Multi-site cooperativity in alkali-metal-exchanged faujasites for the production of biomass-derived aromatics

The catalytic Diels–Alder cycloaddition–dehydration (DACD) reaction of furanics with ethylene is a promising route to bio-derived aromatics. The reaction can be catalyzed by alkali-metal-exchanged faujasites. Herein, the results of periodic DFT calculations based on accurate structural models of alkali-metal-exchanged zeolites are presented, revealing the fundamental roles that confinement and the nature of the exchangeable cations in zeolite micropores have in the performance of faujasite-based catalysts in the DACD reaction. Special attention is devoted to analyzing the effect of functional substituents on furanic substrates (furan, 2,5-dimethylfuran, 2,5-furandicarboxylic acid) on the catalyst behavior. It is demonstrated that the conventional reactivity theories of the Diels–Alder chemistry based on simplistic single-site Lewis acidity and substituent effects do not apply if catalytic processes in the multiple-site confined environment of zeolite nanopores are considered. The nature and cooperativity of the interactions between the multiple exchangeable cations and the substrates determine the reaction energetics of the elementary steps involved in the DACD process

Insight into the formation of nanostructured MFI sheets and MEL needles driven by molecular recognition

Mesoporous and nanostructured zeolite-Based catalysts experience prolonged lifetimes due to increased mass transfer and reduced micropore obstruction by coke formation as compared to their bulky microporous counterparts. Diquaternary ammonium structure-Directing agents (SDAs) can be used to synthesize hierarchical MFI sheet-Like and MEL needle-Like zeolites. An explanation of the underlying molecular-Level details of the synthesis of these nanostructured zeolites is presented on the basis of non-Covalent interactions between the template and zeolite surfaces as well as silicate oligomers studied by means of classical molecular dynamics. Use was made of Si 11 and Si 33 silicate oligomers that contain structural features of the framework to be formed as originally proposed by the Leuven group. Molecular recognition is driven by a combination of strong electrostatic and weaker dispersion interactions. An analysis of the early stage of zeolite formation is necessary, as the template adsorption energies in the fully formed zeolite crystals cannot explain the preferential growth of the MFI sheets or MEL needles. Specifically, it is found that the differences in dispersion interactions between the SDA alkyl chains and the silicate oligomers are decisive in the formation of particular zeolite structures

Multi-site cooperativity in alkali-metal-exchanged faujasites for the production of biomass-derived aromatics

\u3cp\u3eThe catalytic Diels–Alder cycloaddition–dehydration (DACD) reaction of furanics with ethylene is a promising route to bio-derived aromatics. The reaction can be catalyzed by alkali-metal-exchanged faujasites. Herein, the results of periodic DFT calculations based on accurate structural models of alkali-metal-exchanged zeolites are presented, revealing the fundamental roles that confinement and the nature of the exchangeable cations in zeolite micropores have in the performance of faujasite-based catalysts in the DACD reaction. Special attention is devoted to analyzing the effect of functional substituents on furanic substrates (furan, 2,5-dimethylfuran, 2,5-furandicarboxylic acid) on the catalyst behavior. It is demonstrated that the conventional reactivity theories of the Diels–Alder chemistry based on simplistic single-site Lewis acidity and substituent effects do not apply if catalytic processes in the multiple-site confined environment of zeolite nanopores are considered. The nature and cooperativity of the interactions between the multiple exchangeable cations and the substrates determine the reaction energetics of the elementary steps involved in the DACD process.\u3c/p\u3

Shape selectivity in linear paraffins hydroconversion in 10-membered-ring pore zeolites

