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Lignin is a potential non-fossil resource of diverse functionalized phenolic units. The most important lignin-derived monomers are 4-alkylphenols, 4-hydroxybenzaldehydes, 4-hydroxybenzoic acids, and 4-hydroxycinnamic acids/esters. Efficient transformation of lignin and/or its monomers into valuable aromatics and their derivatives is crucial, not only for a sustainable lignocellulose biorefinery, but also to reduce our dependence on fossil feedstocks. This review provides a concise account of the recent advances in lignocellulose fractionation/lignin depolymerization processes towards lignin-derived monomers. Subsequently, numerous potential atom-efficient catalytic routes for upgrading lignin monomers into drop-in chemicals and new polymer building blocks are discussed.

ispartof: Acs Catalysis vol:11 issue:21 pages:13603-13648 status: published

Efficient transformation of biomass to value-added chemicals and high-energy density fuels is pivotal for a more sustainable economy and carbon-neutral society. In this framework, developing potential cascade chemical processes using functionalised heterogeneous catalysts is essential because of their versatile roles towards viable biomass valorisation. Advances in materials science and catalysis have provided several innovative strategies for the design of new appealing catalytic materials with well-defined structures and special characteristics. Promising catalytic materials that have paved the way for exciting scientific breakthroughs in biomass upgrading are carbon materials, metal-organic frameworks, solid phase ionic liquids, and magnetic iron oxides. These fascinating catalysts offer unique possibilities to accommodate adequate amounts of acid-base and redox functional species, hence enabling various biomass conversion reactions in a one-pot way. This review therefore aims to provide a comprehensive account of the most significant advances in the development of functionalised heterogeneous catalysts for efficient biomass upgrading. In addition, this review highlights important progress ensued in tailoring the immobilisation of desirable functional groups on particular sites of the above-listed materials, while critically discussing the role of consequent properties on cascade reactions as well as on other vital processes within the bio-refinery. Current challenges and future opportunities towards a rational design of novel functionalised heterogeneous catalysts for sustainable biomass valorisation are also emphasized. ispartof: Chemical Society Reviews vol:47 issue:22 pages:8349-8402 ispartof: location:England status: published

The oxidation of cyclohexene was studied using nanostructured CeO2, CeO2/SiO2, CeO2-Sm2O3, and CeO2-Sm2O3/SiO2 catalysts. The CeO2-Sm2O3/SiO2 catalyst shows ~ 96% cyclohexene conversion and ~ 91% selectivity to allylic products, namely, 2-cyclohexen-1-one (~ 53%) and 2-cyclohexen-1-ol (~ 38%) owing to favorable redox (oxygen vacancies) and acid sites stimulated by synergistic interactions of dopant (Sm) and support (SiO2) with CeO2. In contrast, only ~ 40, ~ 55, and ~ 78% of cyclohexene conversions were observed, respectively, over CeO2, CeO2/SiO2, and CeO2-Sm2O3 catalysts. With the increase of reaction time, temperature, and molar ratio of cyclohexene/oxidant, the conversion of cyclohexene and the selectivity of 2-cyclohexen-1-one are increased considerably.

Various Cu-Ni/Ti-MCM-41 catalysts by varying Ti amount (Ti/Si = 10, 20 and 30%) were synthesized for hydrodeoxygenation of guaiacol and characterized by XRD, N 2 adsorption-desorption, NH 3 -TPD, H 2 -TPR and Raman spectroscopy. The Ti loading shows an adverse effect on the catalyst surface area. But, more number of acidic sites (5173.55 μmol/g) were found for 20% Ti loaded Cu-Ni/Ti-MCM-41 catalyst. The 20% Ti loaded Cu-Ni/Ti-MCM-41 catalyst shows a high guaiacol conversion (74.2%) and cyclohexane selectivity (48.81%) which is due to abundant acid and redox sites: acid sites initiate removal of oxygen via dehydration, while hydrogenation of guaiacol to cyclohexane is promoted by redox sites.

