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

Flexible and Heat-Resisting Lignin-Based Epoxy Resins by Hardwood Kraft Low-Molecular-Weight Lignin as a Sustainable Substitute for Bisphenol A

Finding renewable substitutes for bisphenol A (BPA) for producing epoxy resin is of high practical significance. While the low cost and high yield of lignin make it a promising candidate, its high molecular weight, low reactivity, and high dispersity limit its applications. Here, we utilized a low-molecular-weight (low-Mw) lignin mainly consisting of syringaresinol and stilbenes with low dispersity and rich hydroxyl groups as a partial substitute for BPA to synthesize epoxy resins. The resulting lignin-based epoxy resins were designed under three different reaction conditions to yield lignin replacement amounts of 20, 40, and 60%. The epoxy resins with a 20% substitute amount exhibited the best properties with enhanced elongation (239%), tensile stress (144%), and thermal stability (48%) compared to the BPA-based epoxy resin. This study revealed that, besides the epoxy value, low-Mw lignin derivatives including syringaresinol, stilbenes, and aldoketones are essential in improving the elongation. Especially, the newly proposed possible open-loop mechanism of syringaresinol (β–β) indicates that the cross-linked network formed by the furan ring-opening after curing could further reinforce the epoxy resins’ mechanical properties. It provides a new sustainable substitute for BPA to design flexible and heat-resistant lignin-based epoxy resins

Ru–MnO_<i>x</i> Interaction for Efficient Hydrodeoxygenation of Levulinic Acid and Its Derivatives

Metal–oxide interaction was widely observed in supported metal catalysts, playing a significant role in tuning the catalytic performance. Here, we reported that the interaction of Ru and MnOx was able to facilitate the hydrodeoxygenation of levulinic acid (LA) to 2-butanol with a high turnover frequency (1.99 × 106 h–1), turnover number (4411), and yield (98.8%). Moreover, this catalyst was capable of removing the hydroxymethyl group of lactones and diol with high yields of products. The high activity of the Ru–MnOx catalyst was due to the strong Ru–MnOx interaction, which facilitated reduction of Ru oxide to Ru0 and Mn oxide to Mn2+. The increased fractions of Ru0 and Mn2+ provided metal and Lewis acid sites, respectively, and therefore facilitated LA hydrodeoxygenation. A linear correlation between the hydrodeoxygenation activity of the Ru–MnOx catalyst and [Mn2+]­ln­([Ru0]) was observed

Silica–Carbon Nanocomposite Acid Catalyst with Large Mesopore Interconnectivity by Vapor-Phase Assisted Hydrothermal Treatment

Mesostructured silica–carbon nanocomposites with large mesopore interconnectivity are created from sucrose as sustainable carbon source using a mild vapor-phase assisted hydrothermal treatment procedure. The resultant mesostructured silica–carbon nanocomposite can be readily sulfonated to provide a strong acid catalyst with high sulfonic acid density, or the carbon phases of the nanocomposite can be removed by calcination to produce a silica material with ultrahigh porosity (Vpore = 1.25 to 1.34 cm3 g–1). A superior catalytic activity is demonstrated for the solvent-less condensation of 2-methylfuran with furfural; both product yield and conversion rate surpass that of reference catalysts such as their counterparts from dry pyrolysis and the commercial strong acid resins. The enhanced catalytic activity is attributed to the higher SO3H acid density (0.64 to 1.08 mmol g–1), the larger and better communicating mesopores (Vmeso = 0.38 to 0.82 cm3 g–1) and the abundant presence of surface oxygen-containing functional groups on the vapor-phase assisted hydrothermally treated samples. The origin of the well-developed large interconnecting mesopores is investigated and discussed. The mild hydrothermal treatment causes local etching of the original mesopores in the precursor material, creating unexpected interconnectivity between the pores, while the original micropores are basically eliminated during the treatment. Therefore, the here specified hydrothermal treatment provides a promising method to conventional pyrolysis for the efficient and eco-friendly synthesis of highly mesoporous silica–carbon nanocomposites and modification of their physicochemical properties

Highly Effective Production of 5‑Hydroxymethylfurfural from Fructose with a Slow-Release Effect of Proton of a Heterogeneous Catalyst

Highly effective production of 5-hydroxymethylfurfural (HMF) from biobased sugar is one of the vital steps for the synthesis of a fuel precursor. In this work, a slow-release effect of proton from a heterogeneous catalyst induced by 1-butyl-3-methylimidazole chloride ([Bmim]­Cl) ionic liquids was found and employed to effectively convert biobased fructose to HMF. Under mild conditions of 100 °C and 60 min, a promising HMF yield of 94.6% with a fructose conversion of 99.1% could be achieved in a tetrahydrofuran solvent over the synergy of Amberlyst-15 with [Bmim]­Cl, owing to the slow-release effect. Based on extensive control experiments and characterizations, a mechanism of slow release of proton induced by [Bmim]Cl was proposed to better understand the catalytic performance, mainly involving that proton in the −SO3H group of Amberlyst-15 was gradually replaced by the [Bmim]+ of the [Bmim]Cl additive. Additionally, the effect of various parameters (e.g., different ionic liquids and solvents, the loading of the ionic liquid and catalyst, and reaction temperature and time) was systematically investigated. Meanwhile, the stability of Amberlyst-15 and [Bmim]Cl was tested; both could be reused four times. This work provides a novel and highly effective strategy for conversion of biomass-based feedstock toward fuel precursors

