Demystifying group-4 polyolefin hydrogenolysis catalysis. Gaseous propane hydrogenolysis mechanism over the same catalysts.

A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h-1 at 150 °C. Both catalysts exhibit approximately linear relationships between rate and [H2] at substoichiometric [H2] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H2], the rates approach zero-order in [H2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular ß-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH‡ = 16.8 ± 0.2 kcal mol-1 and 18.2 ± 0.6 kcal mol-1, respectively, track the relative turnover frequencies, while ΔS‡ = -19.1 ± 0.8 and -16.7 ± 1.4 cal mol-1 K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.

Hydrogenolysis of Cationic Half-Sandwich Zinc Complexes Containing a Chelating Amine: Facile Cleavage of Zinc-Carbon Bond by Dihydrogen to Give Zinc Hydride Cations.

Cationic half-sandwich zinc complexes containing chelating amines [Cp*Zn(Ln)][BAr4 F] (2 a, Cp*=η3-C5Me5, Ln=N,N,N',N'-tetramethylethylenediamine, TMEDA; 2 b, Ln=N,N,N',N'-tetraethylethylenediamine, TEEDA; 2 c, Cp*=η1-C5Me5, Ln=N,N,N',N'',N''-pentamethyldiethylenetriamine, PMDTA; Ar4 F=(3,5-(CF3)2C6H3)4) reacted with dihydrogen (ca. 2 bar) in THF at 80 °C to give molecular zinc hydride cations [(Ln)ZnH(thf)m][BAr4 F] (3 a,b, m=1; 3 c, m=0) previously reported along with Cp*H. Pseudo first-order kinetics with respect to the concentration of 2 b suggests heterolytic cleavage of dihydrogen by the Zn-Cp* bond, reminiscent of σ-bond metathesis. Hydrogenolysis of the zinc cation 2 b in the presence of benzophenone gave the zinc alkoxide [(TEEDA)Zn(OCHPh2)(thf)][BAr4 F] (5 b). Cation 2 b was shown to catalytically hydrogenate N-benzylideneaniline. The PMDTA complex 2 c underwent C-H bond activation in acetonitrile to give a dinuclear µ-κC,κN-cyanomethyl zinc complex [(PMDTA)Zn(CH2CN)]2[BAr4 F]2 (6 c).

Hydrogenolysis of Glycerol over NiCeZr Catalyst Modified with Mg, Cu, and Sn at the Surface Level.

Biomass valorization is an essential strategy for converting organic resources into valuable energy and chemicals, contributing to the circular economy, and reducing carbon footprints. Glycerol, a byproduct of biodiesel production, can be used as a feedstock for a variety of high-value products and can contribute to reducing the carbon footprint. This study examines the impact of surface-level modifications of Mg, Cu, and Sn on Ni-Ce-Zr catalysts for the hydrogenolysis of glycerol, with in situ generated hydrogen. The aim of this approach is to enhance the efficiency and sustainability of the biomass valorization process. However, the surface modification resulted in a decrease in the global conversion of glycerol due to the reduced availability of metal sites. The study found that valuable products, such as H2 and CH4 in the gas phase, and 1,2-PG in the liquid phase, were obtained. The majority of the liquid fraction was observed, particularly for Cu- and Sn-doped catalysts, which was attributed to their increased acidity. The primary selectivity was towards the cleavage of the C-O bond. Post-reaction characterizations revealed that the primary causes of deactivation was leaching, which was reduced by the inclusion of Cu and Sn. These findings demonstrate the potential of Cu- and Sn-modified Ni-Ce-Zr catalysts to provide a sustainable pathway for converting glycerol into value-added chemicals.

Influence of Synthesis Conditions on Catalytic Performance of Ni/CeO2 in Aqueous-Phase Hydrogenolysis of Glycerol without External Hydrogen Input.

