Lewis Base Activation of Lewis Acid: A Detailed Bond Analysis

Adsorption and mechanism study for phenol removal by 10% CO2 activated bio-char after acid or alkali pretreatment

The development of an efficient bio-char used to remove phenol from wastewater holds great importance for environmental protection. In this work, wheat straw bio-char (BC) was acid-washed by HF and activated at 900 °C with 10% CO2 to obtain bio-char (B-Ⅲ-0.1D900). Adsorption experiments revealed that B-Ⅲ-0.1D900 achieved a remarkable phenol removal efficiency of 90% within 40 min. Despite its relatively low specific surface area of 492.60 m2/g, it exhibited a high maximum adsorption capacity of 471.16 mg/g. Furthermore, B-Ⅲ-0.1D900 demonstrated a good regeneration capacity for at least three cycles (90.71%, 87.54%, 84.36%). It has been discovered that HF washing, which removes AAEM and exposes unsaturated functional groups, constitutes one of the essential prerequisites for enhancing CO2 activation efficiency at high temperatures. After 10% CO2 activation, the mesoporous structure exhibited substantial development, facilitating enhanced phenol infiltration into the pores when compared to untreated BC. The increased branching of the bio-char culminated in a more complete aromatic system, which enhances the π-π forces between the bio-char and the phenol. The presence of tertiary alcohol structure enhances the hydrogen bonding forces, thereby promoting intermolecular multilayer adsorption of phenol. With the combination of various forces, B-Ⅲ-0.1D900 has a good removal capacity for phenol. This work provides valuable insights into the adsorption of organic pollutants using activated bio-char.

Investigation into Lewis and Brønsted acid interactions between metal chloride and aqueous choline chloride-oxalic acid for enhanced furfural production from lignocellulosic biomass

Furfural has been identified as a valuable biobased platform chemical that can be further converted into bioenergy and biochemicals. Furfural is derived from lignocellulosic biomass and can also be regarded as a sustainable alternative to petrochemical products. Herein, the performance of trivalent metal chlorides (FeCl₃, AlCl₃) and tetravalent metal chlorides (SnCl₄, TeCl₄) as Lewis acidic cocatalysts was investigated in an aqueous choline chloride-oxalic acid (16.4 wt% H₂O) deep eutectic solvent (DES) system for producing furfural from oil palm fronds (OPFs). The metal chlorides with greater electrical field gradients were stronger Lewis acids that enhanced both furfural production and degradation reactions. The main degradation product in this reaction system was humin, and this result was confirmed by FTIR analysis. By subjecting OPFs to an aqueous DES reaction (120 °C, 45 min) with SnCl₄ (2.50 wt%), a furfural yield of 59.4% was obtained; without incorporated metal chlorides, the furfural yield was 46.1%. Characterization studies showed synergistic Lewis and Brønsted acid interactions between metal chlorides and DES components. Overall, the residual OPFs showed high glucan content, which led to the production of glucose (71.4%) as a byproduct via enzymatic hydrolysis. Additionally, the aqueous DES system was recycled and reused for several additional runs. The proposed aqueous DES system presents a promising biorefinery approach for the conversion of OPFs to biochemicals.

Assessing the Orbital Contribution in the "Spodium Bond" by Natural Orbital for Chemical Valence-Charge Displacement Analysis

The term "spodium bond" (SpB) has been recently proposed to describe the noncoordinative interaction that can be established between a polarized group 12 metal and a mild Lewis base (LB). Most of the systems showing short metal-donor distances compatible with SpB are characterized by the coexistence of multiple weak interactions, including hydrogen and halogen bonding, making the assessment of real importance of SpB difficult. Here, we show that the relative importance of each contribution can be probed by dissecting the orbital component of the interaction through the extended transition state-natural orbital for chemical valence-charge displacement analysis (ETS-NOCV-CD). The latter gives useful information about relative energies and electrons involved, for model systems ([(thiourea)₂MX₂]···LB; M = Zn, Cd, and Hg; X = Cl and I; and LB = CH₂S, CH₂O, CH₃CN, and CO) and a variety of structures extracted from experimentally characterized adducts, allowing us to demonstrate the lack of a direct correlation between a favorable metal-base distance and the presence of an orbital contribution for the SpB.

Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

As known, the electron density of covalently bound halogen atoms is anisotropically distributed, making them potentially able to establish many weak interactions, acting at the same time as halogen bond donors and hydrogen bond acceptors. Indeed, there are many examples in which the halogen and hydrogen bond coexist in the same structure and, if a correct bond analysis is required, their separation is mandatory. Here, the advantages and limitations of coupling the charge displacement analysis with natural orbital for chemical valence method (NOCV-CD) to separately analyze orthogonal weak interactions are shown, for both symmetric and asymmetric adducts. The methodology gives optimal results with intermolecular adducts but, in the presence of an organometallic complex, also intramolecular interactions can be correctly analyzed. Beyond the methodological aspects, it is shown that correctly separate and quantify the interactions can give interesting chemical insights about the systems.

Charge Displacement Analysis-A Tool to Theoretically Characterize the Charge Transfer Contribution of Halogen Bonds

Theoretical bonding analysis is of prime importance for the deep understanding of the various chemical interactions, covalent or not. Among the various methods that have been developed in the last decades, the analysis of the Charge Displacement function (CD) demonstrated to be useful to reveal the charge transfer effects in many contexts, from weak hydrogen bonds, to the characterization of σ hole interactions, as halogen, chalcogen and pnictogen bonding or even in the decomposition of the metal-ligand bond. Quite often, the CD analysis has also been coupled with experimental techniques, in order to give a complete description of the system under study. In this review, we focus on the use of CD analysis on halogen bonded systems, describing the most relevant literature examples about gas phase and condensed phase systems. Chemical insights will be drawn about the nature of halogen bond, its cooperativity and its influence on metal-ligand bond components.

Influence of halogen bonding on gold(i)-ligand bond components and DFT characterization of a gold-iodine halogen bond

A gold(i) complex bearing nitrogen acyclic carbene (NAC) and selenourea (SeU) has been used to verify whether the second-sphere SeI halogen bond (XB) is able to modify the Dewar-Chatt-Duncanson components of the Au-C and Au-Se bonds. The chosen system was found to be thermically unstable but it allowed an in-depth theoretical study by means of Energy Decomposition Analysis, Natural Bond Orbital and Natural Orbitals for Chemical Valence methods, coupled with Charge Displacement analysis. Indeed, in the presence of iodoperfluoroalkanes as XB donors, iodine interacts with the lone pair of the coordinated selenium, enhancing the Au ← C σ donation and depressing the Au → C π back-donation, as demonstrated also by the increase of the rotational barrier of the C-N bond of the NAC (see G. Ciancaleoni and others, Chem. - Eur. J., 2015, 21, 2467). On the other hand, in the presence of N-iodosuccinimide (NIS), the gold directly establishes a XB with the iodine by using its d lone pairs. This AuI XB is favored by the low steric hindrance of the ligands coordinated to the gold and the presence of the amino protons of SeU, which establish additional hydrogen bonds with the NIS. Also in this case, the effect is to increase the σ acidity and decrease the π basicity of the metal.

Interplay between Gold(I)-Ligand Bond Components and Hydrogen Bonding: A Combined Experimental/Computational Study

The influence of weak interactions on the donation/back-donation bond components in the complex [(NHC)Au(SeU)]⁺ (NHC = N-heterocyclic carbene; SeU = selenourea) has been studied by coupling experimental and theoretical techniques. In particular, NMR ¹H and pulsed-field gradient spin-echo titrations allowed us to characterize the hydrogen bond (HB) between the -NH₂ moieties of SeU and the anions PF₆ ^- and ClO₄ ^-, whereas ⁷⁷Se NMR spectroscopy allowed us to characterize the Au-Se bond. Theoretically, the Au-Se and Au-C orbital interactions have been decomposed using the natural orbital for the chemical valence framework and the bond components quantified through the charge displacement analysis. This methodology provides the quantification of the Dewar-Chatt-Duncanson (DCD) components for the Au-C and Au-Se bonds in the absence and presence of the second-sphere HB. The results presented here show that the anion has a dual mode action: it modifies the conformation of the cation by ion pairing (and this already influences the DCD components) and it induces new polarization effects that depend on the relative anion/cation relative orientation. The perchlorate polarizes SeU, enhancing the Se → Au σ donation and the Au → C back-donation and depressing the C → Au σ donation. On the contrary, the hexafluorophosphate depresses both the Se → Au and C → Au σ donations.