No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
From Lignin to Chemicals: Hydrogenation of Lignin Models and Mechanistic Insights into Hydrodeoxygenation via Low Temperature C–O Bond Cleavage
Abstract
The catalytic hydrogenation of a series of lignin model compounds, including anisole, guaiacol, 1,2-dimethoxy benzene, 4-propyl-2-methoxy-phenol and syringol has been investigated in detail, using a Ru/C catalyst in acetic acid as the solvent. Both hydrogenation of the aromatic unit and C–O bond cleavage are observed resulting in a mixture of cyclohexanes and cyclohexanols, together with cyclohexyl acetates due to esterification with the solvent. The effect on product composition of the reaction parameters temperature (80-140 ˚C), pressure (10-40 bar) and reaction time (0.5-4 h) has been evaluated in detail. The lignin model compound 4-propyl-2-methoxy-phenol was converted to 4-propyl cyclohexanol in 4 hours at 140 ˚C and 30 bar H2 pressure with 84 % conversion and 63 % selectivity. Mechanistic studies on the reactivity of reaction intermediates have shown that C–O bond cleavage under these relatively mild conditions does not involve a C–O bond hydrogenolysis reaction, but is due to elimination and hydrolysis reactions (or acidolysis in acetic acid solvent) of highly reactive cyclohexadiene- and cyclohexene-based enols, enol ethers and allyl ethers.
... The lesser arene selectivity of noble metal catalysts is speculated by planar adsorption of aromatic ring over metal surface, which activates the ring more easily and preferably hydrogenated the aromatic ring. Vriamont et al. [30] investigated the mechanistic pathway of hydrodeoxygenation of lignin model compounds such as anisole, guaiacol, 1,2-dimethoxy benzene, 4-propyl-2-methoxy phenol, and syringol catalyzed by Ru/C in acetic acid medium [30]. The study revealed the occurrence of reduction as well as deoxygenation to yield a mixture containing cyclohexane and cyclohexanol. ...
... The lesser arene selectivity of noble metal catalysts is speculated by planar adsorption of aromatic ring over metal surface, which activates the ring more easily and preferably hydrogenated the aromatic ring. Vriamont et al. [30] investigated the mechanistic pathway of hydrodeoxygenation of lignin model compounds such as anisole, guaiacol, 1,2-dimethoxy benzene, 4-propyl-2-methoxy phenol, and syringol catalyzed by Ru/C in acetic acid medium [30]. The study revealed the occurrence of reduction as well as deoxygenation to yield a mixture containing cyclohexane and cyclohexanol. ...
... The study revealed the occurrence of reduction as well as deoxygenation to yield a mixture containing cyclohexane and cyclohexanol. The mechanistic investigation revealed that under the reaction condition, dehydration of cyclohexanol occurs more preferably than hydrogenolysis of C-O linkages [30]. Besides Ru/C, the Pt group metals such as Pd, Pt, and Rh, as well as nonnoble metals such as Cu and Ni supported over neutral carbon have been reported for the hydrodeoxygenation of eugenol. ...
Conversion of biomass into alternative fuel to replace nonrenewable fossil fuels is one of the sustainable goals of growing nations. As a prerequisite to attain this goal, carbon- and hydrogen-rich renewable resources (agricultural biomass) or the postconsumer polymer wastes, which are abundantly available are needed for the generation of fuels. Lignocellulosic material, a major class of biomass and a fascinating organic molecule, is rich in carbon and hydrogen and has been identified as a suitable renewable resource. Through valorization, this biomass can be converted into many value-added fine chemicals including hydrocarbons (alternative fuels) by adopting pyrolysis–hydrogenation methods. However, the inherent polymeric complex structure of lignin makes it a challenging material for chemical conversion into phenolic derivatives and subsequent reduction to hydrocarbons by hydrodeoxygenation. Owing to the complex nature of lignin, a variety of pyrolysis/copyrolysis cum catalytic hydrogenation methods have been developed to achieve maximum conversion of lignin into aromatic/aliphatic hydrocarbons with higher selectivity.
This chapter encompasses the catalytic methods developed for cracking lignin and hydrodeoxygenation. The catalytic methods, viz., porous materials, metal oxides, transition metal–loaded oxides, activated carbon catalysts, functionalized silica, bimetallic oxides, clay, carbon nanotubes, ionic liquid–porous medium, supercritical solvent, etc., have been covered in this chapter. This chapter critically analyzes the advantages and challenges confronted by catalytic systems, discussed from the bird’s eye view. In a nutshell, outcome and future perspectives of the catalytic processing of lignin are summarized in the final section of this chapter.
... Role of Catalysts. Anisole HDO has been investigated using a variety of catalysts including non-noble metals such as Ni, 13,54,67,68 Mo, 69,70 Fe, 64 and Co, 11 noble metals such as Ru, 4,12,71 Pd, 72,73 and Pt, 66 and bimetallic NiGa, 61 RuFe, 62 FeNi, 51 ReMo, 74 NiMo, 56 and CoMo. 65 68 and carbon. ...
... 65 68 and carbon. 71 From the previous discussion, conversion of anisole to deoxygenated products requires both metallic and acidic sites. 66,75,76 Li et al. 67 investigated the effect of porosity of Ni/HZSM-5 on anisole to cyclohexane conversion. ...
... 68,70 The studies on anisole HDO using various catalyst systems are summarized in Table 2 57,81 It is known that the cleavage of the C aryl −O bond becomes pronounced at elevated temperatures. 62,71 Li et al. 67 investigated the effect of change in temperature on anisole conversion and product selectivity using a Ni/HZSM-5 catalyst in a batch reactor. With an increase in temperature from 160 to 200°C, sharp increase in anisole conversion from 37% to 98% and cyclohexane selectivity from 49.6% to 84.2% were observed at a constant H 2 partial pressure of 68 bar. ...
... Nowadays, one of the main trends in upgrading renewable feedstocks is focused on the hydrodeoxygenation (HDO) of lignin-derived substances into hydrocarbons or aromatics by the complete deoxygenation [1][2][3][4][5][6][7][8][9][10][11]. Selective hydrogenation of lignin-derived compounds to valuable products such as phenol, cyclohexanol and its alkyl derivatives is a promising process in green chemistry [12][13][14][15]. Those chemicals can be used in the production of plastics, paints, detergents and in many other applications [16]. ...
... NMR analysis was performed on bio-oil sample and products with each sample being dissolved in 2 mL of dimethyl sulphoxide-d6 (DMSO-d6). The 1 H and, 13 C NMR and HSQC spectra were recorded at 25 • C on a Bruker Avance III HD (400 MHz) NMR spectrometer (Rheinstetten, Germany) operating at a frequency of 400 MHz for 1 H and 100 MHz to detect the presence of 13 C. The 1 H spectra were recorded at 1024 scans and 0.2 s acquisition time. ...
We present the conversion of guaiacol and diphenyl ether (DPhE) with ~100 % selectivity towards hydrocarbons over a ruthenium oxide containing catalyst on micro-mesoporous support (ZM). To elucidate the structure–catalytic...
... Examples of such supports include various carbon materials such as activated carbons [21][22][23], carbon nanotubes [24][25][26], and porous polymers [27][28][29]. Catalysts derived from these supports effectively promote hydrogenation of bio-oil components even in acetic acid media, and this under relatively mild conditions [30]. Some polymeric materials, such as porous aromatic frameworks (PAFs), possess properties close to those of carbon supports [31]. ...
... A nonlinear relationship between guaiacol conversion and temperature was demonstrated, the conversion reaching its maximum at 250°C and further heating boosting the yield of phenol. The guaiacol conversion can be enhanced by decreasing its concentration; in this regard, Ru-PAF- 30 and Ru(COD)-PAF-30 exhibited similar activity. The conversion of various lignin derivatives was then tested in the presence of the Ru(COD)-PAF-30 ruthenium catalyst and the previously synthesized Pt-PAF platinum catalyst. ...
This study focuses on the hydrogenation of model lignin bio-oil components over ruthenium and platinum catalysts synthesized from a porous aromatic framework, namely PAF-30. This PAF represents a polymeric support with developed porosity and high chemical and thermal stability. The effects of the guaiacol concentration, process temperature, and reaction time on the product composition were identified in the catalytic hydrogenation of guaiacol as a common component of lignin bio-oil. Various guaiacol derivatives were hydrogenated, and the hydrogenate composition was investigated. It was demonstrated that, within one hour at 250°C and a hydrogen pressure of 3 MPa, guaiacol can be hydrogenated exhaustively into 2-methoxycyclohexanol (64%) and cyclohexanol (64%) over the ruthenium catalyst and into a mixture of various hydrogenation products over the platinum catalyst.
... Hence copyrolysis synergy elevates the biooil composition by reducing its oxygen content by establishing alternative reaction pathways. Moreover, high reaction temperatures initiate the sequential decomposition of lignin through demethoxylation and hydrogenation reactions intensifying formation of a hydrocarbon pool in the product spectrum [124,125]. Concurrently deoxygenation and decarboxylation of PLA (as discussed in section 3.2) derived ketones, carboxylic acids and, aldehydes further contribute to depletion of oxygenated fractions in the pyrolytic products [126]. Chromatographic analysis at 425 • C further corroborates this shift through an increase in aromatic, hydrocarbon, and phenolic groups with a pronounced decrease in oxygenated intermediates as seen in Fig. 8. ...
Polylactic acid (PLA) is the highest produced bioplastic globally but facing end-life disposal challenges. Pyrolysis proves to be a viable option, but the recovered product profile is not desirable in terms of quality and value. Date Pits (DP), a waste byproduct chemically rich with lignocellulosic fragments, can provide unique carbon-rich precursors which are highly desirable in the pyrolysis process. This study aims to investigate the synergistic effect of DP addition on PLA pyrolysis products. Thermogravimetric data demonstrates that PLA mixing with DP promotes char formation, initiates degradation at lower temperatures, and decreases the peak decomposition temperature (T p) from 362 • C to 343 • C. Primary pyrolysis occurs in the range of (200-400 • C) with 75.5 % weight loss and low heating rate shifts T p toward lower temperatures by averting the development of the thermal lag effect. Chemical structure analysis through FTIR shows that DP addition promotes controlled volatile release through PLA depolymerization hence yielding more uniformed and distinguished peaks for hydroxyl, phenols, and ester-containing groups. Moreover, it promoted free radical reactions that enhanced lactide recovery by restricting aldehyde formation. GCMS profiling indicates that pure PLA pyrolysis majorly yieldes lactide (3,6-Dimethyl-1,4-dioxane-2,5-dione). While the copyrolysis with date pits diversifies this product profile with the production of hydrocarbons (heptane and decane), aromatics (xylene, toluene and styrene), and furans which are highly valued in biorefineries, as drop-in fuels and in petrochemical industries. Kinetic analysis shows that the PLA/DP co-pyrolysis mixture reduces activation energies (E a) by 18 % and also reduces the thermodynamic parameters.
