V. Van Speybroeck

Reliably Modeling the Mechanical Stability of Rigid and Flexible Metal-Organic Frameworks

S.M.J. Rogge, M. Waroquier, V. Van Speybroeck
Accounts of Chemical Research
51 (1), 138-148
2018
A1

Abstract 

Over the past two decades, metal–organic frameworks (MOFs) have matured from interesting academic peculiarities toward a continuously expanding class of hybrid, nanoporous materials tuned for targeted technological applications such as gas storage and heterogeneous catalysis. These oft-times crystalline materials, composed of inorganic moieties interconnected by organic ligands, can be endowed with desired structural and chemical features by judiciously functionalizing or substituting these building blocks. As a result of this reticular synthesis, MOF research is situated at the intriguing intersection between chemistry and physics, and the building block approach could pave the way toward the construction of an almost infinite number of possible crystalline structures, provided that they exhibit stability under the desired operational conditions. However, this enormous potential is largely untapped to date, as MOFs have not yet found a major breakthrough in technological applications. One of the remaining challenges for this scale-up is the densification of MOF powders, which is generally achieved by subjecting the material to a pressurization step. However, application of an external pressure may substantially alter the chemical and physical properties of the material. A reliable theoretical guidance that can presynthetically identify the most stable materials could help overcome this technological challenge.

In this Account, we describe the recent research the progress on computational characterization of the mechanical stability of MOFs. So far, three complementary approaches have been proposed, focusing on different aspects of mechanical stability: (i) the Born stability criteria, (ii) the anisotropy in mechanical moduli such as the Young and shear moduli, and (iii) the pressure-versus-volume equations of state. As these three methods are grounded in distinct computational approaches, it is expected that their accuracy and efficiency will vary. To date, however, it is unclear which set of properties are suited and reliable for a given application, as a comprehensive comparison for a broad variety of MOFs is absent, impeding the widespread use of these theoretical frameworks.

Herein, we fill this gap by critically assessing the performance of the three computational models on a broad set of MOFs that are representative for current applications. These materials encompass the mechanically rigid UiO-66(Zr) and MOF-5(Zn) as well as the flexible MIL-47(V) and MIL-53(Al), which undergo pressure-induced phase transitions. It is observed that the Born stability criteria and pressure-versus-volume equations of state give complementary insight into the macroscopic and microscopic origins of instability, respectively. However, interpretation of the Born stability criteria becomes increasingly difficult when less symmetric materials are considered. Moreover, pressure fluctuations during the simulations hamper their accuracy for flexible materials. In contrast, the pressure-versus-volume equations of state are determined in a thermodynamic ensemble specifically targeted to mitigate the effects of these instantaneous fluctuations, yielding more accurate results. The critical Account presented here paves the way toward a solid computational framework for an extensive presynthetic screening of MOFs to select those that are mechanically stable and can be postsynthetically densified before their use in targeted applications.

Open Access version available at UGent repository
Gold Open Access

Influence of a confined methanol solvent on the reactivity of active sites in UiO-66

C. Caratelli, J. Hajek, S.M.J. Rogge, S. Vandenbrande, E.J. Meijer, M. Waroquier, V. Van Speybroeck
ChemPhysChem
19 (4), 420-429
2018
A1

Abstract 

UiO-66, composed of Zr-oxide bricks and terephthalate linkers, is currently one of the most studied metal-organic frameworks due to its exceptional stability. Defects can be introduced in the structure, creating undercoordinated Zr atoms which are Lewis acid sites. Here, additional Brønsted sites can be generated by coordinated protic species from the solvent. In this contribution, a multilevel modeling approach was applied to unravel the effect of a confined methanol solvent on the active sites in UiO-66. First, active sites were explored with static periodic density functional theory calculations to investigate adsorption of water and methanol. Solvent was then introduced in the pores with grand canonical Monte Carlo simulations, followed by a series of molecular dynamics simulations at operating conditions. A hydrogen-bonded network of methanol molecules is formed, allowing the protons to shuttle between solvent methanol, adsorbed water, and the inorganic brick. Upon deprotonation of an active site, the methanol solvent aids the transfer of protons and stabilizes charged configurations via hydrogen bonding, which could be crucial in stabilizing reactive intermediates. The multilevel modeling approach adopted here sheds light on the important role of a confined solvent on the active sites in the UiO-66 material, introducing dynamic acidity in the system at finite temperatures by which protons may be easily shuttled from various positions at the active sites.

