computational Details All DFT calculations reported here were performed using the Vienna Ab initio

Simulation Package (VASP)38-41. The generalized gradient approximation (GGA) PerdewBurke-Ernzerhof functional (PBE) exchange-correlation functional42 was employed. The DFT-D3 method of Grimme et al.43 is included to account for the vdW interactions. The Brillouin-zone was sampled with a 1 × 1 × 2 k-point mesh based on the Monkhorst-Pack scheme.44 The cutoff energy of 400 eV was used for all calculations. The energy convergence was assumed to be achieved until the force is smaller than 0.05 eV/Å. Gas phase reactions without zeolite are computed using Gaussian program45 based on B3LYP/6-311G(d) level of theory46-48 as reference, showing in supporting information. The ZSM-5 MFI-type unit cell was constructed, which consists of 96 Si and 192 O atoms. All atoms were allowed to relax during the calculations. The optimized lattice constants of the unit cell are a = 20.02 Å, b = 19.90 Å, and c = 13.38 Å, which are within 1% of the experimental values49 (a = 20.07 Å, b = 19.92 Å, c = 13.42 Å). The Si atom close to the intersection of two channels in T12 site50-51 of the main channel is then substituted with an Al and H atom, where the proton represents the Brønsted acid site. The substitution 5 ACS Paragon Plus Environment


of Al and H atom in other T sites are not considered here and the qualitative conclusions may vary with the location of the active site. The ‘Ga-O’ site was introduced to replace the proton to mimic the [GaO]/ZSM-5 catalyst. The GaO+ catalyst site can be stabilized by addition of steam forming a [Ga(OH)2]+ site. The optimized structures of HZSM-5, [GaO]/ZSM-5, and [Ga(OH)2]/ZSM-5 are shown in Figure 1a, Figure 1b and Figure 1c, respectively. The vibrational frequencies analysis of reaction intermediates was performed to compute the entropies and free energies at 873.15 K, which is an experimental temperature21 for furan conversion into aromatics over ZSM-5. To accelerate the vibrational frequency calculations, the zeolite framework is fixed and the catalytic active site (H site in HZSM-5, [GaO]+, and [Ga(OH)2]+) with adsorbate are allowed to relaxed. The nudged elastic band (NEB)52 and dimer methods53 were performed for transition state (TS) searches. The initial guess for the saddle point from NEB calculations was used as the starting point for the dimer calculations.


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Figure 1. The optimized structures of (a) HZSM-5 (b) [GaO]/ZSM-5, (c) [Ga(OH)2]/ZSM5 model catalyst pores and the extra framework site using periodic density functional calculations. Notes: models (b) and (c) are denoted as [GaO]+ site and [Ga(OH)2]+ site, respectively in the manuscript

3

Results and Discussions The products (formyl allene, propyne, allene, ketene) of the furan pyrolysis

considered here are identical to the experimental studies performed in the pyrolysis study of furan via micro-tube reactors54 and the computational study by Sendt et al.34. The desired pyrolysis reaction of furan or any other oxygenates is to enable their conversion to products that preserves the number of hydrogen and carbon atoms; this is essentially a selective deoxygenation reaction. Based on this hypothesis, four pathways are considered for the preliminary gas phase calculations, where furan can be converted to (1) formyl allene, (2) propyne, (3) allene, and (4) ketene. Gas phase calculations as shown in supporting information suggest that the furan formyl allene formation is the most kinetically feasible reaction, consistent with the experimental studies by Urness et al.54 and with the theoretical studies by Tian et al.55 and Sendt et al. 34. Wei et al.56 also proposed a modified mechanism for furan decomposition and indicated that the most important fuel consumption path is through the unimolecular initiation reaction: furan to formyl allene. Therefore, we focused on the furanformyl allene pathway in three catalysts. The computed free energy profiles for this pathway catalyzed by HZSM-5 (section 3.1), [GaO]/ZSM-5 (section 3.2), and [Ga(OH)2]/ZSM-5 (section 3.3) are discussed below.