Heterogeneous catalysis is a promising technology for the valorization of renewable biomass to sustainable advanced fuels and fine chemicals. Porosity and nanostructure are the most versatile features of heterogeneous solid catalysts, which can greatly determine the accessibility of specific active sites, reaction mechanisms, and the selectivity of desirable products. Hence, the precise tuning of porosity and nanostructure has been a potential strategy towards developing novel solid catalysts with indispensable characteristics for efficient biomass valorization. Herein, we present a timely and comprehensive review of the recent advances in catalytic biomass conversions over microporous zeolites, mesoporous silicas, and nanostructured metals/metal oxides. This review covers the catalytic processing of both edible (lipids and starch) and non-edible (lignocellulose) biomass as well as their derived compounds, along with a systematic evaluation of catalyst reusability/kinetic/mechanistic aspects in the relevant processes. The key parameters essential for tailoring particle size, morphology, porosity, acid-base, and redox properties of solid catalysts are emphasized, while discussing the ensuing catalytic effects towards the selective conversion of biomass into desirable chemicals. Special attention has been drawn to understand the role of water in liquid phase biomass conversions as well as the hydrothermal stability and the deactivation of nanoporous catalysts. We believe this comprehensive review will provide new insights towards developing state-of-the-art solid catalysts with well-defined porosity and nanoscale properties for viable biomass conversion. ispartof: Chemical Society Reviews vol:48 issue:8 pages:2366-2421 ispartof: location:England status: published

The structure-activity properties of MnO x /CeO 2 nanorods and MnO x /CeO 2 nanoparticles, with the importance of CeO 2 morphology have been investigated for the solvent-free oxidation of amines using oxygen as a green oxidant. The physicochemical properties of the samples have been investigated using HRTEM, XRD, Raman, BET, XPS, and FT-IR techniques. HRTEM studies reveal that CeO 2 nanorods preferentially expose {1 1 0} and {1 0 0} crystal planes, while CeO 2 nanoparticles expose {1 1 1} planes. The addition of manganese to CeO 2 supports leads to an enhancement in the concentration of Ce 3+ ions and oxygen vacancies, which are more pronounced for the MnO x /CeO 2 nanorods as evidenced by Raman and XPS studies. Another striking observation noticed from XPS studies is that MnO x /CeO 2 nanorods catalyst exhibits Mn 4+ , Mn 3+ , and Mn 2+ species, whereas only Mn 4+ and Mn 3+ are presented in MnO x /CeO 2 nanoparticles catalyst. It was found that MnO x /CeO 2 nanorods catalyst exhibit a two-fold higher activity for the oxidation of benzylamine with superior selectivity to dibenzylimine (∼99%) compared with that of MnO x /CeO 2 nanoparticles catalyst. The MnO x /CeO 2 nanorods catalyst was also found to be effective for the oxidation of various amines, and moderate to good product yields were obtained. Novel probable reaction pathways are proposed for solvent-free oxidation of primary and secondary benzylamines over MnO x /CeO 2 nanorods catalyst. The presence of surface-active Mn 4+ /Mn 2+ couple and the enhanced defect structure of CeO 2 nanorods (i.e., higher numbers of Ce 3+ ions and abundant O vacancies) are found to be key factors for the high catalytic efficiency of the MnO x /CeO 2 nanorods.

