| 1. |
- Engedahl, Unni, 1990, et al.
(författare)
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Reaction mechanism for methane-to-methanol in CU-SSZ-13: First-principles study of the Z 2 [Cu 2 O] and Z 2 [Cu 2 oh] motifs
- 2021
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 11:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- As transportation continues to increase world-wide, there is a need for more efficient utilization of fossil fuel. One possibility is direct conversion of the solution gas bi-product CH4 into an energy-rich, easily usable liquid fuel such as CH3OH. However, new catalytic materials to facilitate the methane-to-methanol reaction are needed. Using density functional calculations, the partial oxidation of methane is investigated over the small-pore copper-exchanged zeolite SSZ-13. The reaction pathway is identified and the energy landscape elucidated over the proposed motifs Z2 [Cu2O] and Z2 [Cu2OH]. It is shown that the Z2[Cu2O] motif has an exergonic reaction path, provided water is added as a solvent for the desorption step. However, a micro-kinetic model shows that neither Z2 [Cu2O] nor Z2 [Cu2OH] has any notable activity under the reaction conditions. These findings highlight the importance of the detailed structure of the active site and that the most stable motif is not necessarily the most active.
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| 2. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Descriptor-based microkinetic modelling for methanol-to-DME in zeotypes
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A descriptor-based approach to study the methanol-to-DME reaction in zeotypes from first-principles is herein used to investigate the different reaction mechanism by means of a mean-field microkinetic model. In this theoretical study we focus on Brønsted acidic zeotype catalysts. We demonstrate that the difference of OH stretch frequency for the bare site and when CO is adsorbed on the site is a good descriptor for the acidic strength of the site. Different types of zeotypes are herein studied; CHA, BEA and MFI zeolite frameworks, where besides the conventional Al exchange we also consider B, Ga and Fe exchanged structures, along with SAPO-34, Mg-AlPO-34, Zn-AlPO-34, and Ti-AlPO-34. These span a wide range of strength of the acidic sites. The top panel in the figure to the right shows the free energy landscape for the two main reaction mechanisms for BEA and B-BEA (which have quite different acidity). The concerted mechanism, where two methanol molecules adsorb simultaneously to form DME and water, is shown in lighter colours, and the stepwise mechanism, where one methanol dissociates to a methyl group, producing water, after which a second methanol molecule is adsorbed to form hydrogenated DME, which then easily dehydrogenates, is shown in the darker shades. The bottom panel in the figure shows the calculated turn-over frequency (TOF) for the two mechanisms as a function of temperature. This shows that the concerted mechanism is dominating at lower temperatures while the stepwise dominates at higher temperatures for both systems.
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| 3. |
- Arvidsson, Adam, 1990, et al.
(författare)
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First-principles microkinetic study of methane and hydrogen sulfide catalytic conversion to methanethiol/dimethyl sulfide on Mo 6 S 8 clusters: Activity/selectivity of different promoters
- 2019
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 9:17, s. 4573-4580
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Tidskriftsartikel (refereegranskat)abstract
- A large fraction of the global natural gas reserves is in the form of sour gas, i.e. contains hydrogen sulfide (H2S) and carbon dioxide (CO2), and needs to be sweetened before utilization. The traditional amine-based separation process is energy-intensive, thereby lowering the value of the sour gas. Thus, there is a need to find alternative processes to remove, e.g., hydrogen sulfide. Mo6S8 clusters are promising candidates for transforming methane (CH4) and hydrogen sulfide into methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), which are high-value sulfur-containing products that can be further used in the chemical industry. Here first-principles microkinetics is used to investigate the activity and selectivity of bare and promoted (K, Ni, Cl) Mo6S8. The results show that methanethiol is produced via two different pathways (direct and stepwise), while dimethyl sulfide is formed via a competing pathway in the stepwise formation of methanethiol. Moreover, there is an increase in activity and a decrease in selectivity when adding an electropositive promoter (K), whereas the reverse behaviour is observed when adding an electronegative promoter (Cl). When adding Ni there is also a decrease in activity and an increase in selectivity; however, Ni is acting as an electron donor. The results provide insights and guidance as to what catalyst formulation is preferred for the removal of hydrogen sulfide in sour gas.