... Zhou et al[14] selectively hydrodeoxygenated the β-O-4 ether bond in lignin into fuels such as naphthenes using NiLa/CNT. Charles et al[15] carried out hydrogenation reactions with ligninlike model compounds (anisole, guaiacol, 1,2-dimethoxybenzene, etc.) and explored the influencing factors such as temperature, pressure and reaction time, and concluded that the cleavage and hydrogenation of the C-O bond do not interfere with each other under appropriate temperature conditions. ...
Biofuels have emerged as a potential substitute for fossil fuels, with production occurring through a range of methods, including physico-chemical processes and other techniques involving biomass. Plant-based biomass has significant potential for biofuel production and holds considerable promise for future applications in the petrochemical industry. Plant-based biomass is composed primarily of two types: lignocellulosic and microalgal. During processing, pyrolysis enhances the efficiency of upgrading; however, the resulting bio-oil contains numerous oxygenated compounds, the presence of which impairs the bio-oil's performance, rendering it unsuitable for direct use as a fuel. Accordingly, the selective utilisation of catalysts for hydrotreating is imperative, as this reduces the oxygen content and enhances the quality of the bio-oil.
... Phenolic substances are acidic, and will not only corrode fuel equipment, but also affect the stability of biomass oil [6]. Therefore, the conversion of phenolic substances into chemically stable and high-quality biomass oil is a scientific problem to be solved urgently [7,8]. ...
This paper studies the catalytic hydrogenation reduction of lignin-derived phenolic compounds, such as catechol, guaiacol (O-methoxyphenol), phenol, P-methylphenol, O-ethylphenol, O-ethoxyphenol, etc. The reaction system focuses on the catalytic performance of hydrodeoxygenation reactions involving the phenolic derivatives of the lignin depolymerization products catechol and guaiacol. A series of Al2O3-TiO2 composite oxide supports with different Al/Ti ratios were prepared by a co-precipitation method, and a 5% Pd/Al2O3-TiO2 bifunctional catalyst was prepared by an impregnation method and characterized with XRD, SEM, BET, NH3-TPD, etc. Among these, the Pd/Al2Ti1 catalyst had the most excellent catalytic performance. At 100 °C and 2 MPa hydrogen pressure, the conversion of catechol was as high as 100%, and at 100 °C and 5 MPa hydrogen pressure, the conversion of guaiacol reached 90%.
... Additionally, lignin is the earth's most common source of aromatics and natural biopolymer, for instance, the pulping industry alone produces over 130 million tons of lignin waste each year (van den Bosch et al. 2018;Poveda-Giraldo et al. 2021). Furthermore, there are several valuable functional groups in this renewable resource, such as benzene rings, phenolic hydroxyl groups, and methoxyl groups (Vriamont et al. 2019;Zhou et al. 2017). The aforementioned evidence demonstrates that lignin has the potential to be a superb starting material in producing biofuels and phenolic fine chemicals. ...
Lignin, a vital renewable biopolymer, serves as the Earth’s primary source of aromatics and carbon. Its depolymerization presents significant potential for producing phenolic fine chemicals. This study assesses promoted Ni-based bimetallic catalysts (Ni-Co/C and Ni-Cu/C) supported on activated carbon in isopropanol for lignin depolymerization, compared to monometallic counterparts. BET, SEM, EDX, and XPS analyses highlight their physicochemical properties and promotional effects, enhancing hydrogenolysis activity and hydrogen transformation. Reaction parameter exploration elucidates the influence on lignin depolymerization, with cobalt and copper as promoters notably increasing conversion and monomer yield. Ni-Co/C exhibits the highest lignin conversion (94.2%) and maximum monomer yield (53.1 wt%) under specified conditions, with lower activation energy (36.1 kJ/mol) and higher turnover frequency (31.6 h⁻¹) compared to Ni/C. FT-IR, GPC, GC-FID, and GC–MS analyses confirm effective depolymerization, identifying 20 monomer products. Proposed reaction mechanisms underscore the potential of Ni-based bimetallic catalysts for lignin valorization, offering insights into developing efficient catalytic systems for lignin hydrogenolysis. This research enhances understanding and facilitates the development of selective catalytic processes for lignin valorization.
... Additionally, HDO was coupled with esterification reactions. Vriamont et al. investigated the HDO reaction of the mixture of GUA and acetic acid over Ru/C catalyst [55]. A mixture of cyclohexanes and cyclohexanols, together with cyclohexyl acetates from esterification reaction, was observed. ...
Hydrodeoxygenation (HDO) is an effective technology for upgrading the quality of bio-oil. Despite the considerable research endeavors in recent years, the reaction conditions employed have been notably severe, and there remains a scarcity of investigations into the potential components in bio-oil. In the present study, a series of CuCoOx catalysts were synthesized for HDO reactions of guaiacol (GUA), vanillin (VAN), and levulinic acid (LA) using isopropanol as a hydrogen donor. Under mild reaction conditions (150-190 °C), the optimal CuCoOx catalyst exhibited good performance, yielding ~99% of the desired products from the respective model compounds. Notably, competitive HDO with esterification was observed at 150-170 °C when co-hydrotreating the mixtures of GUA, VAN, and LA over the CuCoOx catalyst. Upon increasing the reaction temperature to 190 °C, the esterification between compounds in the mixtures was inhibited, and all compounds were converted to products with low oxygen content. Further, various characterizations substantiated the robust adsorption capabilities of the optimal catalyst towards methoxy, carbonyl, and aromatic moieties. In addition, the Cu1Co10Ox-300 catalyst demonstrated the capacity to transform high-oxygen compounds present in pyrolysis oil into species with diminished oxygen content, and in some cases, into hydrocarbon compounds. The present study presents a promising approach for the efficient conversion of single model compounds and mixtures into value-added chemicals and fuels under mild conditions.
... Fig. 7 shows the yields of the main aromatic monomers obtained from Soda lignin. Here, guaiacol, methyl-, ethyl-and propylguaiacol were derived from the G-units or were possibly formed by demethoxylation of the present S-units [68,69], while syringol and acetosyringone were derived only from the S-units in Soda lignin. Acetosyringone is formed independently of the conditions in an earlier step of the reaction, and the yield is not significantly affected by the reactive conditions. ...
The development of effective lignin depolymerization is essential for the extraction of aromatic building blocks from renewable resources. In this study, technical lignins (Soda and Kraft) were depolymerized in supercritical ethanol using Ru/C as a catalyst in an inert (N2) or a reducing (H2) atmosphere. The effects of lignin structure and atmosphere were studied on the depolymerization, and the products formed such as solid residue, oligomeric fragments and monomers. Reasonable yields of oligomers and monomers (32.7 wt% and 4.7 wt%, respectively) were obtained after Ru/C-assisted Soda lignin depolymerization under both tested atmospheres, while in the case of Kraft lignin, H2 and Ru/C were required to increase the yields of depolymerization products (30.9 wt% and 4.4 wt%, respectively). A comprehensive characterization of the reaction products (GC-MS, UV fluorescence and FTIR spectroscopy, 31P and 2D HSQC NMR) was performed to evaluate the monomer/oligomer distribution and to determine the structure and functionalities of the oligomers. The analytical results indicate the formation of more condensed oligomeric lignin structures with predominant 5-5′ inter-unit linkages and the prevalence of demethylation or demethoxylation reactions depending on the type of the technical lignin exposed to hydrodeoxygenation. The oligomeric fragments are catalytically modified and depolymerized lignin with a wide range of new applications (coatings, resins, antioxidant agents, adhesives) contributing to the replacement of the petroleum-derived phenolic building-blocks.
... Biomasses with broad sources and abundant reserves are easily available and renewable, which could be a promising substitute for fossil resources [4]. Among them, lignin is a complex biopolymer with high carbon content and abundant aromatic structures, endowing it with great advantage and potential as feedstock for the production of clean biofuels and highvalue chemicals [5][6][7][8]. Lignin could be converted into small molecule platform chemicals via selective cleavage of the C-C and C-O bonds owing to its unique structure and chemical properties [9][10][11][12]. The C-O bridged linkages primarily involve three model linkages, i.e, α-Ο-4, β-Ο-4 and 4-O-5, respectively, which are richer than C-C linkages in the structure of lignin, and these C-O more likely to be cracked due to their relatively lower dissociation energy [13]. ...
... Despite these challenges, the hydrogenation of lignin has the potential to provide significant benefits in the production of biofuels and other bioproducts. By modifying the chemical and physical properties of lignin, it is possible to improve the efficiency and yield of biofuel production, reduce the environmental impact of biofuel production, and potentially create new markets for lignin-based products [171]. Overall, the hydrogenation of lignin is an exciting area of research with significant potential for the development of sustainable, low-carbon technologies and products. ...
Lignin is a polymer found in the cell walls of plants and is an important component of wood. Lignin-derived fuels have attracted attention as a means of producing biofuels from biomass in recent years. There are two basic methods for converting lignin into fuel: thermochemical and catalytic. Lignin-derived fuels have the potential to reduce dependency on fossil fuels and reduce greenhouse gas emissions. However, more research is needed to optimize the production of lignin-derived fuels and to determine their environmental impact. This review aims to evaluate the development of lignin-derived fuels from an economic and environmental point of view while presenting a broad perspective.
... Improved selectivities have been reported for group 10 metals when they are alloyed with a more oxophillic metal [44,[159][160][161][162]. The reaction pathway for HDO of cresols on these alloy catalysts is sometime reported to involve ring hydrogenation on the metal to form a saturated alcohol, followed by dehydration and dehydrogenation back to the aromatic [163]. ...
With the increasing demands for sustainable energy and growing concerns of global warming, the use of biomass as a replacement for conventional petroleum has received considerable attention. Phenolic and furanic compounds derived from biomass could potentially serve as platform molecules with valuable chemical structures. However, the technologies to upgrade and utilize such molecules are still under development. This thesis aims to study the capabilities of oxides doped with metal-atom catalysts and metal-oxide, inverse catalysts to transform biomass platform molecules into useful chemicals, such as fuel, surfactant, and lubricants. Firstly, NbOx on Pt was found to be extremely active, selective, and stable for the direct deoxygenation of m-cresol, a model compound for phenolics. The metal-oxide overlayer was found to be the active site for the reaction while Pt itself hydrogenated the aromatic ring. This well-defined inverse catalyst structure was prepared by strong metal-support interactions (SMSI). To provide the H2 economically for the deoxygenation process, different oxides doped with isolated Co atoms were investigated for the dehydrogenation of ethane to ethylene. Atomic layer deposition (ALD) was shown to be able to synthesize the single atom structure readily, which is the active site for dehydrogenation. Lastly, aldol condensation of furfural, a furanic model compound, was investigated over the solid-base catalyst, CaO, to increase the carbon chain length. It was found that the aldol condensation rates are high when small ketones were used as the reactants. When larger ketones were used, the Cannizaro reactions could override the aldol condensation and lower the selectivity of furfural to high carbon products. The contributions from these studies should help develop catalytic processes for biomass valorization.