The aqueous-phase hydrogenolysis of glycerol was studied in Ni/CeO2 catalytic systems prepared by incipient wetness impregnation. The operating conditions were 34 bar, 227 ºC, 5 wt.% of glycerol, and a W/mglycerol = 20 g catalyst min/g glycerol without a hydrogen supply. The effect of the catalyst preparation conditions on the catalytic activity and physicochemical properties of the catalysts was assessed, particularly the calcination temperature of the support, the calcination temperature of the catalyst, and the Ni content. The physicochemical properties of the catalysts were determined by N2 adsorption, H2-TPR, NH3-TPD, and XRD, among other techniques. A relevant increase in acidity was observed when increasing the nickel content up to 20 wt.%. The increase in the calcination temperatures of the supports and catalysts showed a detrimental effect on the specific surface area and acid properties of the catalysts, which were crucial to the selectivity of the reaction. These catalysts notably enhanced the yield of liquid products, achieving global glycerol conversion values ranging from 17.1 to 29.0% and carbon yield to liquids ranging from 12.6 to 24.0%. Acetol and 1,2-propanediol were the most abundant products obtained in the liquid stream.

Hydrogen-Induced Formation of Surface Acid Sites on Pt/Al(PO3)3 Enables Remarkably Efficient Hydrogenolysis of C-O Bonds in Alcohols and Ethers.

The hydrogenolysis of oxygenates such as alcohols and ethers is central to the biomass valorization and also a valuable transformation in organic synthesis. However, a mild and efficient catalyst system for the hydrogenolysis of a large variety of alcohols and ethers with various functional groups is still underdeveloped. Here, we report an aluminum metaphosphate-supported Pt nanoparticles (Pt/Al(PO3)3) for the hydrogenolysis of a wide variety of primary, secondary, and tertiary alkyl and benzylic alcohols, and dialkyl, aryl alkyl, and diaryl ethers, including biomass-derived furanic compounds, under mild conditions (0.1-1 atm of H2, as low as 70 °C). Mechanistic studies suggested that H2 induces formation of the surface Brønsted acid sites via its cleavage by supported Pt nanoparticles. Accordingly, the high efficiency and the wide applicability of the catalyst system are attributed to the activation and cleavage of C-O bonds by the hydrogen-induced Brønsted acid sites with the assistance of Lewis acidic Al sites on the catalyst surface. The high efficiency of the catalyst implies its potential application in energy-efficient biomass valorization or fine chemical synthesis.

Hydrogenolysis of Haloboranes: from Synthesis of Hydroboranes to Formal Hydroboration Reactions.

Hydroboranes are versatile reagents in synthetic chemistry, but their synthesis relies on energy-intensive processes. Herein, we report a new method for the preparation of hydroboranes from hydrogen and the corresponding haloboranes. Triethylamine (NEt3) form with dialkylchloroboranes a Frustrated Lewis Pair (FLP) able to split H2 and afford the desired hydroborane with ammonium salts. Unreactive haloboranes were unlocked using a catalytic amount of Cy2BCl, enabling the synthesis of commonly used hydroboranes such as pinacolborane or catecholborane. The mechanisms of these reactions have been examined by DFT studies, highlighting the importance of the base selection. Finally, the system's robustness has been evaluated in one-pot B-Cl hydrogenolysis/hydroboration reactions of C=C unsaturated bonds.

Palladium Single Atom-supported Covalent Organic Frameworks for Aqueous-phase Hydrogenative Hydrogenolysis of Aromatic Aldehydes via Hydrogen Heterolysis.