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| 4. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Influence of Acidity on the Methanol-to-DME Reaction in Zeotypes: A First Principles-Based Microkinetic Study
- 2020
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:27, s. 14658-14663
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Tidskriftsartikel (refereegranskat)abstract
- Acidity is considered a key factor in zeotype-based catalysts. Here, the effect of acidity in the methanol-to-DME reaction is investigated using first-principles calculations and microkinetic modeling, thereby establishing a connection between acididity and kinetics. The CHA, MFI, and BEA frameworks are investigated, and the acidity of the Brønsted hydroxyl group is varied by exchanging a T-site Si with Al, B, Ga, and Fe in the zeolites, along with SAPO-34, Mg-AlPO-34, Zn-AlPO-34, and Ti-AlPO-34 zeotypes with the CHA structure, and as a result, the Brønsted hydroxyl group spans a wide range of acidity. Clear trends in adsorption and transition-state energies are found and by means of linear regression, we obtain scaling relations of relevant energies that are later used as input in a mean-field steady-state microkinetic model. This study confirms that both the shift in frequency of the Brønsted hydroxyl stretch, ΔfOH, caused by adsorption of CO and the ammonia adsorption energy, ΔEammonia, on the Brønsted site are equivalent descriptors for the acidity of the Brønsted acid site and the reactivity of the different zeotypes relevant for the methanol-to-DME reaction. It further shows that a full microkinetic model is needed to accurately describe the reaction over the whole range of temperatures. However, if focusing on low temperatures, where the associative mechanism is dominating the reaction, a simple rate-determining step model is actually able to describe the results with satisfying agreement (deviation of the rate by less than a factor of two).
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| 5. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Insight of the methanol-to-DME reaction from descriptor-based microkinetic modelling
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Using a descriptor-based approach to study the methanol-to-DME reaction in zeotypes from first-principles, we investigate the different reaction mechanism through a microkinetic model. This theoretical study focuses on Brønsted acidic zeotype catalysts. We demonstrate that the shift in OH stretch frequency with and without CO adsorbed on the site is a good descriptor for the strength of the acid site. Different zeotypes were studied, namely CHA, BEA and MFI zeolite frameworks, where besides conventional Al exchange, we also consider B, Ga and Fe, along with SAPO-34, Mg-AlPO-34, Zn-AlPO-34, and Ti-AlPO-34. These span a wide range of strength of the acidic sites. The top figure shows the free energy landscape for the two main reaction mechanisms for BEA and B-BEA (which have quite different acidity), namely the concerted where two methanol molecules adsorb simultaneously to form DME and water, and the stepwise where one methanol dissociates to a methyl group, producing water. The bottom figure shows the calculated turn-over frequency (TOF) for the two mechanisms as a function of temperature. This shows that the concerted mechanism is dominating at lower temperatures while the stepwise dominates at higher temperatures for both systems.
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| 6. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Metal dimer sites in ZSM-5 zeolite for methane-to-methanol conversion from first-principles kinetic modelling: is the [Cu-O-Cu]2+ motif relevant for Ni, Co, Fe, Ag, and Au?
- 2017
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 7:7, s. 1470-1477
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Tidskriftsartikel (refereegranskat)abstract
- Direct methane-to-methanol conversion is a desired process whereby natural gas is transformed into an energy-rich liquid. It has been realised at ambient pressure and temperature in metal ion-exchanged zeolites, where especially copper-exchanged ZSM-5 has shown promising results. The nature of the active sites in these systems is, however, still under debate. The activity has been assigned to a [Cu-O-Cu]2+ motif. One remaining question is whether this motif is general and also active in other metal-exchanged zeolites. Herein, we use first-principles microkinetic modelling to analyse the methane-to-methanol reaction on the [Cu-O-Cu]2+ motif, for Cu and other metals. First, we identify the cluster model size needed to accurately describe the dimer motif. Starting from the [Cu-O-Cu]2+ site, the metal ions are then systematically substituted with Ni, Co, Fe, Ag and Au. The results show that activation of Ag and Au dimer sites with oxygen is endothermic and therefore unlikely, whereas for Cu, Ni, Co and Fe, the activation is possible under realistic conditions. According to the kinetic simulations, however, the dimer motif is a plausible candidate for the active site for Cu only. For Ni, Co and Fe, close-to-infinite reaction times or unreasonably high temperatures are required for sufficient methane conversion. As Ni-, Co- and Fe-exchanged ZSM-5 are known to convert methane to methanol, these results indicate that the Cu-based dimer motif is not an appropriate model system for these metals.