... Milled wood lignin (MWL) is obtained by extraction using 1,4-dioxane: water (96:4 v/v) of extensively groundwood biomass for 24 h. This process is often referred to as Bjorkman process [30] and does not significantly change the lignin structure [31]. ...
Currently, fossil-derived crude oil is the major resource for the production of aromatic commodity chemicals. However, the use of fossil fuels resulting in environmental damages such as global warming and rising CO2 concentration in the atmosphere. One such alternative is lignocellulosic biomass–derived lignin, but the lignin is mainly considered a waste by-product stream in the paper, pulp, and 2G ethanol biorefineries and treated as a heat/power generation source for boilers. However, lignin is abundant and potential aromatic carbon feedstock on the planet earth. Hence, the demand for the production of biobased chemicals from lignin is increasing in biorefineries. Despite its recalcitrant nature and irregularity in lignin structure, many thermochemical strategies have been developed to depolymerize lignin to aromatic chemicals. The present chapter focuses on lignin’s depolymerization to aromatic chemicals using various thermochemical methods, including pyrolysis, hydrothermal liquefaction, solvolysis, reductive, and oxidation processes. The effect of lignin feedstock, catalyst type, and the reaction parameters, including reaction temperature, residence time, pressure, and the solvent medium, on the distribution of degraded lignin products, including low molecular weight monomers, oligomers, and solid char and possible mechanisms, was discussed.
... Continuous concern on the change of global climate and the depletion of fossil fuels currently leads to tremendous interests on the production of biofuels and chemicals from renewable sources [1,2]. Due to its abundant natural phenolic biopolymer, lignin has the potential to replace crude oil or coal as the source of arenes in prospective chemical industries [3][4][5][6]. ...
Selective hydrogenation of aromatic ring represents an essential process for the valorization of lignin in the chemical industry but achieving this process at low temperature is still a challenge. Herein, a series of Ru-based catalysts were investigated. It is found that Ru/γ-Al2O3 with small Ru metal particle size shows the low activity in this system. A unique and strong metal-support interaction for Ru/γ-Al2O3 indicates that the strongly bounded Ru-O-Al sites lead to the positive charge of Ru species. In contrast, Ru/α-Al2O3 with well-dispersed Ru nanoparticles supported on α-Al2O3 can successfully catalyze the selective hydrogenation of lignin-derived aromatic compounds at room temperature. Ru/α-Al2O3 has excellent recyclability, air stability and the highest activity. The selective hydrogenation of aromatic ring for Ru/α-Al2O3 results from the fast dissociation of H2 and the high adsorption energy of aromatic ring as revealed by X-ray absorption spectra, X-ray photoelectron spectroscopy and density functional theory calculations.
... Although tremendous effort has been engaged in lignin depolymerization, fully unlocking lignin's potential for monomeric products still remains challenging, because of: (1) heterogeneity and recalcitrant structure of lignin [11], and (2) recondensation reactions during the depolymerization process [12]. Consequently, the conversion of lignin into monomers usually needs harsh conditions, for example, the catalytic hydrogenation in the presence of catalysts (Pd, Pt, Ru, Rh, V, Ni, Ni-Fe, CoMoS x , zeolite catalyst), high hydrogen pressure (0.5-40 MPa) and long reaction time (1-48 h) [13][14][15][16][17][18][19][20][21][22]. In these reactions, it is highly desirable to have high selectivity and high yield. ...
Lignin is considered as a renewable and sustainable resource for producing value-added aromatic chemicals and functional carbon materials. Herein, we develop a one-step catalyst-free depolymerization strategy to convert lignin into aryl monomers and carbon nanospheres simultaneously. Importantly, microwave-assisted depolymerization (MAD) in conjunction with dichloromethane (CH2Cl2) vapors is developed. The total mass yield of guaiacols reached the highest amount of 225.1 mg/g at 600 °C, and the highest yields of phenols (49.0 mg/g) and aromatic hydrocarbons (155.1 mg/g) were obtained at 700 °C. Hydrogen radicals and hydrogen chloride (HCl) are in-situ formed from CH2Cl2, significantly decreasing the activation barrier and reforming pyrolysis vapors to promote the formation of aryl monomers. Interestingly, uniform carbon nanospheres with an average size of 140 nm were produced as co-products at 700 °C. The microwave “hot-spots”, allied with the continuous surface erosion and the decrease in surface energy of lignin-derived carbon precursors by CH2Cl2 vapor, can be considered the driving force for the ultimate formation of carbon nanospheres. The CH2Cl2/MAD system produces aryl monomers (26.8 wt% yield) and carbon nanospheres (36.6 wt% yield) at 700 °C. We provide a facile, intriguing and scalable approach to convert lignin to valuable aryl monomers and sustainable carbon materials that can be applied in the chemistry, energy and environmental fields.
... This is because the Ni/SiO 2 -Al 2 O 3 catalyst favors: (i) a suitable mechanism (PG hydrodemethoxylation to 4-n-propylphenol, followed by its hydrogenation into PCol); (ii) high reactivity of 4-n-propyl-2-methoxycyclohexanol (obtained from PG hydrogenation) over PCol (because PCol can further react to give hydrocarbons); and (iii) a high carbon balance. Owing to strong redox properties, the Ru/carbon catalyst gave good yields of cyclohexane (via phenolic C-O bond cleavage), along with cyclohexanols, which was also demonstrated by Vriamont and colleagues [97]. Compared with the acidic support (ZSM-5), high guaiacol conversion (96%) with cyclohexanol as the main product was obtained over a mild acidic γ-Al 2 O 3 -supported NiCo catalyst [98]. ...
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.
Lignin, one of the most abundant biopolymers in the world has drawn extensive attention from scientific researchers due to emerging energy challenges. However, due to the complexity of its structure only 1%–2% of lignin can currently be converted into commercially valuable chemicals, the precise conversion of lignin remains a highly challenging scientific problem. This study centers on the transformation of aryl methyl ethers, which serve as representative lignin model compounds, aiming to develop an efficient strategy for acetic acid synthesis. N‐heterocyclic carbene catalyst was synthesized and characterized, the reaction conditions for the directional conversion of lignin model compounds to acetic acid are studied, excellent results were obtained using different kinds of aryl methyl ethers under the standard reaction procedure and a possible reaction mechanism was proposed. Comparing with the previous literature, this research not only provides a new, targeted strategy for the directional conversion of lignin model compounds but also offers a promising pathway for the green synthesis of acetic acid.
The one-pot hydrotreatment of phenols to cyclohexyl ethers is crucial but difficult to achieve for fine chemical synthesis owing to the easy overhydrogenation to cyclohexanols over traditional metal–acid bifunctional catalysts. Herein, surface oxygen-doped carbon-supported Pd nanoparticles (Pd/C-O) were prepared via nitric acid oxidation and subsequent incipient wetness impregnation, demonstrating the tandem hydrogenation–acetalization–hydrogenolysis route of phenol to cyclohexyl methyl ether, achieving an significant yield of 97.9% in a methanol solvent at a low temperature of 110 °C. Catalytic mechanism investigation indicated that the in situ hydrogen spillover from Pd nanoparticles to the Pd–O–C interface formed H⁺–H⁻ pairs, which acted as uncommon active sites for hydrogenation and hydrogenolysis steps and also provided Brønsted acid sites for the acetalization step, thereby triggering the facile preparation of cyclohexyl methyl ether. Furthermore, the prepared catalyst exhibited excellent catalytic generality for synthesizing cyclohexyl ethers from various phenols or alcohol solvents via a similar reaction route and great expansibility from diphenyl ethers via preliminary partial hydrogenation–alcoholysis steps. The study reports an interesting bifunctional catalysis for challenging tandem reaction routes toward cyclohexyl ether synthesis by harnessing an oxygen-doped carbon support to form transient H⁺–H⁻ pairs.
Catalytic refining of lignin holds promise for producing sustainable platform chemicals. In this work, a gaseous hydrogen‐free catalytic hydrodeoxygenation system is developed for upgrading lignin‐derived phenols to alkane chemicals. Commercially available Raney Ni and HZSM‐5 are used as a combinational catalyst, with isopropanol serving as the hydrogen‐donating solvent. By modifying the temperature and the ratio of Raney Ni to HZSM‐5, the reaction pathways for hydrogenation and deoxygenation can be tailored to specific requirements. As a result, a 97.1% yield of alkane fuels is achieved, with 64.4% propylcyclohexane and 32.7% propylbenzene obtained in one‐pot reaction from the hydrodeoxygenation of 2‐methoxy‐4‐propylphenol using a 3:1 mass ratio of Ni to HZSM‐5, further increasing the ratio of HZSM‐5 leads to a selectively production of propylbenzene in 62.0% yield. Through careful regulation of the catalytic system and the design of hydrogenation–deoxygenation pathways, excellent yields of 4‐propylcyclohexanol (92.2%), propylcyclohexene (93.3%), and propylcyclohexane (93.2%) are directionally achieved. The catalyst maintained a conversion rate of over 99% after five cycles, demonstrating excellent robustness. This study offers a strategic system that expedites the selective upgrading of lignin‐derived chemicals, heralding a pathway toward sustainable fuels and chemicals.
Lignin, a vital renewable biopolymer, serves as Earth's primary source of aromatics and carbon. Its depolymerization presents significant potential for producing phenolic fine chemicals. This study assesses promoted Ni-based bimetallic catalysts (Ni-Co/C and Ni-Cu/C) supported on activated carbon in isopropanol for lignin depolymerization, compared to monometallic counterparts. BET, SEM, EDX, and XPS analyses highlight their physicochemical properties and promotional effects, enhancing hydrogenolysis activity and hydrogen transformation. Reaction parameter exploration elucidates the influence on lignin depolymerization, with cobalt and copper as promoters notably increasing conversion and monomer yield. Ni-Co/C exhibits the highest lignin conversion (94.2 %) and maximum monomer yield (53.1 wt. %) under specified conditions, with lower activation energy (36.1 kJ/mol) and higher turnover frequency (31.6 h−1) compared to Ni/C. FT-IR, GPC, GC-FID, and GC-MS analyses confirm effective depolymerization, identifying 20 monomer products. Proposed reaction mechanisms underscore the potential of Ni-based bimetallic catalysts for lignin valorization, offering insights into developing efficient catalytic systems for lignin hydrogenolysis. This research enhances understanding and facilitates the development of selective catalytic processes for lignin valorization.
