Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Bronsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.

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Potential and challenges of zeolite chemistry in the catalytic conversion of biomass

Increasing demand for renewable feedstock-based chemicals is driving the interest of both academic and industrial research to substitute petrochemicals with renewable chemicals from biomass-derived resources. The search towards novel platform chemicals is challenging and rewarding, but the main research activities are concentrated on finding efficient pathways to produce familiar drop-in chemicals and polymer building blocks. A diversity of industrially important monomers like alkenes, conjugated dienes, unsaturated carboxylic acids and aromatic compounds are thus targeted from renewable feedstock. In this context, on-purpose production of 1,3-butadiene from biomass-derived feedstock is an interesting example as its production is under pressure by uncertainty of the conventional fossil feedstock. Ethanol, obtained via fermentation or (biomass-generated) syngas, can be converted to butadiene, although there is no large commercial activity today. Though practised on a large scale in the beginning of the 20th century, there is a growing worldwide renewed interest in the butadiene-from-ethanol route. An alternative route to produce butadiene from biomass is through direct carbohydrate and gas fermentation or indirectly via the dehydration of butanediols. This review starts with a brief discussion on the different feedstock possibilities to produce butadiene, followed by a comprehensive summary of the current state of knowledge regarding advances and achievements in the field of the chemocatalytic conversion of ethanol and butanediols to butadiene, including thermodynamics and kinetic aspects of the reactions with discussions on the reaction pathways and the type of catalysts developed.

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Review of old chemistry and new catalytic advances in the on-purpose synthesis of butadiene

A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass

In contrast to the nonrenewable fossil resources, biomass, the only renewable resource of organic carbon in the nature, is considered as a special kind of inexhaustible feedstocks, which can be used for the synthesis of numerous valuable products in a sustainable manner. Among many biomass-derived products, 5-hydroxymethylfurfural (HMF) is identified to be a crucially important versatile compound due to its marvelous structure that is composed of an aldehyde group, a hydroxyl group and a furan ring. Hence, HMF possesses a very strong chemical reactivity, and it can be further transformed into a wide variety of value-added derivatives. In recent years, the synthetic methods, physicochemical properties and commercial prospects of HMF-based conventional derivatives such as 2,5-dimethylfuran (DMF), 5-ethoxymethylfurfural (EMF), ethyl levulinate (EL), long chain alkane (LLA), levulinic acid (LA), 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA) have been intensively reviewed by many researchers. However, up to now, the preparation of HMF-based innovative derivatives such as 2,5-dihydroxymethylfuran (DHMF), 2,5-dihydroxymethyltetrahydrofuran (DHMTHF), 1,2,6-hexanetriol (HTO), 1,6-hexanediol (HDO), 1-hydroxyhexane-2,5-dione (HHD), 3-hydroxymethylcyclopetanone (HMCPN), furan-2,5-dimethylcarboxylate (FDMC), maleic anhydride (MA), 5-hydroxy-5-(hydroxymethyl)furan-2(5H)-one (HHMFO), 5-alkoxymethylfurfural (AMF), 5,5-oxy-(bismethylene)-2-furaldehyde (OBMF), 5-arylaminomethyl-2-furanmethanol (AAMFM), 2,5-furandiamidine dihydrochloride (FDADHC), 1-alkyl-5-hydroxy-2-(hydroxymethyl)pyridinium (AHHMP), 5,5-bis(hydroxymethyl)furoin (BHMF), 5-(dialkyloxymethyl)-2-furanmethanol (DAMFM), 5-chloromethylfurfural (CMF), 5-alkanoyloxymethylfurfural (AOOMF) and furfuryl alcohol (FFA) has not yet been comprehensively summarized. In order to fill this gap, the latest studies and advancements on the preparation of HMF-based innovative derivatives via various catalytic approaches such as hydrogenation, oxidation, etherification, amination, condensation, halogenation, esterification and decarbonylation are systematically outlined and discussed in this review. Furthermore, a few potential research trends in the future studies are also proposed to provide some useful ideas for the further preparation of HMF-based innovative derivatives in a much more green, simple, efficient and economical way.