Developing a method for the tandem hydrogenative hydrogenolysis of bio-based furfuryl aldehydes to methylfurans is crucial for synthesizing sustainable biofuels and chemicals; however, it poses a challenge due to the easy hydrogenation of the C=C bond and difficult cleavage of the C-O bond. Herein, a palladium (Pd) single-atom-supported covalent organic framework was fabricated and showed a unique 2,5-dimethylfuran yield of up to 98.2% when reacted with 5-methyl furfuryl aldehyde in an unprecedented water solvent at 30°C. Furthermore, it exhibited excellent catalytic universality while converting various furfuryl-, benzyl-, and heterocyclic aldehydes at room temperature. The analysis of the catalytic mechanism confirmed that H2 was heterolytically activated on the Pd-N pair and triggered the keto-enol tautomerism of the covalent organic frameworks (COFs) host, resulting in H--Pd∙∙∙O-H+ sites. These sites served as novel asymmetric hydrogenation sites for the C=O group and hydrogenolysis sites for the C-OH group through a scarce SN2 mechanism. This study demonstrated remarkable bifunctional catalysis through the H2-induced keto-enol tautomerism of COF catalysts for the atypical preparation of methyl aromatics in a water solvent at room temperature.

Consumer Grade Polyethylene Recycling via Hydrogenolysis on Ultrafine Supported Ruthenium Nanoparticles.

Catalytic hydrogenolysis has the potential to convert high-density polyethylene (HDPE), which comprises about 30 % of plastic waste, into valuable alkanes. Most investigations have focused on increasing activity for lab grade HDPEs displaying low molecular weight, with limited mechanistic understanding of the product distribution. No efficient catalyst is available for consumer grades due to their lower reactivity. This study targets HDPE used in bottle caps, a waste form generated globally at a rate of approximately one million units per hour. Ultrafine ruthenium particles (1 nm) supported on titania (anatase) achieved up to 80 % conversion into light alkanes (C1 -C45 ) under mild conditions (498 K, 20 bar H2 , 4 h) and were reused for three cycles. Small ruthenium nanoparticles were critical to achieving relevant conversions, as activity sharply decreased with particle size. Selectivity commonalities and peculiarities across HDPE grades were disclosed by a reaction modelling approach applied to products. Isomerization cedes to backbone scission as the reaction progresses. Within this trend, low molecular weight favor isomerization whilst high molecular weight favor cleavage. Commercial caps obeyed this trend with decreased activity, anticipating the influence of additives in realistic processing. This study demonstrates effective hydrogenolysis of consumer grade polyethylene and provides selectivity patterns for product control.

Selective Cleavage of Cß-O-4 Bond for Lignin Depolymerization via Paired-Electrolysis in an Undivided Cell.

The cleavage of C-O bonds is one of the most promising strategies for lignin-to-chemicals conversion, which has attracted considerable attention in recent years. However, current catalytic system capable of selectively breaking C-O bonds in lignin often requires a precious metal catalyst and/or harsh conditions such as high-pressure H2 and elevated temperatures. Herein, we report a novel protocol of paired electrolysis to effectively cleave the Cß-O-4 bond of lignin model compounds and real lignin at room temperature and ambient pressure. For the first time, "cathodic hydrogenolysis of Cß-O-4 linkage" and "anodic C-H/N-H cross-coupling reaction" are paired in an undivided cell, thus the cleavage of C-O bonds and the synthesis of valuable triarylamine derivatives could be simultaneously achieved in an energy-effective manner. This protocol features mild reaction conditions, high atom economy, remarkable yield with excellent chemoselectivity, and feasibility for large-scale synthesis. Mechanistic studies indicate that indirect H* (chemical absorbed hydrogen) reduction instead of direct electron transfer might be the pathway for the cathodic hydrogenolysis of Cß-O-4 linkage.

Boosting Polyethylene Hydrogenolysis Performance of Ru-CeO2 Catalysts by Finely Regulating the Ru Sizes.

Hydrogenolysis is an effective method for converting polyolefins into high-value chemicals. For the supported catalysts commonly used, the size of active metals is of great importance. In this study, it is discovered that the activity of CeO2 -supported Ru single atom, nanocluster, and nanoparticle catalysts shows a volcanic trend in low-density polyethylene (LDPE) hydrogenolysis. Compared with CeO2 supported Ru single atoms and nanoparticles, CeO2 -supported Ru nanoclusters possess the highest conversion efficiency, as well as the best selectivity toward liquid alkanes. Through comprehensive investigations, the metal-support interactions (MSI) and hydrogen spillover effect are revealed as the two key factors in the reaction. On the one hand, the MSI is strongly related to the Ru surface states and the more electronegative Ru centers are beneficial to the activation of CH and CC bonds. On the other hand, the hydrogen spillover capability directly affects the affinity of catalysts and active H atoms, and increasing this affinity is advantageous to the hydrogenation of alkane species. Decreasing the Ru sizes can promote the MSI, but it can also reduce the hydrogen spillover effect. Therefore, only when the two effects achieve a balance, as is the case in CeO2 -supported Ru nanoclusters, can the hydrogenolysis activity be promoted to the optimal value.