Designing Au nanocatalysts supported on nanosized metal oxides has drawn much attention due to remarkable nanoscale influenced metal-support interactions and their favorable role in heterogeneous catalysis. This work reports development of Au nanocatalysts dispersed on nanosized CeO2 and CeO2-ZrO2 supports for solvent- and base-free oxidative coupling of benzylamines into N-benzylbenzaldimines using O2 as a green oxidant. The physicochemical characterization of nanocatalysts has been undertaken using HRTEM, UV–vis DRS, XRD, Raman, BET, TG-DTA, AAS, and XPS techniques. HRTEM images reveal the formation of nanosized CeO2 and CeO2-ZrO2 supports with an average diameter of ∼10 and 7 nm, respectively. HRTEM images also indicated that Au/CeO2-ZrO2 catalyst has smaller Au nanoparticles (∼2.1 nm) compared with that of Au/CeO2 catalyst (∼3.7 nm). Raman and XPS studies showed that the addition of ZrO2 to CeO2 leads to abundant oxygen vacancies and higher concentration of Ce3+, respectively. The Au/CeO2-ZrO2 catalyst exhibited a higher efficiency in benzylamine conversion (∼95%) followed by Au/CeO2 (∼78%), CeO2-ZrO2 (∼51%), and CeO2 (∼39%). The Au/CeO2-ZrO2 catalyst was also found to effective for oxidative coupling of various benzylamines, and moderate to good product yields were obtained. The presence of smaller Au particles (2.1 nm) and improved surface-defect properties of nanoscale CeO2-ZrO2 support are found to be key factors for high performance of Au/CeO2-ZrO2 catalyst. Additionally, the reaction temperature is one of the important factors for the performance of catalysts. Remarkably, ∼99.6–99.9% selectivity for N-benzylbenzaldimines was found in the amine oxidation, which highlights the significance of present work in the selective oxidation catalysis.

The selective hydrogenolysis of bioglycerol to 1,2-propanediol was investigated over a series of Ce-promoted Cu/Mg catalysts, namely, Cu/Mg (1/9), Cu/Ce/Mg (1/1/5), Cu/Ce/Mg (1/3/5), and Cu/Ce/Mg (1/5/5) prepared by a coprecipitation method. The physicochemical properties of the synthesized catalysts were analyzed by XRD, Raman, BET, BJH, XPS, NH3- and CO2-TPD, and H2-TPR techniques. The XRD and BET surface area results indicated that addition of Ce to Cu/Mg sample remarkably inhibits the crystal growth of CuO and improves the specific surface area. More number of oxygen vacancy defects were found in the Cu/Ce3/Mg sample, as evidenced from Raman studies. The reducible nature of the Cu/Mg sample was significantly enhanced after the Ce-incorporation. The NH3- and CO2-TPD results show that the acid–base properties of the Ce-promoted Cu/Mg samples are highly dependent on the Ce-loading. Among the synthesized samples, the Cu/Ce3/Mg sample exhibited higher concentration and superior strength of acidic sites. The achieved activity order of various catalysts for glycerol hydrogenolysis is Cu/Mg

Selective transformation of levulinic acid (LA) to γ-valerolactone (GVL) using novel heterogeneous catalysts is one of the promising strategies for viable biomass processing. In this framework, we developed a continuous flow process for the selective hydrogenation of LA to GVL using several nanostructured Ni/SiO2 catalysts. The structural, textural, acidic, and redox properties of Ni/SiO2 catalysts, tuned by selectively varying the Ni amount from 5 to 40 wt %, were critically investigated using numerous materials characterization techniques. Electron microscopy images showed the formation of uniformly dispersed Ni nanoparticles on the SiO2 support, up to 30% Ni loading (average particle size is 9.2 nm), followed by a drastic increase in the particles size (21.3 nm) for 40% Ni-loaded catalyst. The fine dispersion of Ni particles has elicited a synergistic metal-support interaction, especially in 30% Ni/SiO2 catalyst, resulting in enhanced acidic and redox properties. Among the various catalysts tested, the 30% Ni/SiO2 catalyst showed the best performance with a remarkable 98% selectivity of GVL at complete conversion of LA for 2 h reaction time. Interestingly, this catalyst showed a steady selectivity to GVL (>97%), with a 54.5% conversion of LA during 20 h time-on-stream. The best performance of 30% Ni/SiO2 catalyst was attributed to well-balanced catalytic properties, such as ample amounts of strong acidic sites and abundant active metal sites. The obtained results show a great potential of applying earth-abundant nickel/silica catalysts for upgrading biomass platform molecules into value-added chemicals and high-energy-density fuels. ispartof: ACS OMEGA vol:3 issue:12 pages:16839-16849 ispartof: location:United States status: published