Recent advances in catalytic transformation of biomass-derived 5-hydroxymethylfurfural into the innovative fuels and chemicals

Lignocellulosic biomass is the most abundant organic carbon source and has received a great deal of interest as renewable and sustainable feedstock for the production of potential biofuels and value-added chemicals with a wide range of designed catalytic systems. However, those natural polymeric materials are composed of short-chain monomers (typically C6 and C5 sugars) and complex lignin molecules containing plenty of oxygen, resulting in products during the downstream processing having low-grade fuel properties or limited applications in organic syntheses. Accordingly, approaches to increase the carbon-chain length or carbon atom number have been developed as crucial catalytic routes for upgrading biomass into energy-intensive fuels and chemicals. The primary focus of this review is to systematically describe the recent examples on the selective synthesis of long-chain oxygenates via different C-C coupling catalytic processes, such as Aldol condensation, hydroalkylation/alkylation, oligomerization, keto...

Carbon-Increasing Catalytic Strategies for Upgrading Biomass into Energy-Intensive Fuels and Chemicals

Mechanism of MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using Sacrificial Alcohols

The synthesis of furoins from biomass-derived furfural and 2-methylfurfural is demonstrated in high yields in green and renewable solvents using N-heterocyclic carbene organocatalysts. The resulting furoin molecules are used as precursors for fuels using cascade catalysis, first by using Pd/C with acidic co-catalysts under very mild conditions to yield oxygenated C12 molecules. Two main products were formed, which we identified as 1,2-bis(5-methyltetrahydrofuran-2-yl)ethane and 1-(5-methyltetrahydrofuran-2-yl)heptanol. The use of a Pd/Zeolite-β catalyst under more extreme conditions resulted in the complete hydrodeoxygenation of 5,5'-dimethylfuroin to dodecanes in high yields (76%) and exceptional selectivity (94%) for n-dodecane.

From Furfural to Fuel: Synthesis of Furoins by Organocatalysis and their Hydrodeoxygenation by Cascade Catalysis

The reactivity of carbon-centered distonic radical ions has been of interest for decades. The existence of distonic radical ions was first postulated by Gross and McLafferty in the early 1970’s.1,2 In 1978, Bouma, MacLeod and Radom reported experimental results that supported theoretical predictions of the existence of a stable ring-opened ethylene oxide distonic radical cation.3 Ions with spatially separated charge and radical sites were coined as “distonic ions” by Yates, Bouma, and Radom4 in 1984. They later refined this definition5 to correspond to radical ions generated by ionization of a zwitterion, ylide, or diradical. Eberlin and co-workers later introduced the term “distonoid”, meaning distonic like, to encompass any radical ion that displays distonic “character” (i.e., ions with a high degree of discrete (non-mandatory) charge-spin separation) and is over-looked as a result of the strict distonic ion definition.6 However, the “distonoid” classification is not commonly used by the scientific community. Currently, the term “distonic” is widely accepted and used to denote ions with formally separated charge and radical sites even if they do not fall into the formal definition.7 According to the conventional valence bond description, the charge and radical sites are on adjacent atoms in α-distonic ions while they are separated by one and two atoms in β- and γ-distonic ions, respectively. A vast amount of experimental and theoretical studies were dedicated to distonic ions from the 1980’s to 1990’s, which have been previously reviewed separately by Hammerum and Kenttamaa.8,9,10

The focus of this review is on gaseous ions with one or more aryl radical sites, a subgroup of distonic radical cations. The interest in these distonic ions was initially sparked by the limited knowledge on the reactivity of neutral phenyl radicals and their diradical counterparts in spite of the vast amount of research dedicated to these reactive intermediates.11–70 Many such mono- and diradicals have been investigated as they are thought to play a vital role in numerous fields, including combustion,11–13 polymerization,14–16 atmospheric chemistry,17–19 interstellar chemistry,20 organic synthesis8 and the biological activity of certain drugs.21–33 In the 1990’s, the formation of such aromatic diradicals in naturally occurring anti-tumor antibiotics was associated with their DNA-cleaving ability.21–33 The two radical sites are thought to abstract a hydrogen atom from each strand of double stranded DNA, thus causing irreversible DNA cleavage. Since then, theoretical and experimental research on aryl mono- and diradicals has boomed. An area of special interest has been the mechanistic understanding of hydrogen atom abstraction by these radicals from small organic and biological molecules both in solution34–45 and in the gas phase.46–70 However, the ability to predict the rates of such seemingly simple reactions has proven challenging due to a poor understanding of the nature of the transition states for these reactions. Further, the examination of the chemical properties of neutral radicals is a challenge due to the difficulty to cleanly generate them both in solution and in the gas phase.