Pd/zeolite-catalyzed hydroconversion of n-hexadecane (n-C16) and n-heptane (n-C7) was studied for 10MR (ZSM-5, ZSM-22), 12MR (ZSM-12), and EMM-23 (21MR × 10MR) zeolites. The catalytic activity depended on the Brønsted acidity and the crystalline domain size. n-C16 hydroconversion benefited from short diffusion lengths in ZSM-5 nanosheets compared to bulk ZSM-5. In general, over-cracking is dominant in ZSM-5 with a cracked product distribution skewed to C4 products, to be explained by a snug fit of particular dibranched isomers at zeolite intersections. This effect is less pronounced for the 1D 10MR pores in ZSM-22, which lacks intersections. Although large pores in ZSM-12 offer relatively high activity, those in EMM-23 do not. Based on selectivity patterns, EMM-23 behaves like ZSM-5, probably because of the trilobe shape of its 21MR pores acting as 10MR pores. Only ZSM-12 offers operation in the ideal hydrocracking regime, in the sense of impediments neither by hydrogenation nor by diffusion. Faster intrazeolite diffusion of n-C7 in comparison to n-C16 leads to a higher yield of isomers for the nanostructured zeolites. Overall, the hydroconversion of the smaller alkane is more substantially impacted by variations in the crystalline zeolite domain size.</p

Insight into the formation of nanostructured MFI sheets and MEL needles driven by molecular recognition

\u3cp\u3e Mesoporous and nanostructured zeolite-Based catalysts experience prolonged lifetimes due to increased mass transfer and reduced micropore obstruction by coke formation as compared to their bulky microporous counterparts. Diquaternary ammonium structure-Directing agents (SDAs) can be used to synthesize hierarchical MFI sheet-Like and MEL needle-Like zeolites. An explanation of the underlying molecular-Level details of the synthesis of these nanostructured zeolites is presented on the basis of non-Covalent interactions between the template and zeolite surfaces as well as silicate oligomers studied by means of classical molecular dynamics. Use was made of Si \u3csub\u3e11\u3c/sub\u3e and Si \u3csub\u3e33\u3c/sub\u3e silicate oligomers that contain structural features of the framework to be formed as originally proposed by the Leuven group. Molecular recognition is driven by a combination of strong electrostatic and weaker dispersion interactions. An analysis of the early stage of zeolite formation is necessary, as the template adsorption energies in the fully formed zeolite crystals cannot explain the preferential growth of the MFI sheets or MEL needles. Specifically, it is found that the differences in dispersion interactions between the SDA alkyl chains and the silicate oligomers are decisive in the formation of particular zeolite structures. \u3c/p\u3

Shape selectivity in linear paraffins hydroconversion in 10-membered-ring pore zeolites

Pd/zeolite-catalyzed hydroconversion of n-hexadecane (n-C16) and n-heptane (n-C7) was studied for 10MR (ZSM-5, ZSM-22), 12MR (ZSM-12), and EMM-23 (21MR × 10MR) zeolites. The catalytic activity depended on the Brønsted acidity and the crystalline domain size. n-C16 hydroconversion benefited from short diffusion lengths in ZSM-5 nanosheets compared to bulk ZSM-5. In general, over-cracking is dominant in ZSM-5 with a cracked product distribution skewed to C4 products, to be explained by a snug fit of particular dibranched isomers at zeolite intersections. This effect is less pronounced for the 1D 10MR pores in ZSM-22, which lacks intersections. Although large pores in ZSM-12 offer relatively high activity, those in EMM-23 do not. Based on selectivity patterns, EMM-23 behaves like ZSM-5, probably because of the trilobe shape of its 21MR pores acting as 10MR pores. Only ZSM-12 offers operation in the ideal hydrocracking regime, in the sense of impediments neither by hydrogenation nor by diffusion. Faster intrazeolite diffusion of n-C7 in comparison to n-C16 leads to a higher yield of isomers for the nanostructured zeolites. Overall, the hydroconversion of the smaller alkane is more substantially impacted by variations in the crystalline zeolite domain size

Correlations between Density-Based Bond Orders and Orbital-Based Bond Energies for Chemical Bonding Analysis

\u3cp\u3eQuantum chemistry-based codes and methods provide valuable computational tools to estimate reaction energetics and elucidate reaction mechanisms. Electronic structure methods allow directly studying the chemical transformations in molecular systems involving breaking and making of chemical bonds and the associated changes in the electronic structure. The link between the electronic structure and chemical bonding can be provided through the crystal orbital Hamilton population (COHP) analysis that allows quantifying the bond strength by computing Hamilton-weighted populations of localized atomic orbitals. Another important parameter reflecting the nature and strength of a chemical bond is the bond order that can be assessed by the density derived electrostatic and chemical (DDEC6) method which relies on an electron and spin density-partitioning scheme. Herein, we describe a linear correlation that can be established between the DDEC6-derived bond orders and the bond strengths computed with the COHP formalism. We demonstrate that within defined boundaries, the COHP-derived bond strengths can be consistently compared among each other and linked to the DDEC6-derived bond orders independent of the used model. The validity of these correlations and the effective model independence of the electronic descriptors are demonstrated for a variety of gas-phase chemical systems, featuring different types of chemical bonds. Furthermore, the applicability of the derived correlations to the description of complex reaction paths in periodic systems is demonstrated by considering the zeolite-catalyzed Diels-Alder cycloaddition reaction between 2,5-dimethylfuran and ethylene. ©\u3c/p\u3