Open Access version available at UGent repository
Gold Open Access

Methane Adsorption in Zr-Based MOFs: Comparison and Critical Evaluation of Force Fields

S. Vandenbrande, T. Verstraelen, J. J. Gutierrez-Sevillano, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry C
121 (45), 25309-25322
2017
A1

Abstract 

The search for nanoporous materials that are highly performing for gas storage and separation is one of the contemporary challenges in material design. The computational tools to aid these experimental efforts are widely available and adsorption isotherms are routinely computed for huge sets of (hypothetical) frameworks. Clearly the computational results depend on the interactions between the adsorbed species and the adsorbent, which are commonly described using force fields. In this paper, an extensive comparison and in-depth investigation of several force fields from literature is reported for the case of methane adsorption in the Zr-based Metal-Organic Frameworks UiO-66, UiO-67, DUT-52, NU-1000 and MOF-808. Significant quantitative differences in the computed uptake are observed when comparing different force fields, but most qualitative features are common which suggests some predictive power of the simulations when it comes to these properties. More insight into to the host-guest interactions is obtained by benchmarking the force fields with an extensive number of ab initio computed single molecule interaction energies. This analysis at the molecular level reveals that especially ab initio derived force fields perform well in reproducing the ab initio interaction energies. Finally, the high sensitivity of uptake predictions on the underlying potential energy surface is explored.

Open Access version available at UGent repository
Gold Open Access

Reactivity of 3-oxo-β-lactams with respect to primary amines - an experimental and computational approach

N. Piens, H. Goossens, D. Hertsen, S. Deketelaere, L. Crul, L. Demeurisse, J. De Moor, E. Van den Broeck, K. Mollet, K. Van Hecke, V. Van Speybroeck, M. D'Hooghe
Chemistry - A European Journal
2017 (23), 1-9
2017
A1

Abstract 

The reactivity of 3-oxo-β-lactams with respect to primary amines was investigated in depth. Depending on the specific azetidin-2-one C4 substituent, this reaction was shown to selectively produce 3-imino-β-lactams (through dehydration), α-aminoamides (through CO elimination) or ethanediamides (through an unprecedented C3-C4 ring opening). In addition to the experimental results, the mechanisms and factors governing these peculiar transformations were also examined and elucidated by means of density functional theory calculations.

A series of sulfonic acid functionalized mixed-linker DUT-4 analogues: synthesis, gas sorption properties and catalytic performance

G. Wang, K. Leus, K. Hendrickx, J. Wieme, H. Depauw, Y-Y Liu, V. Van Speybroeck, P. Van der Voort
Dalton Transactions
46, 14356
2017
A1

Abstract 

In this work, we present the successful synthesis of a series of sulfonic acid functionalized mixed-linker metal–organic frameworks (MOFs) having the DUT-4 topology by using different ratios of 2,6-naphthalenedicarboxylic acid (H2-NDC) and 4,8-disulfonaphthalene-2,6-dicarboxylic acid (H2-NDC-2SO3H) in one-pot reactions. The obtained materials were fully characterized and their CO2 adsorption properties at low and high pressures were studied and compared with those of the pristine DUT-4 material. Generally, the CO2 adsorption capacities range from 3.28 and 1.36 mmol g−1 for DUT-4 to 1.54 and 0.78 mmol g−1 for DUT-4-SO3H (50) up to 1 bar at 273 K and 303 K, respectively. Computational calculations corroborated the structural changes of the material in function of the loading of sulfonic acid groups. Furthermore, due to the strong Brønsted acid character, the resulting sulfonic acid based MOF material was evaluated as a catalyst for the ring opening of styrene oxide with methanol as a nucleophile under mild conditions, showing almost full conversion (99%) after 5 hours of reaction. A hot filtration experiment demonstrated that the catalysis occurred heterogeneously and the catalyst could be recovered and reused for multiple runs without significant loss in activity and crystallinity.

Alternating Copolymer of Double Four Ring Silicate and Dimethyl Silicone Monomer - PSS-1

S. Smet, S. Vandenbrande, P. Verlooy, S. Kerkhofs, E. Breynaert, C. Kirschhock, C. Martineau, F. Taulelle, V. Van Speybroeck, J.A. Martens
Chemistry - A European Journal
23 (47), 11286-11293
2017
A1

Abstract 

A new copolymer consisting of double four ring (D4R) silicate units linked by dimethylsilicone monomer referred to as polyoligosiloxysilicone number one (PSS-1) was synthesized. The D4R building unit is provided by hexamethyleneimine cyclosilicate hydrate crystals, which were dehydrated and reacted with dichlorodimethylsilane. The local structure of D4R silicate units and dimethyl silicone monomers was revealed by multidimensional solid-state NMR, FTIR and modeling. On average, D4R silicate units have 6.8 silicone linkages. Evidence for preferential unidirectional growth and chain ordering within the PSS-1 copolymer was provided by STEM and TEM. The structure of PSS-1 copolymer consists of twisted columns of D4R silicate units with or without cross-linking. Both models are consistent with the spectroscopic, microscopic and physical properties. PSS-1 chains are predicted to be mechanically strong compared to silicones such as PDMS, yet more flexible than rigid silica materials such as zeolites.