The cleavage of C-C bonds in oxidized lignin model compounds is a highly effective methodology for lignin depolymerizaiton, as well the generation of N-substituted aromatics. Density functional theory calculations have...
A review of recent reports that focus on the selective hydrodeoxygenation of lignin biomass derived aromatic compounds is presented. Obtaining high value chemical feedstocks and fuels from lignin is recognized as an essential aspect of the economic feasibility of biorefineries. Lignin is both a non-edible part of biomass and a potential source of aromatic commodity chemicals. The selective catalytic conversion of lignin derived compounds to deoxygenated aromatic molecules represents the most direct route towards high value chemicals while also maximizing hydrogen use efficiency. This review aims to give an overview of reports within the last 3–4 years that have focused on the selective hydrodeoxygenation of lignin derived or lignin inspired oxygenated aromatic compounds. Many of these lignin derived dimers and monomers can be selectively obtained via reductive catalytic fractionation (RCF) of native lignin, thus a section of this review is dedicated to recent advancements of RCF. The observed trends with respect to catalyst composition and reaction conditions in these reports along with an outlook for selective catalytic hydrodeoxygenation is presented.
Lignin, Lignin is a valuable carbon-rich feedstock and a by-product of the lignocellulosic biorefinery and paper industry. Despite being widely available, it is an under-utilized material. Lignin is the second most prevalent biopolymer with a polyaromatic structure present in the cell wall of vascular plants. However, its complex molecular structure and selective solubility in some solvents make its commercial feasibility largely unexplored. In recent decades, lignin has gained attention as a renewable raw biomaterial for synthesizing industrially important value-added materials such as adhesives, resins, foams, lubricants, composites, and high-value chemicals. Despite this, a significant portion of lignin produced in the pulp and paper mill ends up as boiler fuel for power and heat production. The production of lignin from different methods and sources is often compared and questioned due to handling and environmental issues resulting from excessive production. This review paper discusses the structure of lignin, its types, extraction methods, modification via thermal and chemical approaches, applications, prospects, and challenges.
Rare earth catalysts have gained considerable attention in the catalytic depolymerization of lignin. However, their structure-performance relationship was rarely reported. This work demonstrated that the different morphologies of Sm2O3 exhibited remarkable structure-activity relationships in the catalytic oxidation of lignin. Four different morphologies of samarium oxides, namely rod-like, tube-like, octahedral, and multilayer were investigated. Rod-like Sm2O3 catalyst exhibited optimum activity in the oxidation of lignin model molecules and real lignin. The {222} crystal surface of the Sm2O3 catalyst was proved to play a key role. β-O-4, Cα-Cβ, and Csp2-Csp2 chemical bonds in lignin were synchronously broken.
A Co-modified zeolite (Co/TS-1) catalyst is prepared and evaluated for the catalytic hydrodeoxygenation (HDO) of the lignin-derived model compound guaiacol under mild conditions at 1 MPa H2 and 200 °C. According to the systemic experimental investigations including reduction temperatures, metal loading and reaction time, it is found that the guaiacol conversion and cyclohexanol selectivity over guaiacol HDO are significantly improved by regulating the Ti content, Co content and reduction temperature, in which 5.0%Co/TS-1 (Si/Ti = 110) reduced in 600 °C shows the superior HDO performance with 94.3% guaiacol conversion and 88.7% cyclohexanol selectivity. Combined with XRD, TEM, BET, NH3-TPD, Py-IR, H2-TPR, UV–Vis and XPS characterizations, it is confirmed that the introduction of appropriate Ti could modulate the catalyst Lewis acid sites and the interaction between Co and support to affect the metal reduction degree, to realize the improvement of the substrate adsorption and exposing more metallic Co0 active sites, resulting in higher HDO performance. In addition, the reaction pathway of guaiacol HDO to cyclohexanol over Co/TS-1 is proposed via demethoxylation of guaiacol to generate reaction intermediate phenol.
Cyclohexanols are widely used chemicals, which are mainly produced by oxidation of fossil feedstocks. Selective hydrodeoxygenation of lignin derivatives has great potential for producing these chemicals but is challenging to obtain high yields. Here, we report that CeO2-supported Ru single-atom catalysts (SACs) enabled the hydrogenation of the benzene ring and catalyzed etheric C-O(R) bond cleavage without changing the C-O(H) bond, which could afford 99.9% yields of cyclohexanols. As far as we know, this is the first to report that SACs catalyze hydrogenation of the aromatic ring. The reaction mechanism was studied by control experiments and density functional theory calculations. In the catalysts, the Ru-O-Ce sites were formed and one Ru atom was coordinated with about four O atoms. These catalytic sites could realize both the hydrogenation and deoxygenation reactions efficiently, and thus desired cyclohexanols were generated. This work pioneers the single-atom catalysis in aromatic transformation and provides a novel route for synthesis of cyclohexanols.
Current catalytic hydrogenolysis of lignin C-O bonds, which is crucial for lignin valorization, often requires a noble metal catalyst or/and harsh conditions such as elevated temperatures and high pressures. Herein, we report a highly selective electrochemical protocol to reductively cleave the benzylic C-O bond of the α-O-4 lignin model compound benzyl phenyl ether (BPE) at room temperature and ambient pressure. Nearly complete conversion of BPE to toluene and phenol in methanol was achieved in an undivided cell using Ni foam at both the anode and cathode, with yields of 97% and 30%, respectively. Using a divided cell, yields of 90% (toluene) and 84% (phenol) could be achieved using inexpensive carbon paper as the cathode when Ni(ii) salts were added to the cathode chamber. Notably, other divalent metal salts did not lead to any product formation, suggesting a unique role of Ni ions in benzylic C-O bond cleavage. Further, a substrate scope study revealed the suitability of the method for a variety of substituted BPEs. This work provides an economical and environmentally friendly method for selective cleavage of C-O bonds in benzylic ethers as model compounds for lignin.
Decrease in the metal dispersion of noble-metal-loaded catalysts with high metal loading is a major factor reducing noble-metal efficiency. Herein, we investigated the enhancement in metal efficiency of Ru/TiO2 catalyst by utilizing hydrogen spillover in the liquid-phase. Ru was highly dispersed at low loadings (0.1 and 0.5 wt.%), while larger nanoparticles were formed at higher loadings (1∼5 wt.%). The hydrogen spillover in liquid phase was activated at reaction temperature (100 °C) as the Ru dispersion decreased, which was confirmed through physical dilution experiments, hydrogen temperature-programmed-reduction, and kinetic analysis. Isotope experiment was conducted using D2O, observing inverse kinetic isotope effect (IKIE) for the high-Ru-loading catalysts. Based on the understanding of the hydrogen spillover in the liquid phase, the low metal efficiency of high-Ru-loading catalysts resulting from low dispersion could be compensated simply by physically mixing pristine TiO2, which played a role as new active sites when liquid-phase hydrogen spillover was activated.
Lignin-derived compounds provide a platform for the production of value-added chemicals. Herein, we reported metal phosphide (Pd-Ni-P) catalyzed the conversion of diphenyl ether (DPE) via a restricted pathway involving partial hydrogenation-hydrolysis-hydrogenation. Cyclohexanol, an important raw material for the polymer industry, was therefore obtained as the main product in 87% mole yield with a high production rate of 14.65 mol·g⁻¹Pd·h⁻¹. The results of X-ray diffraction, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy confirmed that Ni and P atoms were inserted into the face-centred cubic Pd lattices, leading to the expanded Ni-Ni lattices and Pd-P covalent bonds. The electronic deficient Pd contributes to the partial hydrogenation of DPE to cyclohexyl phenyl ether (CHPE), and the inserted Ni facilitates the hydrolysis of CHPE to monocycles. Various control experiments including deuterium experiment and reaction kinetic study revealed the strong hydrolysis ability of Pd-Ni-P catalyst accounting for the high production rate of cyclohexanol. Besides, several lignin-derived aryl ethers could undergo the same reaction pathway to produce alcohol chemicals with high yields ranging from 50% to 94%.
Advances reported in the past two decades (1996–2021) in the field of heterogeneous catalytic hydrogenolysis have been reviewed. Due to its practical importance hydrogenolysis remained a major contributor to synthetic, process, industrial, medicinal, and green chemistry. Hydrogenolysis on heterogeneous catalysts is a common process tool in the development of environmentally benign processes. In this chapter the hydrogenolysis of all common CC and C-heteroatom bonds is discussed focusing on CO, CN, C-halogen, CSi, CS bonds; however, the hydrogenolysis of heteroatom-heteroatom bonds, such as NO, NN, or SiO bonds will also be reviewed. At the end of the chapter the hydrogenolysis of biosourced materials is also reviewed.
Lignin, which represents the most abundant renewable aromatic biomass resource on the earth, is promising as an alternative for the production of chemicals. We presented a modified method that provides the hydrogen source in the reaction with a mixed solvent of alcohol, acid and water, replacing the original method that directly provides a hydrogen source with external hydrogen. In addition, a new cobalt supported on nitrogen doped carbon catalyst preparation method (one-pot) was presented. We proposed one-step method in which corn stover lignin is converted to target monomers over Co/AC-N_one-pot catalyst and mixed solvent. The role of the alcohol to acid and water mixed solvent not only extract lignin fragments from the matrix but also provide the hydrogen source. Cobalt which has electronic interaction with the nitrogen reformed alcohols to obtain hydrogen and stabilized lignin intermediates during the reaction by hydrogenation of active bonds. Different kinds of nitrogen including pyridinic and pyrrolic nitrogen functional groups plays a critical role in stabilizing Co. Nearly complete delignification (91%) and high yield of target monomers (23.8%) can be obtained under the conditions (5wt% Co/Ac-N_one-pot, 10:1:1mixture solvent consist of isopropanol/water/ formic acid, 235 °C, 200 min, 0.1 MPa N2), and the selectivity of target monomers was 85%. Circulation experiment of catalyst was performed directly without any operation of the residue, and it could maintain 90% activity in the first four cycles.
Direct cleavage of the C-C bond in lignin linkages is a promising route to afford value-added aromatics, which, however, is usually related to metal-based catalyst and harsh conditions. Here, a...