Pt-Loaded Nb─W Metal Composite Oxide for Selective Cleavage of Secondary C─O Bonds.

Selective hydrogenolysis of glycerol to 1,3-propanediol (1,3-PDO) is recognized as one of the most promising reactions for the valorization of biomass. Precise activation of C─O bonds of glycerol molecule is the key step to realize the high yield of catalytic conversion. Here, a Pt-loaded Nb-W composite oxides with crystallographic shear phase for the precise activation and cleavage of secondary C─O (C(2)─O) bonds are first reported. The developed Nb14 W3 O44 with uniform structure possesses arrays of W-O-Nb active sites that totally distinct from individual WOx or NbOx species, which is superior to the adsorption and activation of C(2)─O bonds. The Nb14 W3 O44 support with rich reversible redox couples also promotes the electron feedback ability of Pt and enhances its interaction with Pt nanoparticles, resulting in high activity for H2 dissociation and hydrogenation. All these favorable factors confer the Pt/Nb14 W3 O44 excellent performance for selective hydrogenolysis of glycerol to 1,3-PDO with the yield of 75.2% exceeding the record of 66%, paying the way for the commercial development of biomass conversion. The reported catalysts or approach can also be adopted to create a family of Nb-W metal composite oxides for other catalytic reactions requiring selective C─O bond activation and cleavage.

Metal-Free Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with "Inverse" Frustrated Lewis Pairs.

The challenging metal-free catalytic hydrogenolysis of silyl chlorides to hydrosilanes is unlocked by using an inverse frustrated Lewis pair (FLP), combining a mild Lewis acid (Cy2 BCl) and a strong phosphazene base (BTPP) in mild conditions (10 bar of H2 , r. t.). In the presence of a stoichiometric amount of the base, the hydrosilanes R3 SiH (R=Me, Et, Ph) are generated in moderate to high yields (up to 95 %) from their chlorinated counterparts. A selective formation of the valuable difunctional monohydride Me2 SiHCl is also obtained from Me2 SiCl2 . A mechanism is proposed based on stoichiometric experiments and DFT calculations; it highlights the critical role of borohydride species generated by the heterolytic splitting of H2 .

Unraveling the Lignin Structural Variation in Different Bamboo Species.

The structure of cellulolytic enzyme lignin (CEL) prepared from three bamboo species (Neosinocalamus affinis, Bambusa lapidea, and Dendrocalamus brandisii) has been characterized by different analytical methods. The chemical composition analysis revealed a higher lignin content, up to 32.6% of B. lapidea as compared to that of N. affinis (20.7%) and D. brandisii (23.8%). The results indicated that bamboo lignin was a p-hydroxyphenyl-guaiacyl-syringyl (H-G-S) lignin associated with p-coumarates and ferulates. Advanced NMR analyses displayed that the isolated CELs were extensively acylated at the γ-carbon of the lignin side chain (with either acetate and/or p-coumarate groups). Moreover, a predominance of S over G lignin moieties was found in CELs of N. affinis and B. lapidea, with the lowest S/G ratio observed in D. brandisii lignin. Catalytic hydrogenolysis of lignin demonstrated that 4-propyl-substituted syringol/guaiacol and propanol guaiacol/syringol derived from ß-O-4' moieties, and methyl coumarate/ferulate derived from hydroxycinnamic units were identified as the six major monomeric products. We anticipate that the insights of this work could shed light on the sufficient understanding of lignin, which could open a new avenue to facilitate the efficient utilization of bamboo.