Asymmetric synthesis of 3,4-disubstituted 2-(trifluoromethyl)pyrrolidines through rearrangement of chiral 2-(2,2,2-trifluoro-1-hydroxyethyl)azetidines

J. Dolfen, E. Birsen Boydas, V. Van Speybroeck, S. Catak, K. Van Hecke, M. D'Hooghe
Journal of Organic Chemistry
82 (19), 10092–10109
2017
A1

Abstract 

Enantiopure 4-formyl-β-lactams were deployed as synthons for the diastereoselective formation of chiral 2-(2,2,2-trifluoro-1-hydroxyethyl)azetidines via trifluoromethylation through aldehyde modification followed by reductive removal of the β-lactam carbonyl moiety. Subsequent treatment of the (in situ) activated 2-trifluoroethylated azetidines with a variety of nitrogen, oxygen, sulfur and fluorine nucleophiles afforded chiral 3,4-disubstituted 2-(trifluoromethyl)pyrrolidines in good to excellent yields (45-99%) and high diastereoselectivities (dr > 99/1, 1H NMR) via interception of bicyclic aziridinium intermediates. Furthermore, representative pyrrolidines were N,O-debenzylated in a selective way and used for further synthetic elaboration to produce e.g. a CF3-substituted 2-oxa-4,7-diazabicyclo[3.3.0]octan-3-one system.

Hydrogen transfer versus methylation: on the genesis of aromatics formation in the Methanol-To-Hydrocarbons over H-ZSM-5

J. S. Martínez-Espín, K. De Wispelaere, T. V. Janssens, S. Svelle, K. P. Lillerud, P. Beato, V. Van Speybroeck, U. Olsbye
ACS Catalysis
7, 5773–5780
2017
A1

Abstract 

The catalytic conversion of methanol (MeOH) and dimethyl ether (DME) into fuels and chemicals over zeolites (MTH process) is industrially emerging as an alternative route to conventional oil-derived processes. After 40 years of research, a detailed mechanistic understanding of the intricate reaction network is still not fully accomplished. The overall reaction is described as two competitive catalytic cycles, dominated by alkenes and arenes, which are methylated and cracked or dealkylated to form effluent products. Herein, we present the reaction of isobutene with methanol and DME as an efficient tool for measuring the relative formation rates of alkenes and arenes, and we provide detailed mechanistic insight into the hydrogen-transfer reaction. We provide experimental and theoretical evidence that manifest a strong competition of methylation and hydrogen transfer of isobutene by methanol, while methylation is substantially favored by DME. Experiments performed at higher conversion facilitate projection of the results to the product distribution obtained when using MeOH or DME as feedstock during the MTH reaction.

Mechanical properties of a gallium fumarate metal-organic framework: a joint experimental-modelling exploration

P. Ramaswamy, J. Wieme, E. Alvarez, L. Vanduyfhuys, J.-P. Itié, P. Fabry, V. Van Speybroeck, C. Serre, P.G. Yot, G. Maurin
Journal of Materials Chemistry A
5 (22), 11047-11054
2017
A1

Abstract 

A gallium analogue of the commercially available Al-fumarate MOF A520 - recently identified as isotypic to MIL-53(Al)-BDC - has been synthesized and further characterized in its hydrated and dehydrated forms. The structural response under applied mechanical pressure of this MIL-53(Ga)-FA solid was investigated using advanced experimental techniques coupled with computational tools. Hg porosimetry and high-pressure X-Ray Powder Diffraction (XRPD) experiments evidenced that the pristine dehydrated large pore form undergoes an irreversible structure contraction upon an applied pressure of 85 MPa with an associated volume change of ca. 14% which makes this material promising for mechanical energy storage applications, in particular as a shock absorber. The breathing behavior was further rationalized performing a series of periodic Density Functional Theory (DFT) calculations with the construction of an energy profile as a function of volume for both MIL-53(Ga)-FA and its Aluminum analogue. As such we could fully unravel the microscopic origin of the difference in pressure-induced behavior for the aluminum and gallium fumarate based materials.

Design of a thermally controlled sequence of triazolinedione-based click and transclick reactions

H.A. Houck, K. De Bruycker, S. Billiet, B. Dhanis, H. Goossens, S. Catak, V. Van Speybroeck, J.M. Winne, F. Du Prez
Chemical Science
8 (4), 3098-3108
2017
A1

Abstract 

The reaction of triazolinediones (TADs) and indoles is of particular interest for polymer chemistry applications, as it is a very fast and irreversible additive-free process at room temperature, but can be turned into a dynamic covalent bond forming process at elevated temperatures, giving a reliable bond exchange or ‘transclick’ reaction. In this paper, we report an in-depth study aimed at controlling the TAD – indole reversible click reactions through rational design of modified indole reaction partners. This has resulted in the identification of a novel class of easily accessible indole derivatives that give dynamic TAD-adduct formation at significantly lower temperatures. We further demonstrate that these new substrates can be used to design a directed cascade of click reactions of a functionalized TAD moiety from an initial indole reaction partner to a second indole, and finally to an irreversible reaction partner. This controlled sequence of click and transclick reactions of a single TAD reagent between three different substrates has been demonstrated both on small molecule and macromolecular level, and the factors that control the reversibility profiles have been rationalized and guided by mechanistic considerations supported by theoretical calculations.

Open Access version available at UGent repository
Green Open Access

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