The development of promising solid acid catalysts alternative to hazardous liquid acids is essential towards a sustainable chemical industry. This work reports the synthesis of nanostructured CeO2–MoO3/SiO2 and CeO2–WO3/SiO2 solid acids, along with CeO2–MoO3, CeO2–WO3 and CeO2 for continuous-flow oxidative coupling of benzylamine using O2 as a green oxidant. A systematic physicochemical characterization has been undertaken using XRD, Raman, N2 adsorption–desorption, TEM, NH3-TPD, and XPS techniques. It was found that the dispersion of CeO2–MoO3 and CeO2–WO3 species on the SiO2 support leads to remarkable structural and acidic properties, due to the synergetic effect of the respective components. TEM analysis reveals the presence of highly dispersed WO3 (0.8–1.2 nm) and MoO3 (0.8–1 nm) nanoparticles in the synthesized catalysts. Among the various catalysts developed, the CeO2–MoO3/SiO2 sample exhibited higher BET surface area (248 m2 g−1), abundant oxygen vacancy defects, and large amounts of strong acidic sites. Owing to improved properties, the CeO2–MoO3/SiO2 solid-acid showed a superior catalytic performance in the continuous-flow oxidative coupling of benzylamine: the obtained benzylamine conversions for 1 h are ∼11.8, 55, 70, 76, and 96%, respectively, for CeO2, CeO2–WO3, CeO2–WO3/SiO2, CeO2–MoO3, and CeO2–MoO3/SiO2 catalysts. Importantly, the CeO2–MoO3/SiO2 solid acid exhibited a remarkable steady performance in terms of benzylamine conversion (∼88–96%) and selectivity of N-benzylbenzaldimine product (∼96–97.8%) up to 6 h. The outstanding catalytic performance of CeO2–MoO3/SiO2 solid acid coupled with the application of continuous-flow synthesis, economical benefits of the respective oxides, and eco-friendly oxidant is expected to bring new opportunities in the design of industrially-favourable chemical processes.

This work has been undertaken with the aim of designing promising noble-metal-free catalysts for efficient hydrodeoxygenation (HDO) of dibenzofuran (DBF) into fuel grade hydrocarbons. For this, various Ni/Al-SBA-15 catalysts with different Si/Al (50, 60, 70 and 80) mole ratios were synthesized and their catalytic performance was tested for HDO of DBF in a batch reactor. The catalysts were systematically characterized using XRD, N2-adsorption–desorption, Raman, H2-TPR, NH3-TPD, XRF, and FESEM techniques. The activity results showed that the HDO of DBF proceeds via hydrogenation of benzene on the Ni sites followed by cleavage of C–O bonds on the acidic sites of the catalyst to yield unsaturated hydrocarbons. Further hydrogenation of unsaturated hydrocarbons on the Ni sites gives bicyclohexane as the major product. Remarkably, a 100% DBF conversion was found for all the catalysts except for Ni/SBA-15 and Ni/Al-SBA-15(80) (Si/Al mole ratio = 80) catalysts, which showed 97.97 and 99.31%, respectively. A significant observation noticed in this study is that the incorporation of Al into Ni/SBA-15 results in an outstanding improvement in the selectivity of the bicyclohexane product. Among the catalysts tested, the Ni/Al-SBA-15(50) (Si/Al mole ratio = 50) catalyst showed the highest efficiency, with superior selectivity of ∼87% for bicyclohexane and ∼96% degree of deoxygenation at 10 MPa, 260 °C and 5 h. The obtained structure–activity results reveal the synergetic effect of Ni and support in HDO of DBF reaction: the concentration of acidic sites has a significant effect on the selectivity of the desired products.

The development of promising magnetic nanocatalysts is one of the key research topics in the field of catalysis. This is because of their versatile surface physicochemical, magnetic, and size-dependent catalytic properties. Herein, an optimization strategy for the synthesis of high-value fuel grade chemicals from hydro-deoxygenation of biomass-derived furfural and vanillin using a nanostructured magnetic Fe(NiFe)O4-SiO2 catalyst, synthesized by a facile one-pot procedure, was presented. Accordingly, effects of calcination temperature from 400, 500, 600 to 700 &deg, C on the structure-activity properties of the magnetic Fe(NiFe)O4-SiO2 catalyst was systematically studied. The magnetic Fe(NiFe)O4-SiO2 catalyst calcined at 500 &deg, C exhibited the best catalytic performance, giving full conversions of vanillin and furfural, with good selectivity of 63 and 59% to cyclohexane and n-pentane (fuel grade chemicals), respectively. The prowess of this catalyst was attributed to its abundant acid properties in addendum to high BET surface area.

