Branching-First: Synthesizing C–C Skeletal Branched Biobased Chemicals from Sugars

A novel strategy to biobased chemicals with a branched carbon skeleton is introduced. Hereto, small sugars, such as 1,3-dihydroxyacetone, are coupled catalytically to obtain branched C₆ sugars, such as dendroketose, in high yield at mild conditions. By bringing this branching step up front, at the level of the sugar feedstock (<i>branching-first</i>), new opportunities for the synthesis of useful chemicals arise. Here, we show that the branched sugar can be efficiently valorized into (i) new branched polyols and (ii) short branched alkanes. The first route preserves most of the original sugar functionality by hydrogenation with Ru/C and renders access to branched polyols with three primary alcohol groups. These molecules are potentially interesting as plasticizers, cross-linkers, or detergent precursors. The second valorization route demonstrates a facile hydrodeoxygenation of the branched sugars toward their corresponding branched alkanes (e.g., 2-methylpentane). The highest alkanes yields (>65 mol % C) are obtained with a Rh/C redox metal catalyst in a biphasic catalytic system, following a HDO mechanism. In the short term, commercial integration of these monobranched alkanes, in contrast to branched polyols, is expected to be straightforward because of their drop-in character and well-known valuable octane booster role when present in gasoline. Accordingly, the <i>branching-first</i> concept is also demonstrated with other small sugars (e.g., tetroses) enabling the production of branched C₆–C₈ sugars and thus also branched C₅–C₈ alkanes after HDO

Impact of Hydrotalcite’s Basic Sites on the Catalyst Stability and the Branching Selectivity in α‑Hydroxyketone Aldolization

Reconstructed hydrotalcites serve as powerful catalysts for the aldolization of α-hydroxyketones, exemplified here by the glycerol-derived dihydroxyacetone (DHA), toward the formation of branched hexoses (dendroketoses). Due to the multichemical functionality of these hydroxyketones, various competitive reactions such as keto-aldehyde isomerization (e.g., dihydroxyacetone (DHA)/glyceraldehyde (GLA) equilibrium) are possible, reducing the branching selectivity of the aldolization reaction. This study reveals that the nature of the basic sites (as determined via CDCl3 probe FT-IR and CO2-TPD experiments) strongly affects the branching selectivity of the condensation reaction as well as the stability of the hydrotalcite catalyst. For instance, strong basic sites not only allow the undesired keto-aldehyde isomerization but also promote the Cannizzaro reactions toward the formation of organic carboxylic acids leading to the catalyst leaching and waste generation. Thus, subsequent chemical transformations of branched sugars cannot occur without prior purification. For instance, the hydrogenation toward biobased branched polyol, which is the industrial target in casu base condensation of DHA. Tuning the basic properties of the reconstructed hydrotalcite based on this knowledge ultimately led to an active, selective, and stable catalyst with improved regeneration possibility

Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites

Two [Cu₂O]²⁺ cores have been identified as the active sites of low temperature methane hydroxylation in the zeolite Cu-MOR. These cores have similar geometric and electronic structures, yet different reactivity with CH₄: one reacts with a much lower activation enthalpy. In the present study, we couple experimental reactivity and spectroscopy studies to DFT calculations to arrive at structural models of the Cu-MOR active sites. We find that the more reactive core is located in a constricted region of the zeolite lattice. This leads to close van der Waals contact between the substrate and the zeolite lattice in the vicinity of the active site. The resulting enthalpy of substrate adsorption drives the subsequent H atom abstraction stepa manifestation of the “nest” effect seen in hydrocarbon cracking on acid zeolites. This defines a mechanism to tune the reactivity of metal active sites in microporous materials

Identification of α‑Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations

α-Fe is the precursor of the reactive Fe^IVO core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe­(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV–vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm^–1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe­(II) located at a 6MR with an Al–Si–Si–Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm^–1 in the experimental spectra, assigned to spectator Fe­(II), are attributed to six-coordinated trigonal-prismatic Fe­(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe­(II) model complex FeL₂ (with L = [Al­(OH)₄]⁻). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe­(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3d_<i>z</i>² orbital with the higher lying 4s

Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam

This contribution studies the steam-assisted dealkylation of 4-<i>n</i>-propylphenol (4-<i>n</i>-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (<i>e.g</i>., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (<i>i</i>) analyzing products distribution under varying temperature and contact time conditions, (<i>ii</i>) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (<i>iii</i>) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed to a pore confinement, avoiding disproportionation/transalkylation as a result of a transition state shape selectivity. The presence of water maintains a surprisingly stable catalysis, especially for ZSM-5 with low acid density. The working hypothesis of this stabilization is that water precludes diphenyl ether(s) formation in the pores by reducing the lifetime of the phenolics at the active site due to the high heat of adsorption of water on H-ZSM-5, besides counteracting the equilibrium of the phenolics condensation reaction. The water effect is unique for the combination of (alkyl)­phenols and ZSM-5

Miniaturized Layer-by-Layer Deposition of Metal–Organic Framework Coatings through Digital Microfluidics

Miniaturized Layer-by-Layer Deposition of Metal–Organic Framework Coatings through Digital Microfluidic

An Inner-/Outer-Sphere Stabilized Sn Active Site in β‑Zeolite: Spectroscopic Evidence and Kinetic Consequences