The hydrodeoxygenation (HDO) of m-cresol was studied on Pt/Nb2O5/MgAl2O4 and Pt/TiO2/MgAl2O4 catalysts to understand the effects of Strong Metal Support Interactions (SMSI). The Nb2O5 and TiO2 supports were prepared as 0.7-nm films on MgAl2O4 by Atomic Layer Deposition (ALD) to ensure that the structures of the catalyst were the same. When reduced at 773 K to place Pt in the encapsulated state, Pt/Nb2O5/MgAl2O4 was much less active than Pt/MgAl2O4 at 573 K but much more active at 623 K. While Pt/MgAl2O4 deactivated rapidly due to coking, Pt/Nb2O5/MgAl2O4 showed significantly better coke tolerance and was almost 100% selective towards toluene production. Pt/Nb2O5/MgAl2O4 reduced at lower temperatures exhibited intermediate catalytic properties. The effect of reduction temperature on Pt/TiO2/MgAl2O4 was much less and this catalyst was more similar to Pt/MgAl2O4 than its Nb2O5 counterpart. The implications of these results for understanding the nature of oxide promoters on HDO of m-cresol are discussed.
In the past five years, biomass‐derived biofuels and biochemicals were widely studied both in academia and industry as promising alternatives to petroleum. In this Review, the latest progress of the synthesis and fabrication of porous nanocatalysts that are used in catalytic transformations involving hydrogenolysis of lignin is reviewed in terms of their textural properties, catalytic activities, and stabilities. A particular emphasis is made with regard to the catalyst design for the hydrogenolysis of lignin and/or lignin model compounds. Furthermore, the effects of different supports on the lignin hydrogenolysis/hydrogenation are discussed in detail. Finally, the challenges and future opportunities of lignin hydrogenolysis over nanomaterial‐supported catalysts are also presented.
We report a cascade synthetic route to directly obtain diethyl terephthalate, a replacement for terephthalic acid, from biomass-derived muconic acid, ethanol, and ethylene. The process involves two steps: First, a substituted cyclohexene system is built through esterification and Diels-Alder reaction; then, a dehydrogenation reaction provides diethyl terephthalate. The key esterification reaction leads to improved solubility and modulates the electronic properties of muconic acid, thus promoting the Diels-Alder reaction with ethylene. With silicotungstic acid as the catalyst, nearly 100 % conversion of muconic acid was achieved, and the cycloadducts were formed with more than 99.0 % selectivity. The palladium-catalyzed dehydrogenation reaction preferentially occurs under neutral or mildly basic conditions. The total yield of diethyl terephthalate reached 80.6 % based on the amount of muconic acid used in the two-step synthetic process.
The transition from a petroleum-based infrastructure to an industry which utilises renewable resources is one of the key research challenges of the coming years. Biomass, consisting of inedible plant material that does not compete with our food production, is a suitable renewable feedstock. In recent years, much research has been focused on developing new chemical strategies for the valorisation of different biomass components. In addition to the many heterogeneous and enzymatic approaches, homogenous catalysis has emerged as an important tool for the highly selective transformation of biomass, or biomass derived platform chemicals. This Perspective provides an overview of the most important recent developments in homogeneous catalysis towards the production and transformation of biomass and biomass related model compounds. The chemical valorisation of the main components of lignocellulosic biomass -lignin and (hemi) cellulose is reviewed. In addition, important new catalyst systems for the conversion of triglycerides and fatty acids are presented.
Exploiting biomass as an alternative to petrochemicals for the production of commodity plastics is vitally important if we are to become a more sustainable society. Here, we report a synthetic route for the production of terephthalic acid (TPA), the monomer of the widely used thermoplastic polymer poly(ethylene terephthalate) (PET), from the biomass-derived starting material furfural. Biobased furfural was oxidised and dehydrated to give maleic anhydride, which was further reacted with biobased furan to give its Diels-Alder (DA) adduct. The dehydration of the DA adduct gave phthalic anhydride, which was converted via phthalic acid and dipotassium phthalate to TPA. The biobased carbon content of the TPA was measured by accelerator mass spectroscopy and the TPA was found to be made of 100% biobased carbon.
The partial hydrogenation of benzene to cyclohexene is an economically interesting and technically challenging reaction. Over the last four decades, a lot of work has been dedicated to the development of an exploitable process and several approaches have been investigated. However, environmental constraints often represent a limit to their industrial application, making further research in this field necessary. The goal of this review is to highlight the main findings of the different disciplines involved in understanding the governing principles of this reaction from a sustainable chemistry standpoint. Special emphasis is given to ruthenium-catalyzed liquid phase batch hydrogenation of benzene.
Current biomass utilization processes do not make use of lignin beyond its heat value. Here we report on a bimetallic Zn/Pd/C catalyst that converts lignin in intact lignocellulosic biomass directly into two methoxyphenol products, leaving behind the carbohydrates as a solid residue. Genetically modified poplar enhanced in syringyl (S) monomer content yields only a single product, dihydroeugenol. Lignin-derived methoxyphenols can be deoxygenated further to propylcyclohexane. The leftover carbohydrate residue is hydrolyzed by cellulases to give glucose in 95% yield, which is comparable to lignin-free cellulose (solka floc). New conversion pathways to useful fuels and chemicals are proposed based on the efficient conversion of lignin into intact hydrocarbons.
Through catalytic hydrogen transfer reactions, a new biorefining method results in the isolation of depolymerized lignin-a non-pyrolytic lignin bio-oil-in addition to pulps that are amenable to enzymatic hydrolysis. Compared with organosolv lignin, the lignin bio-oil is highly susceptible to further hydrodeoxygenation under low-severity conditions and therefore establishes a unique platform for lignin valorization by heterogeneous catalysis. Overall, the potential of a catalytic biorefining method designed from the perspective of lignin utilization is reported.
Significance
Current, plant-based polyethylene terephthalate (PET) is produced from biomass-derived ethylene glycol [the terephthalic acid (PTA) used is not from biomass]. To have a 100% biomass-derived PET, PTA must be produced from biomass. Here, pathways for the production of renewable PTA, using Diels-Alder reactions between ethylene and oxidized derivatives of 5-hydroxymethylfurfural, a biomass-derived chemical, are reported. These pathways are enabled by new catalytic chemistry that may provide routes for the production of 100% biomass-derived PET.
Cheap fossil oil resources are becoming depleted and crude oil prices are rising. In this context, alternatives to fossil fuel-derived carbon are examined in an effort to improve the security of carbon resources through the development of novel technologies for the production of chemicals, fuels, and materials from renewable feedstocks such as biomass. The general concept unifying the conversion processes for raw biomass is that of the biorefinery, which integrates biofuels with a selection of pivot points towards value-added chemical end products via so-called “platform chemicals”. While the concept of biorefining is not new, now more than ever there is the motivation to investigate its true potential for the production of carbon-based products. A variety of renewable chemicals have been proposed by many research groups, many of them being categorized as drop-ins, while others are novel chemicals with the potential to displace petrochemicals across several markets. To be competitive with petrochemicals, carbohydrate-derived products should have advantageous chemical properties that can be profitably exploited, and/or their production should offer cost-effective benefits. The production of drop-ins will likely proceed in short term since the markets are familiar, while the commercial introduction of novel chemicals takes longer and demands more technological and marketing effort.
Research and development activities directed toward commercial production of cellulosic ethanol have created the opportunity
to dramatically increase the transformation of lignin to value-added products. Here, we highlight recent advances in this
lignin valorization effort. Discovery of genetic variants in native populations of bioenergy crops and direct manipulation
of biosynthesis pathways have produced lignin feedstocks with favorable properties for recovery and downstream conversion.
Advances in analytical chemistry and computational modeling detail the structure of the modified lignin and direct bioengineering
strategies for future targeted properties. Refinement of biomass pretreatment technologies has further facilitated lignin
recovery, and this coupled with genetic engineering will enable new uses for this biopolymer, including low-cost carbon fibers,
engineered plastics and thermoplastic elastomers, polymeric foams, fungible fuels, and commodity chemicals.
Biomass represents an abundant carbon-neutral renewable resource for the production of bioenergy and biomaterials, and its
enhanced use would address several societal needs. Advances in genetics, biotechnology, process chemistry, and engineering
are leading to a new manufacturing concept for converting renewable biomass to valuable fuels and products, generally referred
to as the biorefinery. The integration of agroenergy crops and biorefinery manufacturing technologies offers the potential
for the development of sustainable biopower and biomaterials that will lead to a new manufacturing paradigm.
The incentive for use of renewable resources to replace fossil sources is motivating extensive research on new and alternative fuel sources. Bio-oils derived from biomass offer the prospect of becoming a major feedstock for production of fuels and chemicals, and lignin is a plentiful, underutilized source. Lignin conversion requires depolymerization and removal of oxygen. Likely processes for lignin conversion involve depolymerization (e.g., by pyrolysis) and catalytic upgrading of the resultant bio-oils. A major goal of the upgrading is catalytic hydrodeoxygenation (HDO), which involves reactions with hydrogen that produce hydrocarbons and water. The aim of this review is to present a critical introduction to HDO chemistry focused on compounds derived from lignin. This article provides a summary of the reactions of HDO and those that accompany it, with a comparison of catalysts and a discussion of their stabilities. The reactions are evaluated in terms of reaction pathways of compounds representative of lignin-derived bio-oils, including anisole, guaiacol, and phenol. The review includes recommendations for further research and an attempt to place HDO in a context of options for renewable fuels and chemicals, but it does not provide an economic assessment.
Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.
In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.
http://pubs.rsc.org/en/content/articlelanding/2018/cs/c7cs00566k#!divAbstract
Aqueous phase hydrogenolysis of renewable biomass at low H2 pressures is an attractive route to selectively produce renewable fuels and valuable chemicals. Here, we showed that the dispersive Ru and Ni nanoparticles (NPs) on HZSM-5 with an optimum H· radical transfer catalyzed a rapid rate (152 mmol·g−1·h−1) in hydrogenolysis of C–O bonds in lignin-derived guaiacol at 240 °C and 2 bar H2 pressure in water. The co-impregnated individual Ru and Ni nanoparticles (NPs) on HZSM-5 were highly dispersed and did not present an alloy structure, but the individual Ru and Ni NPs were in a close proximity. The guaiacol hydrogenolysis rates were proportional to the amounts of the adjacent RuO2 and NiO NPs on the calcined samples, suggesting the closely contacted Ru and Ni NPs on HZSM-5 are the active sites. In the water phase at low H2 pressures, Ru dissociated the hydrogen molecules to H· radicals (H·), and then such radicals were transferred to adjacent Ni atoms for activating the capability of inert Ni centers. The adjustment of the H· transfer length between Ru and Ni NPs led to shorter H· transfer lengths, which resulted in activities as high as 118 mmol·g−1·h−1. The transferring and anchoring of H· radicals was considered to be achieved by the Si-OH groups and their defects on HZSM-5, as demonstrated by temperature programmed desorption of hydrogen coupled with mass spectroscopy (TPD/H2-MS) experiment. To further shorten the H· transfer length over uniformly formed Ru and Ni nanoparticles, the isolated Ni islands were removed through the incorporation of a Ru precursor that initially occupied the Brönsted acid sites on HZSM-5. By fully activating the two metals in the aqueous phase via an H· transfer mechanism at low H2 pressures, the rational design of bimetallic quasi-alloy catalysts provides a promising approach for achieving substantially high rates in selective hydrogenolysis steps.