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| 7. |
- Arvidsson, Adam, 1990, et al.
(författare)
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Modelling partial oxidation of methane over ZSM-5 and Mo6S8 catalysts
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Partial oxidation of methane is an interesting but difficult reaction. Experimentally, methane can be partially oxidized to methanol over metal-exchanged zeolites. In particular the ZSM-5 zeolite has been widely studied and has been shown to be active when exchanged with Cu, Ni, Co, and Fe [1-4]. A clear [Cu-O-Cu] 2+ candidate for active site for the methane-to-methanol reaction has been put forth for Cu-ZSM-5 [1,5]. A first question to ask is whether this [Cu-O-Cu] 2+ motif is generic and could work also with Cu interchanged with Ni, Co, or Fe. Herein, we employ first-principles calculations and micro-kinetic modelling to compare the performance of Ni, Co, and Fe in this motif to the Cu one. Our finding is that the methane-to-methanol reaction can only realistically happen for Cu on this motif. Thus, this particular motif can be excluded as an active site candidate for Ni-, Co-, and Fe-ZSM-5 [6]. Another catalyst that is interesting for partial methane oxidation is the Mo6S8 cluster. However, instead of the methane-to-methanol reaction we use H2S as an oxidant and transform methane into hydrogen and CH3SH. Using electronic structure calculations and mean-field micro-kinetic modelling to study this reaction, we successfully capture the experimentally observed trend, i.e. that promoting the Mo 6 S 8 cluster with K increases the selectivity towards CH 3 SH, and Ni enhances the hydrogen formation [7]. [1] M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs and R. A. Schoonheydt, J. Am. Chem. Soc., 127, 1394–1395 (2005). [2] J. Shan, W. Huang, L. Nguyen, Y. Yu, S. Zhang, Y. Li, A. I. Frenkel and F. F. Tao, Langmuir, 30, 8558–8569 (2014). [3] N. V. Beznis, B. M. Weckhuysen, and J. H. Bitter, Catal. Lett., 136, 52–56 (2010). [4] E. V. Starokon, M. V. Parfenov, L. V. Pirutko, S. I. Abornev and G. I. Panov, J. Phys. Chem. C, 115, 2155–2161 (2011). [5] J. S. Woertink, P. J. Smeets, M. H. Groothaert, M. A. Vance, B. F. Sels, R. A. Schoonheydt, and E. I. Solomon, Proc. Natl. Acad. Sci. U. S. A., 106, 18908–18913 (2009). [6] A. A. Arvidsson, V. P. Zhdanov, P.-A. Carlsson, H. Grönbeck, and A. Hellman, Catal. Sci. Technol., 7, 1470, (2017). [7] O. Y. Gutiérrez, L. Zhong, Y. Zhu, J. A. Lercher, ChemCatChem, 5, 3249–3259 (2013).