The Role of Copper in the Hydrogenation of Furfural and Levulinic Acid.

Currently, there is a great interest in the development of sustainable and green technologies for production of biofuels and chemicals. In this sense, much attention is being paid to lignocellulosic biomass as feedstock, as alternative to fossil-based resources, inasmuch as its fractions can be transformed into value-added chemicals. Two important platform molecules derived from lignocellulosic sugars are furfural and levulinic acid, which can be transformed into a large spectrum of chemicals, by hydrogenation, oxidation, or condensation, with applications as solvents, agrochemicals, fragrances, pharmaceuticals, among others. However, in many cases, noble metal-based catalysts, scarce and expensive, are used. Therefore, an important effort is performed to search the most abundant, readily available, and cheap transition-metal-based catalysts. Among these, copper-based catalysts have been proposed, and the present review deals with the hydrogenation of furfural and levulinic acid, with Cu-based catalysts, into several relevant chemicals: furfuryl alcohol, 2-methylfuran, and cyclopentanone from FUR, and γ-valerolactone and 2-methyltetrahydrofuran from LA. Special emphasis has been placed on catalytic processes used (gas- and liquid-phase, catalytic transfer hydrogenation), under heterogeneous catalysis. Moreover, the effect of addition of other metal to Cu-based catalysts has been considered, as well as the issue related to catalyst stability in reusing studies.

Ruthenium/TiO2 -Catalyzed Hydrogenolysis of Polyethylene Terephthalate: Reaction Pathways Dominated by Coordination Environment.

Polyethylene terephthalate (PET) hydrogenolysis can produce benzene, toluene, and xylene (BTX), where the selectivity control is challenging. We report a reaction pathway dictated by the Ru coordination environment by examining the binding geometries of adsorbates on differently coordinated Ru centers and their evolution during PET hydrogenolysis. A BTX yield of 77 % was obtained using a Ru/TiO2 with a Ru coordination number of ca. 5.0 where edge/corner sites are dominant, while more gas and saturated products were formed for Ru/TiO2 containing primarily terrace sites. Density functional theory and isotopic labelling revealed that under-coordinated Ru edge sites favor "upright" adsorption of aromatic adsorbates while well-coordinated Ru sites favor "flat-lying" adsorption, where the former mitigates ring hydrogenation and opening. This study demonstrates that reaction pathways can be directed through controlled reactant/intermediate binding via tuning of the Ru coordination environment for efficient conversion of PET to BTX.

Highly Efficient Depolymerization of Waste Polyesters Enabled by Transesterification/Hydrogenation Relay Under Mild Conditions.

The efficient depolymerization of polyesters under mild conditions remains a significant challenge. Herein, we demonstrate a highly efficient strategy for the degradation of a diverse array of waste polyesters as low to 80 °C, 1 bar H2 . The key to the success of this transformation relied on the initial transesterification of macromolecular polyester into more degradable oligomeric fragments in the presence of CH3 OH and the subsequent hydrogenation by the use of the rationally designed quinaldine-based Ru complex. Controlled experiments and preliminary mechanistic studies disclosed the quinaldine-based catalysts could be hydrogenated to the eventually active species, which has been confirmed by X-ray diffraction analysis and directly used as a catalyst in the hydrogenolysis of polyester. The strong viability and high activity of this new species in protic solvent were explained in detail. Besides, the crucial role of CH3 OH in promoting reaction efficiency during the whole process was also elucidated. The synthetic utility of this method was further illustrated by preparing 1,4-cyclohexanedimethanol (CHDM) from waste polyethylene terephthalate (PET).

Ester Reduction with H2 on Bifunctional Metal-Acid Catalysts: Implications of Metal Identity on Rates and Selectivities.