The present investigation was undertaken to know the influence of different dopants on the physicochemical properties and catalytic behavior of nano-Au/CeO2 catalyst for CO oxidation. Accordingly, various metal ions namely, Fe3+, La3+ and Zr4+ were incorporated into the ceria lattice by a facile coprecipitation approach using ultra-high dilute aqueous solutions. An anion adsorption method was used to prepare the Au/doped-CeO2 catalysts in the absence of any base, reducing and protective agents. The physicochemical characterization was performed by XRD, BET surface area, ICP-AES, TG-DTA, FT-IR, TEM, UV–vis DRS, Raman, XPS and TPD techniques. Doped CeO2 exhibited smaller crystallite size, higher BET surface area and larger amount of oxygen vacancies than the pure CeO2. These remarkable properties showed a beneficial effect toward gold particle size as confirmed by XRD and TEM studies. XPS results revealed that Au is present in the metallic state and Ce in both +3 and +4 oxidation states. Incorporation of Zr into the Au/CeO2 resulted in high CO oxidation activity attributed to the presence of more Ce3+ ions and oxygen vacancies. In contrast, the La-incorporation caused an opposite effect due to the presence of carbonate species on the surface of Au/CeO2–La2O3 catalyst, which blocked the active sites essential for CO oxidation. It was shown that accumulation of carbonate species strongly depends on the acid–base properties of the supports. The catalytic performance of Au catalysts is highly dependent on the nature of the support.

The development of an efficient doped CeO2 material is an active area of intense research in environmental catalysis. In this study, we prepared highly promising Ce–Fe nano-oxides by a facile coprecipitation method and their catalytic performance was studied for CO oxidation. Various characterization techniques, namely, XRD, BET surface area, pore size distribution, Raman, FT-IR, TEM, H2-TPR, and XPS were used to correlate the structure–activity properties of the Ce–Fe catalysts. XRD results confirmed the formation of nanocrystalline Ce1−xFexO2−δ solid solution due to doping of Fe3+ into the CeO2 lattice. The BET surface area and lattice strain of CeO2 are significantly improved after the Fe-incorporation. Raman studies revealed the presence of abundant oxygen vacancies in the Ce–Fe sample. TEM images evidenced the formation of nanosized particles with an average diameter of 5–20 nm in the prepared samples. Interestingly, despite the thermal treatment at higher temperatures, the Ce–Fe sample showed remarkable reducible nature compared to pure CeO2 ascribed to existence of strong interaction between the CeO2 and FeOx. The synthesized Ce–Fe nano-oxides calcined at 773 K exhibited excellent CO oxidation performance (T50 = 480 K), with a huge difference of 131 K with respect to pure CeO2 (T50 = 611 K). The outstanding activity of the Ce–Fe catalyst is mainly due to smaller crystallite size, facile reduction, enhanced lattice strain, and ample oxygen vacancies. The superior CO oxidation performance of Ce–Fe nano-oxides with the advantages of low cost and easy availability could make them potential alternatives to noble metal-based oxidation catalysts.

A nanosized Ce–Sm/SiO2 catalyst was found to show an outstanding performance in the oxidation of benzylamines into valuable dibenzylimine products with almost 100% selectivity with O2 as the green oxidant under solvent-free conditions, which is attributed to the presence of abundant strong acidic sites, enhanced oxygen vacancy concentration, and superior BET surface area.