A highly active Sn site with Lewis acid properties is identified in post-synthetically synthesized Sn/DeAlβ catalyst, prepared by liquid-phase Sn grafting of a dealuminated β-zeolite. Though apparently similar Sn active-site structures have been reported for the post-synthetic and the conventional hydrothermal Snβ, detailed study of the electronic structure and redox behavior of Sn with EXAFS, XANES, DR UV–vis, and TPR clearly reveals dissimilarities in geometry and electronic properties. A model of the active Sn site is proposed using a contemporary interpretation of inner-/outer-sphere coordination, assuming inner-sphere coordination of Sn^IV with three framework SiO^– and one outer-sphere coordination by a distant charge-balancing SiO^–, resulting in a separated Lewis acid–base pair. Stabilization of this geometry by a nearby water molecule is proposed. In comparison with active Sn sites in a hydrothermally synthesized Snβ, those in the grafted dealuminated material are sterically less demanding for substrate approach, while the low inner-sphere coordination of Sn leads to a stronger Lewis acidity. Proximate silanols in the active-site pocket, identified by FTIR, ²⁹Si MAS NMR, ¹H–²⁹Si CP MAS NMR, DR NIR, and TGA, may impact local reagent concentration and transition states stabilization by hydrogen bonding. The structural dissimilarity of the active Sn site leads to a different kinetic behavior. Kinetic experiments using two Lewis-acid-catalyzed reactions, Baeyer–Villiger and Meerwein–Ponndorf–Verley, show differences that are reaction-type dependent and have different entropic (like sterical demand and hydrogen bonding) and enthalpic contributions (Lewis acid strength). The active-site model, containing both inner- and outer-sphere ligands with the zeolite framework, may be considered as a general model for other grafted Lewis acid single sites

Influence of Acidic (H₃PO₄) and Alkaline (NaOH) Additives on the Catalytic Reductive Fractionation of Lignocellulose

Reductive catalytic fractionation of lignocellulose is a promising “lignin-first” biorefinery strategy wherein lignin is solvolytically extracted from the cell wall matrix and simultaneously disassembled, resulting in a stable lignin oil and a solid carbohydrate-rich residue. Herein, we report on the different influence of acidic (H₃PO₄) and alkaline (NaOH) additives on the Pd/C-catalyzed reductive processing of poplar wood in methanol (MeOH). It was found that the addition of small quantities of H₃PO₄ results in three rather than two product streams, since under acidic conditions both delignification and alcoholysis of hemicellulose are promoted, leaving behind a cellulose-rich pulp. The simultaneous acid-catalyzed fractionation of the carbohydrates into separate cellulose and hemicellulose streams provides opportunities for more efficient downstream conversion, as processing parameters can be tailored to the needs of both streams. Alkaline conditions, on the other hand, also enhance delignification, but additionally cause (i) the formation of lignin products other than those obtained under neutral and acidic conditions, (ii) a hampered degree of lignin depolymerization, and (iii) substantial loss of cellulose from the pulp. Further on, a modified process descriptor (LFFE: <i>lignin first fractionation efficiency</i>) was applied to evaluate the fractionation efficiency of lignocellulose in its three major constituents. According to this new efficiency measure, mildly acidic conditions performed best

Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation

The highly reactive binuclear [Cu2O]2+ active site in copper zeolites activates the inert C–H bond of methane at low temperatures, offering a potential solution to reduce methane flaring and mitigate atmospheric methane levels. While substantial progress has been made in understanding the activation of methane by this core, one critical aspect, the active site’s spin, has remained undetermined. In this study, we use variable-temperature, variable-field magnetic circular dichroism spectroscopy to define the ground state spin of the [Cu2O]2+ active sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective determination of the magnetic exchange coupling between the two copper centers of specific [Cu2O]2+ cores in a heterogeneous mixture, circumventing the drawbacks of bulk magnetic techniques. These experimental findings are coupled to density functional theory calculations to elucidate magnetostructural correlations in copper zeolites that are different from those of homogeneous binuclear Cu(II) complexes. The different spin states for the [Cu2O]2+ cores have different reactivities governed by how methane approaches the active site. This introduces a new understanding of zeolite topological control on active site reactivity

Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation

The industrial implementation of a direct methane-to-methanol process would lead to environmental and economic benefits. Copper zeolites successfully execute this reaction at relatively low temperatures, and mordenite zeolites in particular enable high methanol production. When loaded to a Cu/Al ratio of 0.45, mordenite (Si/Al 5–9) has been shown to host three active sites: two [CuOCu]2+ sites labeled MOR1 and MOR2 and a mononuclear [CuOH]+ site. Also at low copper loadings (Cu/Al < 0.20), mordenite has been demonstrated to activate methane, but its active site has never been reported. Here, we investigate Na+ mordenite with varying copper loadings to better understand copper speciation in mordenite. At low copper loadings, we uncover an unidentified active site (“MOR3”) with a strong overlap with the [CuOH]+ site’s spectroscopic signal. By changing the co-cation, we selectively speciate more MOR3 relative to [CuOH]+, allowing its identification as a [CuOCu]2+ site. Active site identification in heterogeneous catalysts is a frequent problem due to signal overlap. By changing cation composition, we introduce an innovative method for simplifying a material to allow better analysis. This has implications for the study of Cu zeolites for methane-to-methanol and NOx catalysis, but also for studying and tuning heterogeneous catalysts in general