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| 8. |
- Arvidsson, Adam, 1990
(författare)
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Partial methane oxidation from electronic structure calculations
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Investigating catalytic reactions with computational methods is a powerful approach to understand fundamental aspects of catalytic reactions and find ways to guide catalytic design. Partial methane oxidation is one example of a reaction with intriguing challenges, where a detailed atomistic approach may help to unravel the bottlenecks of this, as of yet, inefficient reaction. Although methane only needs one oxygen atom for conversion to methanol, the direct oxidation is difficult; it is in fact so difficult that at many oil extraction sites, the methane that inevitably accompanies the crude oil is flared into carbon dioxide and water as gas-phase methane is too inconvenient to store and transport.The main challenge with partial oxidation of methane is to selectively control the oxidation and steer it towards methanol and prevent over-oxidation to CO2. There exist natural enzymes that can partially oxidize methane to methanol at ambient pressure and temperature, although very slowly. One inorganic analogue to these naturally occurring enzymes are zeolites, a porous material that can readily be synthesized and that have been shown to convert methane to methanol at ambient conditions with a high selectivity (>90 %). This has been realized for zeolites ion-exchanged with different metals, such as iron, cobalt, nickel, and copper. Although there have been many attempts to determine the active site for the reaction, there is still no consensus. One candidate that has been put forth is a [Cu-O-Cu]2+ motif experimentally characterized in the ZSM-5 zeolite. In this thesis, partial oxidation of methane is investigated, focusing on this dimer motif. By combining density functional theory calculations with microkinetic modelling, the catalytic performance of the dimer motif is investigated with a simple reaction mechanism for copper, but also with the copper atoms exchanged with nickel, cobalt, iron, silver, or gold. From these results, it is clear that this particular dimer site is a relevant candidate only for copper, and can be excluded in the continued search for active sites in nickel, cobalt, and iron ion-exchanged ZSM-5.To further understand how methanol is formed and interacts with Cu-ZSM-5, experimental and calculated infrared frequencies are compared for methanol and other adsorbates. The partial oxidation of methane is also studied for other systems with oxidants other than oxygen. In particular, methane oxidation with H2S to CH3SH and H2 is explored on molybdenum sulfide clusters.
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| 9. |
- Arvidsson, Adam, 1990
(författare)
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Partial methane oxidation: insights from first principles and micro-kinetics calculations
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Partial methane oxidation is a much-desired reaction with some intriguing challenges. Not only is there a need to activate methane and oxygen, but there is also a need to control the selectivity and prevent over-oxidation to thermodynamically more stable products, like carbon dioxide and water. In fact, this is so difficult that at many oil extraction sites, methane, which inevitably accompanies the welled crude oil, is today flared since gas-phase methane is too inconvenient to store and transport. In nature, there are enzymes that can partially oxidize methane to methanol at ambient pressure and temperature, although at a very slow rate. An often studied class of material with the potential of being an inorganic analogue to these enzymes are zeolites. Zeolites are a porous class of material that can readily be synthesized and that have been shown to convert methane to methanol at ambient conditions with a high selectivity, but with a low conversion. Unravelling the bottlenecks of this, as of yet, inefficient reaction, calls for an atomistic understanding of what is in fact controlling activity and selectivity of the catalysts at hand. In this thesis, zeolites and chemically related structures, zeotypes, are studied using first-principles calculations combined with micro-kinetic modelling. As a first step, a candidate for the active site in these materials, the [Cu-O-Cu]2+ motif, which is found primarily in the ZSM-5 zeolite, and its relevance for Cu, Ni, Co, Fe, Ag, and Au is investigated. Using a straightforward first-principles based micro-kinetic model, we find that this motif is only relevant for copper. Vibrational IR-spectra and temperature programmed desorption spectra are also calculated for monomer and dimer copper motifs in the ZSM-5 and SSZ-13 zeolites, and the results support experimental conclusions. When studying the continued reaction of methanol to dimethyl ether, large-scale trends in activity correlated to the acidity of the acid sites in three zeolite framework types have been determined, indicating that tuning acidity will change the selectivity between methanol and dimethyl ether. The partial oxidation of methane is also studied using molybdenum sulfide clusters, Mo6S8 . These small clusters enables studies of a wider reaction network, similar to the one in zeolites, where the oxygenated species are replaced by their sulfur-contaning counterparts. In this way, it allows investigation of the activity and selectivity towards methanethiol and dimethyl sulfide using different reaction mechanisms and different promoters. The reaction of methane with H2S is used when cleaning sour natural gas, which is why, in this case, H2S is used as the oxidant instead of oxygen. Our results show that the presence of some promoters on the sulfide clusters affect activity, while others affect selectivity. Furthermore, the results show that diffusion is important to include in the kinetic model.
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| 10. |
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