Electronic structure analysis of the Diels-Alder cycloaddition catalyzed by alkali-exchanged faujasites

\u3cp\u3eThe Diels-Alder cycloaddition (DAC) reaction is a commonly employed reaction for the formation of C-C bonds. DAC catalysis can be achieved by using Lewis acids and via reactant confinement in aqueous nanocages. Low-silica alkali-exchanged faujasite catalysts combine these two factors in one material. They can be used in the tandem DAC/dehydration reaction of biomass-derived 2,5-dimethylfuran (DMF) with ethylene toward p-xylene, in which the DAC reaction step initiates the overall reaction cycle. In this work, we performed periodic density functional theory (DFT) calculations on the DAC reaction between DMF and C \u3csub\u3e2\u3c/sub\u3eH \u3csub\u3e4\u3c/sub\u3e in low-silica alkali(M)-exchanged faujasites (MY; Si/Al = 2.4; M = Li \u3csup\u3e+\u3c/sup\u3e, Na \u3csup\u3e+\u3c/sup\u3e, K \u3csup\u3e+\u3c/sup\u3e, Rb \u3csup\u3e+\u3c/sup\u3e, Cs \u3csup\u3e+\u3c/sup\u3e). The aim was to investigate how confinement of reactants in MY catalysts changed their electronic structure and the DAC-reactivity trend among the evaluated MY zeolites. The conventional high-silica alkali-exchanged isolated site model (MFAU; Si/Al = 47) served as a reference. The results show that confinement leads to initial-state (IS) destabilization and transition-state (TS) stabilization. Among the tested MY, most significant IS destabilization is found in RbY. Only antibonding orbital interactions between the reactants/reactive complex and cations were found, indicating that TS stabilization arises from ionic interactions. Additionally, in RbY the geometry of the transition state is geometrically most similar to that of the initial and final state. RbY also exhibits an optimal combination of the confinement-effects, resulting in having the lowest computed DAC-activation energy. The overall effect is a DAC-reactivity trend inversion in MY as compared to the trend found in MFAU where the activation energy correlates with the Lewis acidity of the exchangeable cations. \u3c/p\u3

Mechanistic insight into the [4 + 2] Diels-Alder cycloaddition over first row d-block cation-exchanged faujasites

The Diels-Alder cycloaddition (DAC) is a powerful tool to construct C-C bonds. The DAC reaction can be accelerated in several ways, one of which is reactant confinement as observed in supramolecular complexes and Diels-Alderases. Another method is altering the frontier molecular orbitals (FMOs) of the reactants by using homogeneous transition-metal complexes whose active sites exhibit d-orbitals suitable for net-bonding orbital interactions with the substrates. Both features can be combined in first row d-block (TM) exchanged faujasite catalysts where the zeolite framework acts as a stabilizing ligand for the active site while confining the reactants. Herein, we report on a mechanistic and periodic DFT study on TM-(Cu(I), Cu(II), Zn(II), Ni(II), Cr(III), Sc(III), V(V))exchanged faujasites to elucidate the effect of d-shell filling on the DAC reaction between 2,5-dimethylfuran and ethylene. Two pathways were found: one being the concerted one-step and the other being the stepwise two-step pathway. A decrease in d-shell filling results in a concomitant increase in reactant activation as evidenced by increasingly narrow energy gaps and lower activation barriers. For models holding relatively small d-block cations, the zeolite framework was found to bias the DAC reaction toward an asynchronous one-step pathway instead of the two-step pathway. This work is an example of how the active site properties and the surrounding chemical environment influence the reaction mechanism of chemical transformations.