3.1

Furan ring opening catalyzed by the HZSM-5



Scheme 2. Schematic of the formation of formyl allene from furan via protonation route

In Scheme 2, acid catalyzed formyl allene formation from furan molecule is shown. The sequence of the reaction is the following: (a) protonation of α-carbon of the furan molecule, (b) cleavage of CH2-O bond, (c) deprotonation. Using the hypothesis shown in Scheme 2, we have investigated the zeolite catalyzed furan ring opening reaction mechanism by computing the energies of intermediates and transition states (TSs). Figure 2 shows the computed energetics of furan to formyl allene in a HZSM-5 catalyst along with the optimized structures of reactive intermediates and TSs at 873.15 K.


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Figure 2. Computed reaction mechanism and energetics (eV) of furan to formyl allene conversion catalyzed by HZSM-5 at 873.15 K. Labels 2A, 2B, 2D, 2F, 2H, and 2I denote the computed reaction intermediates and 2CTS, 2ETS, 2GTS denote the computed TS structures. The ‘2’ in the name of reaction intermediates and TS represents the species in Figure 2. For clarity, only the active site of the periodic model in the zeolite is shown. In Figure 2, labels 2A, 2B, 2D, 2F, 2H, and 2I denote the computed reaction intermediates and 2CTS, 2ETS, 2GTS denote the computed TS structures. The intermediate 2A represents the reference state (0.0 eV), where the HZSM-5 and furan molecule are infinitely separated. The intermediate 2B is the initial adsorption complex formed between furan and HZSM-5 active site. From intermediate 2B, the 2CTS corresponds to the hydrogen transfer from HZSM-5 to the α-carbon of the furan. This initial protonation reaction step of furan molecule (2B2CTS2D) is spontaneous reaction (∆G = -0.38 eV) and requires an energy barrier of 0.22 eV. The subsequent step is the furan ring opening via cleaving the CH2-O bond (2D→2ETS →2F). This reaction step is endergonic by 0.95 eV and requires an energy barrier of 1.70 eV (2D2ETS). Upon the CH2-O bond breaking, deprotonation of intermediate 2F by the HZSM-5 results in the formation of formyl allene. This reaction step (2F2H) is exergonic (∆G = -0.36 eV) and the computed energy barrier for this reaction (2F2GTS) is 0.94 eV. The bond distance between H in HZSM-5 and O in formyl allene is 1.38 Å, suggesting a hydrogen bond type structure, leading to a stable adsorption of 2H. From 2H (-0.17 eV), a complete separation of formyl allene from the catalyst (2I) is energetically uphill by 1.09 eV. Thus, from the free energy diagram, the rate-controlling step is CH2-O bond cleavage upon protonation (2D2ETS, 1.70 eV).

3.2

Furan ring opening catalyzed by [GaO]/ZSM-5 Figure 3 presents the catalytic cycle of furan to formyl allene over [GaO]/ZSM-5

at 873.15 K and the corresponding optimized structures of reaction intermediates (3A, 3B, 3D, 3F, 3H, 3I) and TSs (3CTS, 3ETS, 3GTS). The initial step (3A3B) is the binding of furan with the [GaO]/ZSM-5 catalyst. This is followed by a hydrogen abstraction by the ‘O’ site of [GaO]+ via TS 3CTS, resulting in the formation of a stable intermediate 3D (9 ACS Paragon Plus Environment