The replacement of current petroleum-based plastics with sustainable alternatives is a crucial but formidable challenge for the modern society. Catalysis presents an enabling tool to facilitate the development of sustainable polymers. This review provides a system-level analysis of sustainable polymers and outlines key criteria with respect to the feedstocks the polymers are derived from, the manner in which the polymers are generated, and the end-of-use options. Specifically, we define sustainable polymers as a class of materials that are derived from renewable feedstocks and exhibit closed-loop life cycles. Among potential candidates, aliphatic polyesters and polycarbonates are promising materials due to their renewable resources and excellent biodegradability. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in catalytic transformations that lower the technological barriers for developing more sustainable replacements for petroleum-based plastics.
This Viewpoint presents a perspective on recent successes, challenges and research opportunities in the catalytic reduction of renewable carbon feedstocks, such as CO2 and biomass. Operating a shift from petrochemistry to utilize sustainable carbon resources imposes the development of efficient reduction methods, able to promote the conversion of C-O bonds to C-H bonds in an energy and atom efficient manner. Current reductive processes based on the utilization of main group hydrides, H2, formic acid and Hantzsch esters are compared to highlight their potential and limitations. Catalysis is key in present and future developments of sustainable reductive transformations and the design of efficient catalytic systems is examined to overcome the challenges facing each class of reductant.
High-pressure, vapor-phase, hydrodeoxygenation (HDO) reactions of dihydroeugenol (2-methoxy-4-propylphenol), as well as other phenolic, lignin-derived compounds, were investigated over a bimetallic platinum and molybdenum catalyst supported on multi-walled carbon nanotubes (5%Pt2.5%Mo/MWCNT). Hydrocarbons were obtained in 100% yield from dihydroeugenol, including 98% yield of the hydrocarbon propylcyclohexane. The final hydrocarbon distribution was shown to be a strong function of hydrogen partial pressure. Kinetic analysis showed three main dihydroeugenol reaction pathways: HDO, hydrogenation, and alkylation. The major pathway occurred via Pt catalyzed hydrogenation of the aromatic ring and methoxy group cleavage to form 4-propylcyclohexanol, then Mo catalyzed removal of the hydroxyl group by dehydration to form propylcyclohexene, followed by hydrogenation of propylcyclohexene on either the Pt or Mo to form the propylcyclohexane. Transalkylation by the methoxy group occurred as a minor side reaction. Catalyst characterization techniques including chemisorption, scanning transmission electron microscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize the catalyst structure. Catalyst components identified were Pt particles, bimetallic PtMo particles, a Mo carbide-like phase, and Mo oxide phases.
Bio-oil has a complex composition including acids and aldehydes, which lead to undesirable properties, such as high corrosiveness and instability. Upgrading was needed to improve bio-oil quality for its further application as transportation fuels. Hydrogenation and one step hydrogenation-esterification (OHE) were effective methods to convert acids and bio-oil to combustive alcohols and esters. Herein, various CNT-supported (carbon nanotube) non-sulfide catalysts were prepared and catalytic activity was investigated through hydrogenation of acetic acid and OHE of acetic acid and acetaldehyde. Mo-promoted NiMo/CNT catalysts exhibited better catalytic activity in both hydrogenation and OHE reaction for removing acetic acid. The optimal acetic acid conversion rate was up to 17%, with a selectivity of 85% toward ethanol during hydrogenation. OHE showed better efficiency for the conversion of acetic acid compared with hydrogenation. The effect of process parameters on esterification of acetic acid and acetaldehyde together with the selectivity of ethyl acetate were investigated in detail. It was found that addition of extra hydrogen during the OHE reaction was beneficial for improving both acetaldehyde conversion and ethyl acetate selectivity. When extra hydrogen was added twice, the acetaldehyde conversion rose from 66.0% to 81.4%, and the ethyl acetate selectivity improved from 21.8% to 36.8%.
Recent lignin-first catalytic lignocellulosic biorefineries produce large quantities of two potential platform chemicals, 4-n-propylguaiacol (PG) and 4-n-propylsyringol (PS). Since conversion into 4-n-propylcyclohexanol (PCol), a precursor for novel polymer building blocks, presents a promising valorization route, reductive demethoxylation of PG was examined here in the liquid-phase over three commercial hydrogenation catalysts, viz. 5 wt% Ru/C, 5 wt% Pd/C and 65 wt% Ni/SiO2-Al2O3, at elevated temperatures ranging from 200 to 300 °C under hydrogen atmosphere. Kinetic profiles suggest two parallel conversion pathways: Pathway I involves PG hydrogenation to 4-n-propyl-2-methoxycyclohexanol (PMCol), followed by its demethoxylation to PCol, while Pathway II constitutes PG hydrodemethoxylation to 4-n-propylphenol (PPh), followed by its hydrogenation into PCol. Slowest step in the catalytic formation of PCol is the reductive methoxy removal from PMCol. PCol may under the reaction circumstances react further into hydrocarbons. Following criteria are therefore essential to reach a high PCol yield: i) Catalytic pathway II is preferred as this route does not involve stable intermediates; ii) Reactivity of PMCol should be higher than that of PCol, and iii) the overall carbon balance should be high. Both the catalyst type and the reaction conditions have a substantial impact on the PCol yield. Only the commercial Ni catalyst meets the three criteria, provided the reaction is performed at 250 °C in hexadecane. Additional advantages of this solvent choice are a high boiling point (low operational pressure in closed reactor systems), high solubility of PG and derived products, high thermal, reductive stability, and easy derivability form fatty biomass feedstock. This Ni catalyst also showed an excellent stability in recycling runs and is capable of converting highly concentrated (up to 20 wt%) PG in hexadecane. Ru and Pd on carbon showed a low PCol yield as they are not conform the three criteria. Low hydrogen pressure favors Pathway II, resulting in a very high PCol yield of 85% at 10 bars. Catalytic conversion of guaiacol, 4-methyl- and 4-ethylguaiacol in comparable circumstances showed similarly high yields of the corresponding cyclohexanols.
Vanillic acid and syringic acid were converted to terephthalic acid via a novel two-step process using a fixed-bed reactor. The cascade route includes hydrogenation demethoxylation and carboxylation reactions. Activated carbon (AC) supported MoWBOx and PdNiOx were found suitable catalyst precursors for the process. An intermediate of p-hydroxybenzoic acid (HBA) was produced from the demethoxylation in 71.6% selectivity, and the terephthalic acid was obtained in 58.7% yield with 66.4% HBA conversion.
Catalytic hydrogenation is considered as an efficient technique for upgrading pyrolysis bio-oil. High flammability of hydrogen gas in contact with air leads to difficult control of high pressurized hydrogen gas in large-scale systems. Meanwhile, molecular hydrogen production is a costly industrial process. Thus, hydrogenation study using hydrogen donor (H-donor) material as alternative for hydrogen gas could be useful in terms of cost and safety control. In this study, the potential of decalin and tetralin for use as hydrogen source was investigated in transfer hydrogenation of renewable lignin-derived phenolic compounds (phenol, _o_-cresol and guaiacol) and a simulated phenolic bio-oil over Pd/C and Pt/C catalysts. Reaction mechanisms of H-donor dehydrogenation and phenolics hydrogenation were studied. Catalytic activity of Pt/C for transfer hydrogenation of the phenolic compounds was higher than that of Pd/C at reaction temperature of 275 °C. Decalin as hydrogen source showed to be more efficient for hydrogenation of the phenolic compounds compared to tetralin. In addition, the influence of water content on transfer hydrogenation activity was studied by employing the water to donor ratios of 0/100, 25/75, 50/50 and 75/25 g/g. Maximum hydrogenation of phenol as bio-oil model compound was observed at water to donor ratio of 50/50 g/g.
Selective hydrogenations of (hetero)arenes represent essential processes in the chemical industry, especially for the production of polymer intermediates and a multitude of fine chemicals. Herein, we describe a new type of well-dispersed Ru nanoparticles supported on a nitrogen-doped carbon material obtained from ruthenium chloride and dicyanamide in a facile and scalable method. These novel catalysts are stable and display both excellent activity and selectivity in the hydrogenation of aromatic ethers, phenols as well as other functionalized substrates to the corresponding alicyclic reaction products. Furthermore, reduction of the aromatic core is preferred over hydrogenolysis of the C-O bond in the case of ether substrates. The selective hydrogenation of biomass-derived arenes, such as lignin building blocks, plays a pivotal role in the exploitation of novel sustainable feedstocks for chemical production and represents a notoriously difficult transformation up to now.
Process via acetic acid fermentation has been proposed as a potential way to produce bioethanol from lignocellulosics with improved carbon utilization efficiency as compared to the conventional alcoholic fermentation. This article reports Ru-Sn/TiO2 as an effective hydrogenation catalyst for aqueous acetic acid as the last step of this process. Although a larger loading of Ru to TiO2 rather increased the gas by-production, the additional use of Sn significantly improved the ethanol selectivity by the stronger suppression effect on the gas by-production. Finally, the loading levels of 2-4 wt% of Sn to 4 wt%Ru/TiO2 were found to be optimal for efficient and selective hydrogenation of acetic acid to ethanol in water. With 4 wt%Ru-4 wt%Sn/TiO2 catalyst, aqueous acetic acid solutions (10, 45 and 100 g/L) were converted to ethanol in 98.2, 92.7 and 92.8 mol%, respectively, at 170°C/H2 15 MPa. This catalytic system would make the acetic acid-based bioethanol production practical.
There have been considerable efforts to produce renewable polymers from biomass. Poly(ethylene terephthalate) (PET) is one of the most versatile bulk materials used in our daily lives. Recent advances in the new catalytic process for conversion of biomass have allowed us to design more technically effective and cheaper methods for the synthesis of green PET monomers. This review analyses recent advances in the synthesis of PET monomers from biomass. Different routes for ethylene glycol (EG) and purified terephthalic acid (PTA) synthesis are systematically summarized. The advantages and drawbacks of each route are discussed in terms of feedstock, reaction pathway, catalyst, economic evaluation and technology status, trying to provide some state-of-the-art information on green PET monomer synthesis. Finally, an outlook is presented to highlight the challenges, opportunities and on-going trends, which may serve as guidelines for designing novel synthetic routes to green polymers from fundamental science to practical use.