Esters reduce to form ethers and alcohols on contact with metal nanoparticles supported on Brønsted acidic faujasite (M-FAU) that cleave C-O bonds by hydrogenation and hydrogenolysis pathways. Rates and selectivities for each pathway depend on the metal identity (M=Co, Ni, Cu, Ru, Rh, Pd, and Pt). Pt-FAU gives propyl acetate consumption rates up to 100 times greater than other M-FAU catalysts and provides an ethyl propyl ether selectivity of 34 %. Measured formation rates, kinetic isotope effects, and site titrations suggest that ester reduction involves a bifunctional mechanism that implicates the stepwise addition of H* atoms to the carbonyl to form hemiacetals on the metal sites, followed by hemiacetal diffusion to a nearby Brønsted acid site to dehydrate to ethers or decompose to alcohol and aldehyde. The rates of reduction of propyl acetate appear to be determined by the H* addition to the carbonyl and by the C-O cleavage of hemiacetal.

Rapid Polyolefin Hydrogenolysis by a Single-Site Organo-Tantalum Catalyst on a Super-Acidic Support: Structure and Mechanism.

The novel electrophilic organo-tantalum catalyst AlS/TaNpx (1) (Np=neopentyl) is prepared by chemisorption of the alkylidene Np3 Ta=CHt Bu onto highly Brønsted acidic sulfated alumina (AlS). The proposed catalyst structure is supported by EXAFS, XANES, ICP, DRIFTS, elemental analysis, and SSNMR measurements and is in good agreement with DFT analysis. Catalyst 1 is highly effective for the hydrogenolysis of diverse linear and branched hydrocarbons, ranging from C2 to polyolefins. To the best of our knowledge, 1 exhibits one of the highest polyolefin hydrogenolysis activities (9,800 (CH2 units) ⋅ mol(Ta)-1 ⋅ h-1 at 200 °C/17 atm H2 ) reported to date in the peer-reviewed literature. Unlike the AlS/ZrNp2 analog, the Ta catalyst is more thermally stable and offers multiple potential C-C bond activation pathways. For hydrogenolysis, AlS/TaNpx is effective for a wide variety of pre- and post-consumer polyolefin plastics and is not significantly deactivated by standard polyolefin additives at typical industrial concentrations.

Palladium-Catalyzed Asymmetric Hydrogenolysis of Aryl Triflates for Construction of Axially Chiral Biaryls.

Here we report the first palladium-catalyzed asymmetric hydrogenolysis of readily available aryl triflates via desymmetrization and kinetic resolution for facile construction of axially chiral biaryl scaffolds with excellent enantioselectivities and s selectivity factors. The axially chiral monophosphine ligands could be prepared from these chiral biaryl compounds and were further applied to palladium-catalyzed asymmetric allylic alkylation with excellent ee values and high branched and linear ratio, which demonstrated the potential utility of this methodology.

Entropy Confinement Promotes Hydrogenolysis Activity for Polyethylene Upcycling.

Chemical upcycling that catalyzes waste plastics back to high-purity chemicals holds great promise in end-of-life plastics valorization. One of the main challenges in this process is the thermodynamic limitations imposed by the high intrinsic entropy of polymer chains, which makes their adsorption on catalysts unfavorable and the transition state unstable. Here, we overcome this challenge by inducing the catalytic reaction inside mesoporous channels, which possess a strong confined ability to polymer chains, allowing for stabilization of the transition state. This approach involves the synthesis of p-Ru/SBA catalysts, in which Ru nanoparticles are uniformly distributed within the channels of an SBA-15 support, using a precise impregnation method. The unique design of the p-Ru/SBA catalyst has demonstrated significant improvements in catalytic performance for the conversion of polyethylene into high-value liquid fuels, particularly diesel. The catalyst achieved a high solid conversion rate of 1106 g ⋅ gRu -1 ⋅ h-1 at 230 °C. Comparatively, this catalytic activity is 4.9 times higher than that of a control catalyst, Ru/SiO2 , and 14.0 times higher than that of a commercial catalyst, Ru/C, at 240 °C. This remarkable catalytic activity opens up immense opportunities for the chemical upcycling of waste plastics.