In this work, we systematically investigated the structure–activity performance of nanosized Au/CeO2 and Au/Ce0.9Fe0.1O2−δ catalysts, along with nanocrystalline CeO2 and Ce0.9Fe0.1O2−δ supports, for the oxidation of carbon monoxide and benzylamine. An extensive physicochemical characterization was undertaken using XRD, BET surface area, BJH analysis, TG-DTA, XPS, TEM, Raman, AAS and CHN analyses. XRD studies confirmed the formation of smaller sized Ce0.9Fe0.1O2−δ nanocrystallites due to the incorporation of Fe3+ ions into the CeO2 lattice. Interestingly, Raman analysis revealed that the addition of Au remarkably improves the structural properties of the supports, evidenced by F2g peak shift and peak broadening, a significant observation in the present work. TEM images revealed the formation of smaller Au particles for Au/Ce0.9Fe0.1O2−δ (∼3.6 nm) compared with Au/CeO2 (∼5.3 nm), attributed to ample oxygen vacancies present on the Ce0.9Fe0.1O2−δ surface. XPS studies indicated that Au and Fe are present in metallic and +3 oxidation states, respectively, whereas Ce is present in both +4 and +3 oxidation states (confirming its redox nature). Activity results showed that the incorporation of Fe outstandingly enhances the efficacy of the Au/CeO2 catalyst for both CO oxidation and benzylamine oxidation. A 50% CO conversion was achieved at ∼349 and 330 K for Au/CeO2 and Au/Ce0.9Fe0.1O2−δ catalysts, respectively. As well, the Au/Ce0.9Fe0.1O2−δ catalyst showed ∼99% benzylamine conversion with ∼100% dibenzylimine selectivity for 7 h reaction time and 403 K temperature, whereas only 81% benzylamine conversion was achieved for the Au/CeO2 sample under similar conditions. The excellent performance of the Au/Ce0.9Fe0.1O2−δ catalyst is mainly due to the existence of smaller Au particles and an improved synergetic effect between the Au and the Ce0.9Fe0.1O2−δ support. It is confirmed that the oxidation efficiency of the Au catalysts is highly dependent on the preparation method.

Acetylation of glycerol with acetic acid was investigated over ZrO2, TiO2–ZrO2, WOx/TiO2–ZrO2 and MoOx/TiO2–ZrO2 solid acid catalysts to synthesize monoacetin, diacetin and triacetin having interesting applications as bio-additives for petroleum fuels. The prepared catalysts were characterized by means of XRD, BET surface area, ammonia-TPD and FT-Raman techniques. The effect of various parameters such as reaction temperature, molar ratio of acetic acid to glycerol, catalyst wt.% and time-on-stream were studied to optimize the reaction conditions. Among various catalysts investigated, the MoOx/TiO2–ZrO2 combination exhibited highest conversion (~ 100%) with best product selectivity, and a high time-on-stream stability.

A nanocrystalline Ce-Sm-oxide solid solution, with an excellent redox property and remarkable oxygen storage/release capacity, has been synthesized by means of a simple and highly practicable coprecipitation method. To understand the thermal and textural stability, the synthesized catalyst was subjected to calcination at various temperatures (773-1073 K). Physicochemical characterization was achieved using XRD, HRTEM, BET surface area, Raman, ICP-OES, XPS, TG-TDA, UV-vis DRS, TPR, and FTIR techniques, and the catalytic performance was evaluated for the oxidation of CO. Coprecipitation of Ce 4+ and Sm3+ ions through ultra-high dilute solutions provided the single phase Ce0.6Sm0.4O2-δ solid solution in the nanoscale range, as confirmed by XRD and TEM studies. Raman studies revealed two types of lattice defects, namely, oxygen vacancies and MO8 complex defects due to disparity in the oxidation state and ionic radius of Sm3+ and Ce4+, respectively. Calculations made from XPS atomic ratios (Ce/Sm) and Raman band intensity ratios (A D1/AF2g) indicated migration of Sm from the bulk to the surface at elevated temperatures that caused a negative effect on the oxygen vacancy concentration. The doping of Sm3+ into the ceria lattice effectively enhanced the reduction behaviour of ceria by shifting the surface and bulk reduction to lower temperatures. Remarkably, Sm-incorporation showed an optimistic influence on the oxygen storage ability and CO oxidation efficiency of ceria attributed to profound lattice defects and enhanced bulk oxygen mobility. The salient features of physicochemical characterization versus catalytic CO oxidation efficiency of Ce-Sm-oxide solid solutions have been elaborated in this article. Refereed/Peer-reviewed