Ruthenium nanoparticles immobilized on acid-functionalized supported ionic liquid phases (Ru NPs@SILPs) act as efficient bifunctional catalysts in the hydrodeoxygenation of phenolic substrates under batch and continuous flow conditions. A synergistic interaction between the metal sites and acid groups within the bifunctional catalyst leads to enhanced catalytic activities for the overall transformation as compared to the individual steps catalyzed by the separate catalytic functionalities.
Hydrodeoxygenation of methoxybenzenes such as guaiacol over Ru catalyst was studied. Guaiacol was demethoxylated and then hydrogenated over carbon black supported Ru-MnOx catalyst (Ru-MnOx/C) forming cyclohexanol and methanol in good yield (81% and 86%, respectively) under relatively mild conditions (433K, H2 1.5MPa). Over Ru-MnOx/C, yield of demethoxylated products (cyclohexanol and cyclohexane) was almost the same as that of methanol, suggesting that the methoxy group is eliminated by demethoxylation to form methanol. Other methoxybenzenes such as 2,6-dimethoxyphenol and anisole were also converted to demethoxylated saturated compounds such as cyclohexanol and cyclohexane. The reaction scheme was proposed where demethoxylation and total hydrogenation of aromatic ring from partially-hydrogenated adsorbed guaiacol proceeded in parallel. Lower H2 pressure and higher reaction temperature were advantageous to demethoxylation. Addition of MnOx species slowed down the reaction rate of total hydrogenation of aromatic ring, which increased the relative rate of the elimination of methoxy group to that of total hydrogenation before the elimination. The catalyst can be reused without significant loss of activity. The nanoparticles of Ru and Mn were highly dispersed, and the state of Mn species on Ru-MnOx/C during the reaction was weakly basic MnO.
Catalytic reduction of pyrolyzed biomass is required to remove oxygen and produce transportation fuels, but limited knowledge of how hydrodeoxygenation (HDO) catalysts work stymies the rational design of more efficient and stable catalysts, which in turn limits deployment of biofuels. This work reports results from a novel study utilizing both isotopically labeled phenol (which models the most recalcitrant components of biofuels) with D2O and DFT calculations to provide insight into the mechanism of the highly efficient HDO catalyst, Ru/TiO2. The data point to the importance of interface sites between Ru nanoparticles and the TiO2 support and suggest that water acts as a cocatalyst favoring a direct deoxygenation pathway in which the phenolic OH is replaced directly with H to form benzene. Rather than its reducibility, we propose that the amphoteric nature of TiO2 facilitates H2 heterolysis to generate an active site water molecule that promotes the catalytic C-O bond scission of phenol. This work has clear implications for efforts to scale-up the hydrogen-efficient conversion of wood waste into transportation fuels and biochemicals.
We studied the C-O cleavage of phenolate and catecholate at step sites of a Ru catalyst using periodic DFT methods at the GGA level. Both C-O scission steps are associated with activation barriers of about 75 kJ mol(-1), hence are significantly more facile than the analogous reactions on Ru terraces. With these computational results, we offer an interpretation of recent experiments on the hydrodeoxygenation of guaiacol (2-methoxyphenol) over Ru/C. We hypothesize that the experimentally observed dependency of the product selectivity on the H2 pressure is related to the availability of step sites on a Ru catalyst.
In the coming decades major changes are expected in the chemical industry regarding the utilized raw material inputs. Depleting fossil resources will gradually be replaced by renewable feedstocks wherever possible. Because of this transition, new and efficient methodologies are required that enable depolymerization and defunctionalization of these complex, highly oxygenated biopolymers. Additionally, utilization of all components of lignocellulose is of great importance. In particular, depolymerization of lignin into its aromatic subunits or defined aromatic platform chemicals has proven challenging. Various approaches to overcome these difficulties have been attempted and resulted in new and exciting developments in many fields. In this review we will give an overview of bond cleavage strategies relevant for lignin depolymerization using homogeneous catalysts, focusing especially on reductive and hydrogen transfer methods.
Kinetic flow reactor experiments have been carried out to study acetic acid hydrogenation on a Ru/C catalyst in both three-phase (catalyst, aqueous, and gaseous) and two-phase (catalyst and gaseous) regimes. In addition, density functional theory calculations have been performed and combined with microkinetic modeling to better understand the activity and selectivity observed in the experiments. Our experiments show that ethanol selectivity varies strongly from <10% to a maximum of 70% with increasing hydrogen partial pressure (pH2) at 185 °C in the three-phase reactor. Co-fed water also enhances ethanol selectivity, from 60% to 70% in the two-phase reactor and 40% to 65% in the three-phase reactor, at 185 °C, but only up to a certain concentration. The aqueous phase is not necessary for high ethanol selectivity. The first-principles microkinetic analysis is able to reasonably capture the apparent activation energy, ethanol selectivity, and reaction orders of acetic acid and ethanol with respect to pH2, providing a theoretical explanation for the crucial role that hydrogen plays in the selectivity of this reaction. Our findings provide insights into why high activity and selectivity for acetic acid hydrogenation to ethanol can be achieved on Ru, which may have general relevance to the catalytic hydrogenation of organic oxygenates on Ru and other metals.
Selective demethoxylation from aqueous guaiacol proceeded over Ru catalysts at relatively lower temperatures (≤433 K). Addition of MgO to the reaction media suppressed the unselective C–O dissociation. Cyclohexanol and methanol were obtained in high yield (>80%). A reaction route is proposed where partially hydrogenated guaiacol is decomposed into methanol and phenol, which is further hydrogenated to cyclohexanol.
Catalytic bio-oil upgrading to produce renewable fuels has attracted increasing attention in response to the decreasing oil reserves and the increased fuel demand worldwide. Herein, the catalytic hydrodeoxygenation (HDO) of guaiacol with carbon-supported non-sulfided metal catalysts was investigated. Catalytic tests were performed at 4.0 MPa and temperatures ranging from 623 to 673 K. Both Ru/C and Mo/C catalysts showed promising catalytic performance in HDO. The selectivity to benzene was 69.5 and 83.5 % at 653 K over Ru/C and 10Mo/C catalysts, respectively. Phenol, with a selectivity as high as 76.5 %, was observed mainly on 1Mo/C. However, the reaction pathway over both catalysts is different. Over the Ru/C catalyst, the OCH3 bond was cleaved to form the primary intermediate catechol, whereas only traces of catechol were detected over Mo/C catalysts. In addition, two types of active sites were detected over Mo samples after reduction in H2 at 973 K. Catalytic studies showed that the demethoxylation of guaiacol is performed over residual MoOx sites with high selectivity to phenol whereas the consecutive HDO of phenol is performed over molybdenum carbide species, which is widely available only on the 10Mo/C sample. Different deactivation patterns were also observed over Ru/C and Mo/C catalysts.
The Ru/MMT@IL-SO3H catalyst was prepared by immobilizing Ru nanoparticles onto montmorillonite (MMT) with the assistance of an acidic ionic liquid (1-sulfobutyl-3-methylimidazolium hydrosulfate, IL-SO3H). Transmission electron microscopy examination indicated that the loaded Ru species were distributed uniformly on the MMT support with a particle size of about 1.3 nm, which existed mainly in the form of a metallic state as confirmed by X-ray photoelectron spectroscopy analysis. X-Ray diffraction analysis indicated that the interlayer distance of MMT was increased due to the impregnation of IL-SO3H. The activity of Ru/MMT@IL-SO3H for hydrodeoxygenation (HDO) of various phenolic compounds was investigated. It was demonstrated that the as-prepared catalyst served as a bifunctional catalyst and displayed high efficiency for HDO reactions of a series of phenolic compounds to cycloalkanes.
Four groups of catalysts have been tested for hydrodeoxygenation (HDO) of phenol as a model compound of bio-oil, including oxide catalysts, methanol synthesis catalysts, reduced noble metal catalysts, and reduced non-noble metal catalysts. In total, 23 different catalysts were tested at 100 bar H2 and 275 °C in a batch reactor. The experiments showed that none of the tested oxides or methanol synthesis catalysts had any significant activity for phenol HDO under the given conditions, which were linked to their inability to hydrogenate the aromatic ring of phenol. HDO of phenol over reduced metal catalysts could effectively be described by a kinetic model involving a two-step reaction in which phenol initially was hydrogenated to cyclohexanol and then subsequently deoxygenated to cyclohexane. Among reduced noble metal catalysts, ruthenium, palladium, and platinum were all found to be active, with activity decreasing in that order. Nickel was the only active non-noble metal catalyst. For nickel, the effect of support was also investigated and ZrO2 was found to perform best. Pt/C, Ni/CeO2, and Ni/CeO2-ZrO2 were the most active catalysts for the initial hydrogenation of phenol to cyclohexanol but were not very active for the subsequent deoxygenation step. Overall, the order of activity of the best performing HDO catalysts was as follows: Ni/ZrO2 > Ni-V2O5/ZrO2 > Ni-V2O5/SiO2 > Ru/C > Ni/Al2O3 > Ni/SiO2 Pd/C > Pt/C. The choice of support influenced the activity significantly. Nickel was found to be practically inactive for HDO of phenol on a carbon support but more active than the carbon-supported noble metal catalysts when supported on ZrO2. This observation indicates that the nickel-based catalysts require a metal oxide as a carrier on which the activation of the phenol for the hydrogenation can take place through heterolytic dissociation of the O–H bond to facilitate the reaction.
A systematic study of the comparative performances of various supported noble metal catalysts for the aqueous phase hydrogenation of acetic acid (as a model carboxylic acid in bio-oils) by itself and in combination with p-cresol (as a model phenolic compound in bio-oils) is presented. It was found that Ru/C catalyst shows the highest activity for acetic acid hydrogenation among the tested catalysts, followed by Ru/Al2O3, Pt/C, Pt/Al2O3, Pd/Al2O3, and Pd/C. CH4 and CO2 were observed to be the major products on all of these catalysts at typical hydroprocessing temperatures (300 °C). A systematic study on parametric effects with the Ru/C catalyst shows that the product distribution is dependent upon the temperature and presence of water. At low temperatures (150 °C), acetic acid hydrogenation is favored with higher selectivity to ethanol, while at high temperatures (300 °C), acetic acid decomposition and ethanol reforming/hydrogenolysis dominate with CO2 and CH4 as the major products. When water is replaced with n-heptane at otherwise similar conditions, the esterification reaction is favored over ethanol reforming/hydrogenolysis, resulting in substantial formation of ethyl acetate. With a mixed feed of acetic acid and p-cresol over the Ru/C catalyst, acetic acid hydrogenation is suppressed and p-cresol hydrodeoxygenation is favored, as inferred from the observed high selectivity to methylcyclohexane.