2.84 eV) through formation of ‘O-H’ bond (0.97 Å) and ‘Ga-C’ bond (1.94 Å). The computed reaction barrier required for the hydrogen abstraction (3B3CTS) is 0.55 eV. This hydrogen transfer step (3B3D) is very exergonic (∆G = -2.07 eV). The subsequent step from low energy intermediate 3D is an internal hydrogen transfer, in which the ring opens based on the optimized structure from DFT calculations. A NEB structure of finding the transition state (3ETS) is shown in Supporting Information, Figure S5. This internal hydrogen transfer and ring opening step (3D→3ETS →3F) is endergonic by 0.56 eV and requires a high-energy barrier (3.38 eV). The formed intermediate 3F then goes through another hydrogen transfer from ‘Ga-OH’ group to the carbonyl group forming the aldehyde group (3H) via a TS 3GTS. This hydrogen transfer (3F3H) is energetically uphill by 1.62 eV and requires an activation barrier (3F3GTS) of 1.97 eV. The intermediate 3H is a complex between formyl allene and [GaO]/ZSM-5. Further removal of the formyl allene from intermediate 3H requires energy of 1.58 eV. Based on the free energy profile, hydrogen transfer from intermediate 3D requires a large barrier ( > 3.0 eV) and the following two steps are largely endergonic, indicating the [GaO]+ site in the catalyst is not likely catalytically active for furan ring opening reaction, which also inactive to alkane dehydrogenation reaction57.


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Figure 3. Computed catalytic cycle and energetics (eV) of furan to formyl allene over [GaO]/ZSM-5 at 873.15 K. Labels 3A, 3B, 3D, 3F, 3H, and 3I denote the computed reaction intermediates and 3CTS, 3ETS, 3GTS denote the computed TS structures. The ‘3’ in the name of reaction intermediates and TS represents the species in Figure 3.

3.3

Furan conversion to formyl allene over [Ga(OH)2]/ZSM-5 The synthesis of Ga-promoted ZSM-5 are through ion-exchange, incipient wetness,

hydrothermal synthesis in aqueous solution.21,

37

The presence of water forms the

dihydroxyl species ([Ga(OH)2]+) due to the susceptible of [GaO]+ to water.36 From DFT calculations, formation of [Ga(OH)2]+ from GaO+

and a water molecule

is

thermodynamically favorable (∆E = -3.19 eV). Therefore, the formation of [Ga(OH)2]/ZSM-5 site is preferred if water present in the system, which is consistent with previous study36. The formation of [Ga(OH)2]+ is almost barrierless (~0.01 eV)), indicating the likelihood of the Ga(OH)2 site in the catalyst. 11 ACS Paragon Plus Environment


The computed geometries of intermediates (4A, 4B, 4D, 4F, 4H, 4I ), TSs (4CTS, 4ETS, 4GT) and free-energy profile for formyl allene formation from furan over [Ga(OH)2]/ZSM-5 catalyst at 873.15 K are presented in Figure 4. All energies are computed with respect to the energy of [Ga(OH)2]/ZSM-5 and an isolated furan molecule in the gas phase (4A). The furan molecule is activated via the binding with [Ga(OH)2]/ZSM-5. This is followed by a hydrogen transfer from one of the ‘Ga-OH’ groups in [Ga(OH)2]/ZSM-5 to the α-carbon

of the furan (4B4CTS4D). This

protonation reaction (4B4D) is endoergic (∆G = 0.44 eV) and requires an energy barrier of 1.54 eV (4B4CTS). Upon the protonation of furan, the ring opening via CH2-O bond cleavage is the next step (4D4ETS4F) to form acyclic intermediate. This step (4D4F) is exergonic (∆G= -0.31 eV) and requires an energy barrier of 1.75 eV. The subsequent step (4F4GTS4H) from intermediate 4F is a hydrogen transfer via the TS structure, 4GTS, forming a complex of formyl allene adsorbed with [Ga(OH)2]/ZSM-5. This reaction step (4F4H) is endoergic in nature (∆G = 0.85 eV) and the computed energy barrier for this reaction is 2.48 eV. From intermediate 4H, the free energy required to form formyl allene and release from the catalyst is 0.68 eV. From this free energy profile, the rate-determining step is the hydrogen transfer from intermediate 4F to 4HTS that requires a reaction barrier of 2.48 eV.