Direct Hydrogenation of Furfural to 2‑Methyltetrahydrofuran over an Efficient Cu–Pd/HY Bimetallic Catalyst

The direct one-pot conversion of furfural to 2-methyltetrahydrofuran (2-MTHF) was investigated in liquid phases using H2 as a hydrogen source over a bimetallic Cu–Pd/HY catalyst. This catalyst showed excellent catalytic reactivity toward the formation of 2-MTHF with a yield of 83.1% under optimized reaction conditions. By adjusting the Cu/Pd ratio in the catalyst, the desired product could be obtained selectively. This was due to (1) selective catalysis of Cu toward C=O bonds in furfural, (2) excellent hydrogenation ability of Pd, and (3) the synergistic effects between Cu, Pd, and the acidic sites of the support HY. The influences of other parameters on conversion and selectivity were also investigated. Mechanism studies revealed that reactions mainly perform through the hydrogenation of furfural to furfuryl alcohol and then hydrodeoxygenation to 2-methylfuran followed by furan ring hydrogenation to 2-MTHF. Finally, after five recycling runs, this catalyst still displayed high catalytic behavior and stability, which provided a certain foundation for future research of furfural catalytic hydrogenation to 2-MTHF

Photocatalytic Conversion of Diluted CO₂ into Tunable Syngas via Modulating Transition Metal Hydroxides

The conversion of diluted CO2 into tunable syngas via photocatalysis is critical for implementing CO2 reduction practically, although the efficiency remains low. Herein, we report the use of graphene-modified transition metal hydroxides, namely, NiXCo1–X-GR, for the conversion of diluted CO2 into syngas with adjustable CO/H2 ratios, utilizing Ru dyes as photosensitizers. The Ni(OH)2-GR cocatalyst can generate 12526 μmol g–1 h–1 of CO and 844 μmol g–1 h–1 of H2, while the Co(OH)2-GR sample presents a generation rate of 2953 μmol g–1 h–1 for CO and 10027 μmol g–1 h–1 for H2. Notably, by simply altering the addition amounts of nickel and cobalt in the transition metal composite, the CO/H2 ratios in syngas can be easily regulated from 18:1 to 1:4. Experimental characterization of composites and DFT calculations suggest that the differing adsorption affinities of CO2 and H2O over Ni(OH)2-GR and Co(OH)2-GR play a significant role in determining the selectivity of CO and H2 products, ultimately affecting the CO/H2 ratios in syngas. Overall, these findings demonstrate the potential of graphene-modified transition metal hydroxides as efficient photocatalysts for CO2 reduction and syngas production

A Critical Revisit of Zeolites for CO₂ Desorption in Primary Amine Solution Argues Its Genuine Catalytic Function

Zeolites are the most widely reported solid materials that are used in primary amine-containing postcombustion CO2 capture processes for quicker solvent regeneration at a lower energy consumption. Catalytic solvolysis of the carbamate intermediate, assisted by the Brönsted acid sites (BAS), is commonly accepted as an explanation. However, there is little, if any, attention given to the regeneration of BAS in such basic amine-rich solution. Herein, we revisit the role of zeolite for CO2 desorption in primary amine solution at room to moderately elevated temperature ranges. We noticed indeed an accelerating effect on the CO2 desorption rate in the presence of BAS. Both their numbers as well as their accessibility for the amine-CO2 adduct (i.e., carbamate) (direct) or amine (indirect pathway) are important. However, we also noticed, using spectroscopic techniques and by zeolite reuse, a very strong chemical interaction between BAS and the regenerated amine. This suggests that BAS recovery to close the catalytic cycle is difficult and that zeolites can hardly be considered as genuine catalysts, but rather, this study concludes a merely stoichiometric effect of the zeolites for the overall desorption process, and this is in contrast to reference oxides such as TiO(OH)2

Enhanced Catalytic Activity for Levulinic Acid Esterification Using Covalent Organic Framework Heterogenized Heteropolyacids

Alkyl levulinates act as crucial additives in gasoline and biodiesel and are largely produced through esterification of levulinic acid (LA) with alcohols using homogeneous acids suffering from equipment corrosion and low reusability issues. Here, in this work, a homogeneous phosphotungstic acid (HPW) catalyst was heterogenized via incorporation on a 2D imine-based covalent organic framework material (LZU1) to obtain a heterogeneous HPW-LZU1 catalyst. The material synthesis process was comprehensively monitored through 1H NMR, solid 13C MAS NMR, ATR-FTIR, etc. When the HPW/LZU1 mass ratio was 8.6, the HPW-LZU1 catalyst exhibited an excellent catalytic activity, with >90% conversion of LA and 100% selectivity of alkyl levulinates obtained, comparable to the HPW catalyst. Catalytic performance without obvious decrease after five cycles demonstrated excellent stability and reusability of the as-prepared catalyst. In situ FTIR study shows that LA was dominantly adsorbed on the Brönsted acid sites, while methanol was adsorbed on the Brönsted and Lewis acid sites. The HPW-π interaction-induced electron delocalization effect over the catalyst effectively enhanced the adsorption of LA and alcohols and the attack of alcohols to LA through AAc1 mechanism, attributing to the high catalytic performance of the catalyst