The influence of reaction conditions (temperature, acidity) on the catalytic performance of supported Pt, Pd and Ru catalysts for the aqueous phase hydrodeoxygenation (HDO) of lignin model compounds was systematically investigated. Phenol conversion proceeds via hydrogenation of the aromatic ring resulting in cyclohexanone, which is subsequently converted to cyclohexanol and cyclohexane. Although aromatic ring hydrogenation has a higher rate for Pt and Pd-based catalysts, the rate of hydrogenation of the polar C=O moiety in cyclohexanone is faster for Ru/C. The complete HDO of phenol to cyclohexane on noble-metal catalysts can only be achieved in the presence of a Brønsted acid co-catalyst. In guaiacol conversion, efficient demethoxylation and ring hydrogenation can be achieved within 0.5 h on Pt/C. Under acidic conditions, selectivity of nearly 90% to cyclohexane at a conversion of 75% was achieved in 4 h. To get an insight into the possibility to cleave covalent linkages between aromatic units in lignin under HDO conditions, the reactivity of dimeric model substances such as diphenyl ether, benzyl phenyl ether, diphenyl methane and biphenyl was investigated. Although dimeric oxygen-bridged model compounds such as benzylphenyl ether and diphenyl ether can be readily converted to monomeric species in the presence of noble metal catalysts, cleavage of C–C bonds in diphenyl methane and biphenyl was not observed. Plausible reaction mechanisms are proposed.
Introduction The decrease in fossil fuel reserves along with the problems caused by our dependence on fossil fuels has accelerated research on biofuels. Aqueous-phase hydrogenation (APH) reactions are a crucial component of a number of strategies for the conversion of biomass into fuels and chemicals. This includes the hydrogenation of targeted functionalities of biomass including acids, sugars, aldehydes, furans and alkenes. Aqueous phase hydrogenation reactions are used for ethanol production from organic acids [1], gasoline production from bio-oils [2], and alkane production from carbohydrates [3]. The objective of this study is to gain a better understanding of the fundamentals of aqueous-phase hydrogenation by combining experimental results with insight learned from theoretical calculations. Acetic acid was chosen as a model compound because the acid functionality is one of the more difficult ones to hydrogenate. Materials and Methods Experimental studies were done in a continuous-flow packed-bed reactor. Several well-characterized monometallic catalysts (Ni, Cu, Ru, Rh, Pd, Ir and Pt) were tested for their activity and selectivity in APH of acetic acid into ethanol at 750 psi and over a temperature range of 100-260°C. Theoretical studies were carried out through spin-polarized periodic density functional theory (DFT) calculations performed in the generalized gradient approximation using the Vienna Ab initio Simulation Package [4-6]. Results and Discussion Our experiments showed that ruthenium is the most active and selective catalyst for the aqueous-phase hydrogenation of acetic acid to ethanol. The catalytic activity decreased as Ru > Pt ~ Rh > Ir > Pd > Ni > Cu.
This paper focuses on the fundamental chemical aspects of hydrogen transfer reactions with RANEY® Ni and propan-2-ol. It aims at novel process options for defunctionalization and hydrodeoxygenation of phenolic and aromatic biorefinery feeds under low-severity conditions. A series of 32 model substrates were explored, providing a comprehensive description of the reactivity of RANEY® Ni toward transfer hydrogenation and transfer hydrogenolysis. In addition, the aspects related to the catalyst stability were addressed in detail. With regard to the processing of a model-substrate mixture, important features of the chemoselectivity of RANEY® Ni were also revealed. Herein, we also demonstrate that hydrogen transfer reactions could hold the key to the upgrade of bio-oil under unusual, low-severity conditions. Indeed, bio-oil was easily upgraded to cyclohexanols and less functionalized alkylphenols, with RANEY® Ni and propan-2-ol, at 120 °C. Full saturation of bio-oil to cyclic alcohols, cyclohexane-1,2-diols and other products with reduced oxygen content was achieved at 160 °C under autogenous pressure.
First-order rate constants for the acid hydrolysis of vinyl and alk-1-enyl ethers have been determined. Hydrolysis of n-butyl vinyl ether is slower in deuterium oxide than in water by a factor of 2-63. A mechanism of hydrolysis consistent with the results obtained is proposed.
Selected aromatic compounds were hydrogenated over a ruthenium catalyst to evaluate the effect of a substituting group on the reactivity of an aromatic ring or a carbonyl function. In case of the aromatic aldehydes preferential hydrogenation of a carbonyl group was observed. When carbonyl group was not present, reactivity of the aromatic ring depended on the type of substituent.
Les seuls composes solides consideres pour la realisation du diagramme sont RuO 2 et RuO 4 . Les autres especes sont dissoutes en solution: H 2 RuO 5 , HRuO 5 − , RuO 4 − et RuO 4 2− . On donne les parametres de formation des composes de Ru
The importance of hydrodeoxygenation (HDO) which occurs during hydroprocessing depends on the origin of feeds. HDO plays a minor role in the case of the conventional feeds, whereas for the feeds derived from coal, oil shale, and, particularly from the biomass, its role can be rather crucial. The mechanism of HDO was established using a wide range of model compounds. Complexities in the HDO kinetics have been attributed to the self-inhibiting effects of the O-containing compounds as well as inhibiting and poisoning effects of the S- and N-containing compounds present in the feeds. This is a cause for some uncertainties in establishing the order of the relative HDO reactivities of the O-containing compounds and/or groups of the compounds as well as relative rates of the removal of S, O and N. Complexities arise particularly for real feeds. This is supported by deviations from the established order such as HDS>HDO>HDN. The cases for which the overall HDN was greater than HDO were also observed. In this case, distribution of the O- and N-containing compounds in the feed and the type of catalyst are of a primary importance.HDO is the main reaction which occurs during hydroprocessing of the bio-feeds. The current research activities in HDO are predominantly in this area. Apparently, more stable catalysts are needed to make production of the commercial fuels from the bio-feeds more attractive.
Ru/C catalysts were prepared from commercial activated carbon submitted to different treatments. The catalysts were prepared by incipient wetness impregnation, through an aqueous solution of the precursor RuCl3·xH2O. After impregnation, some catalysts were submitted to direct reduction treatment under H2 flow at the temperature of 150°C, in order to evaluate the effects of activation. The supports were characterized by N2 adsorption, Boehm and potentiometric titration. The X-ray photoelectron spectroscopy was used to study the supports and catalysts surfaces, while scanning electron microscopy allowed us to determine the chemical composition and observe the catalysts morphology. Ru/C catalysts performance was evaluated within the hydrogenation reaction of benzene in liquid phase, using a Parr reactor. The reaction was conducted under total pressure of 5.0MPa of H2, at a temperature of 100°C with water in the reaction medium. The obtained results indicate that the Ru/C system catalytic performance is influenced for determined functional groups present on the activated carbon surface. The carbonyl groups decrease the activity and selectivity of the reactions, while an increase of the carboxylic groups leads to more active catalysts and the highest yield of cyclohexene.
The thermal decomposition of anisole as a prototype of the aryl-methyl-ether linkage of lignin and coals has been studied under supercritical conditions using tetralin as hydrogen donor solvent. The effect of homogenous Lewis acid catalysts have also been studied under the same conditions. The main reaction products are phenol, benzene, toluene and cresols. At high tetralin to anisole ratios the selectivity to phenol is almost 80% with little or no cresol production. This selective conversion can be carried out rapidly and cleanly at high temperature (> 450 °C). Kinetic studies were undertaken using pyrolytic, donor solvent hydrogenolytic and Lewis acid catalysed regimes in the temperature range 400–500 °C. The kinetics of anisole decomposition in a large excess of tetralin have been found to be in good agreement with those published in the literature. The Lewis acid catalysts lower the activation energy relative to the pyrolytic and hydrogenolytic cases. The kinetic studies and their mechanistic interpretation lead to a mechanism involving surprisingly few radical species: methyl, phenoxy, phenoxymethyl and phenyl radicals. In the presence of FeCl3, the selectivity towards phenols and cresols is enhanced, though a side reaction leads to polymerization at low (400–420 °C) temperatures. It is concluded that the aryl-O-methyl ether linkage in anisole can easily be broken at high temperatures, 450–500 °C, in supercritical hydrogen donor solvent to give phenol in high yield and selectivity.
The solvent isotope effect (kD2O/kH2O) of 1.3 observed in the acid-catalyzed isomerization of 3-methyl-3-cyclohexen-1-one to 3-methyl-2-cyclohexen-1-one in aqueous sulfuric acid demonstrates that this reaction occurs through a rate-determining enolization. However, the isomerization of 3-cyclohexen-1-one to 2-cyclohexen-1-one has a solvent isotope effect of 0.2, and the rate of hydrogen exchange (enolization) at C-2 is much faster than the rate of isomerization. Thus, it is concluded that this latter reaction occurs through a rate-determining protonation of the enol. The nature of the rate-limiting step in acid-catalyzed double bond migrations in unsaturated ketones is determined by the alkyl substitution at the carbon β to the carbonyl.
The relative thermodynamic stabilities of 1,3-cyclohexadiene and 1,4-cyclohexadiene, together with a number of their alkoxy and other derivatives and some related seven-membered cyclic dienes, have been determined by chemical equilibration in Me2SO and cyclohexane solution at various temperatures. The values of the thermodynamic parameters ΔG⊖, ΔH⊖, and ΔS⊖ for the isomerization processes involved are discussed and show clearly that the apparently conjugated diene system of 1,3-cyclohexadiene is devoid of conjugation, contrary to that of 1,3-cycloheptadiene.
Typescript. Thesis (Ph. D.)--University of Wisconsin--Madison, 1942. Vita. Includes bibliographical references (leaves 66-67).
Bio-oil (product liquids from fast pyrolysis of biomass) is a complex mixture of oxygenates derived from the thermal breakdown of the biopolymers in biomass. In the case of lignocellulosic biomass, the structures of three major components, cellulose, hemicellulose and lignin, are well-represented by the bio-oil components. To study the chemical mechanisms of catalytic hydroprocessing of bio-oil, three model compounds were chosen to represent those components. Guaiacol represents the large number of mono- and dimethoxy phenols found in bio-oil derived from soft- or hardwood, respectively. Furfural represents a major pyrolysis product group from cellulosics. Acetic acid is a major product from biomass pyrolysis, derived from the hemicellulose, which has important impacts on the further processing of the bio-oil because of its acidic character. These three compounds were processed using a palladium or ruthenium catalyst over a temperature range from 150 to 300 °C. The batch reactor was sampled during each test over a period of 4 h. The samples were analyzed by gas chromatography with both a mass selective detector and a flame ionization detector. The products were determined, and the reaction pathways for their formation are suggested on the basis of these results. Both temperature and catalyst metal have significant effects on the product composition.