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Figure 4. The [Ga(OH)2]/ZSM-5 catalyzed reaction mechanism and the computed energetics (eV) for the formation of formyl allene from furan at 873.15 K. Labels 4A, 4B, 4D, 4F, 4H, and 4I denote the computed reaction intermediates and 4CTS, 4ETS, 4GTS denote the computed TS structures. The ‘4’ in the name of reaction intermediates and TS represents the species in Figure 4.

3.4

Comparison of free energy landscapes of HZSM-5, [GaO]/ZSM-5, and

[Ga(OH)2]/ZSM-5 catalysts In Figure 5, we compared the free energy profiles of furan  formyl allene over HZSM-5, [GaO]/ZSM-5, and [Ga(OH)2]/ZSM-5 catalysts. We selected only the highest and lowest points in the energy profiles shown in Figure 2, Figure 3, and Figure 4 to simplify the comparison. Based on the energy profile, the highest energy point is 2GTS 13 ACS Paragon Plus Environment


while the lowest energy point is 2D on HZSM-5, which has difference of 1.89 eV. For [Ga(OH)2]/ZSM-5, the energy difference between highest energy point (4GTS) and lowest energy point (4B) is 2.61 eV. [GaO]/ZSM-5 site is not catalytically active, as the reaction with the catalyst site leads to low energy intermediates (3D) and further reactions and desorption are prohibitively endoergic.

Figure 5. Comparison of free energy profiles of furan ring opening to form formyl allene over HZSM-5, [GaO]/ZSM-5, and [Ga(OH)2]/ZSM-5 through presenting the highest and lowest points in the energy profiles in Figure 2, Figure 3, and Figure 4. A is the energy reference (furan and catalyst) and I is the product (formyl allene and catalyst).

4

Summary Using periodic DFT calculations, we reported energetics and the structures

associated with the catalytic ring opening reaction of furan to form formyl allene in the gas phase by HZSM-5, [GaO]/ZSM-5 and [Ga(OH)2]/ZSM-5. Based on this mechanistic investigation, the following conclusions can be drawn: 1.

In the HZSM-5 enabled furan ring opening path (via Bronsted acid like pathway),

the rate controlling step is the CH2-O of furan bond cleavage upon protonation (2D2ETS, 14 ACS Paragon Plus Environment

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1.70 eV). The computed energy difference between the highest and lowest points in the free energy diagram is 1.89 eV (2D2GTS). 2.

The Ga(OH)2/ZSM-5 is the likely catalytic center for the gallium incorporated

ZSM-5 catalyst. The rate- determining step is a hydrogen transfer reaction, which requires 2.48 eV (4F 4GTS) energy barrier. The computed energy difference between the highest and lowest points in the free energy profile is 2.61 eV (4B 4GTS). 3.

The ‘GaO’ site of GaO/ZSM-5 is not catalytically active for the ring opening

reaction of furan due to high barriers. These mechanistic investigations provide new understanding about the role of catalytic extra-framework zeolite sites in the catalytic upgrading of oxygenates. Future studies may include investigations of reactivity of furans/intermediates and other mixed oxygenates in the presence of multi-nuclear metal sites and their roles in the catalysis, as well as deriving micro-kinetic models for catalytic vapor phase upgrading reactions.

Supporting Information Thermochemistry of gas-phase furan decomposition reactions, details regarding Nudge Elastic Band (NEB) calculations, and additional references are provided in the supporting information.

Acknowledgements This work was conducted as part of the Computational Chemistry Physics Consortium (CCPC), which is supported by the Bioenergy Technologies Office (BETO) of Energy Efficiency & Renewable Energy (EERE). We gratefully acknowledge the computing resources provided on “BEBOP”, a computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory (ANL). This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231; the Center for Nanoscale Materials, which was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357, and the Beocat Research Cluster at Kansas State 15 ACS Paragon Plus Environment


University, which is funded in part by NSF grants CHE-1726332, CNS-1006860, EPS1006860, and EPS-0919443.

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A First-Principles Investigation of Gas-Phase Ring-Opening Reaction

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