In order to address the above difficulties, studies were carried out in the early 1990’s on distonic radical cations’ ion-molecule reactions inside mass spectrometers as ions can be easily manipulated in this environment.46–70 Distonic radical cations that have a phenyl radical site spatially separated from a chemically inert charge site were found to almost exclusively undergo radical reactions at the radical site(s) in the gas phase46–70 and lately also in solution.45 Hence, examination of these distonic ions will provide information on the properties of phenyl mono- and diradicals.

A special benefit of using mass spectrometry to study above species is that the desired charged radical can be isolated before examining its reactivity. Hence, the precursors to any products formed in these gas-phase experiments are known, which is not always true for solution experiments wherein highly reactive molecules cannot be isolated. The chemical properties of many aryl mono-, di- and triradicals have been successfully examined in mass spectrometers by using this ‘distonic ion approach’.49,50,52 The results obtained in these studies provide valuable information on the relative reactivities of mono- and polyradicals, which would otherwise not be available. This paper reviews the current knowledge of the properties and reactivity of distonic radical ions with aryl radical sites and the mechanisms of these reactions. Distonic phenyl radical ions generated within peptides are not included due to space limitations and also since these radicals are usually generated as precursors to less reactive nonaromatic peptide radicals that are the true interest of the researchers. However, this is an important and exciting new field of distonic ion research that should be reviewed separately.

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

Mass spectrometric methodology was developed for the determination and manipulation of the primary products of fast pyrolysis of carbohydrates. To determine the true primary pyrolysis products, a very fast heating pyroprobe was coupled to a linear quadrupole ion trap mass spectrometer through a custom-built adaptor. A home-built flow tube that simulates pyrolysis reactor conditions was used to examine the secondary reactions of the primary products. Depending on the experiment, the pyrolysis products were either evaporated and quenched or allowed to react for a period of time. The quenched products were ionized in an atmospheric pressure chemical ionization (APCI) source infused with one of two ionization reagents, chloroform or ammonium hydroxide, to aid in ionization. During APCI in negative ion mode, chloroform produces chloride anions that are known to readily add to carbohydrates with little bias and little to no fragmentation. On the other hand, in positive ion mode APCI, ammonium hydroxide forms ammonium adducts with carbohydrates with little to no fragmentation. The latter method ionizes compounds that are not readily ionized upon negative ion mode APCI, such as furan derivatives. Six model compounds were studied to verify the ability of the ionization methods to ionize known pyrolysis products: glycolaldehyde, hydroxyacetone, furfural, 5-hydroxymethylfurfural, levoglucosan, and cellobiosan. The method was then used to examine fast pyrolysis of cellobiose. The primary fast pyrolysis products were determined to consist of only a handful of compounds that quickly polymerize to form anhydro-oligosaccharides when allowed to react at high temperatures for an extended period of time.

On-line mass spectrometric methods for the determination of the primary products of fast pyrolysis of carbohydrates and for their gas-phase manipulation.

Cellulose, a major component of renewable biomass, is a polymer of glucose. Abundant and cheap iron salts promote the conversion of glucose to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA). In this study, glucose transformations catalyzed by iron(III) chloride (FeCl3) in aqueous and in biphasic media (water and 2-methyltetrahydrofuran (MeTHF)) were investigated. Speciation via mass spectrometry (MS), UV-Vis, and X-ray absorption spectroscopy (XAS) show that FeIII is reduced to FeII (over 95%) readily in the early stage of carbohydrate conversion. The reaction time profiles of reactants (glucose and fructose) as well as products (HMF and LA) were modeled using MATLAB to obtain reaction rate constants. The contributions of iron and the intrinsic Bronsted acidity of iron salts in the sugar conversion are discussed. The kinetic study of sugar conversion indicated that the water–MeTHF biphasic system hinders the conversion of sugars to humins and unknown byproducts and increases the yields of HMF and LA. By adjusting concentrations of FeII and Bronsted acidity, yields of 88% LA (FeCl3, pH = 1) or 56% HMF (FeSO4, pH = 2) from glucose in a water–MeTHF biphasic system are achieved. The optimized reaction conditions proved effective in the conversion of milled poplar biomass to LA (53% yield based on glucose content) and furfural (64% yield based on xylan content) using iron salt, outperforming aluminum and chromium salts.

Linan Yang

Papers

Mechanism of MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using Sacrificial Alcohols

From Furfural to Fuel: Synthesis of Furoins by Organocatalysis and their Hydrodeoxygenation by Cascade Catalysis

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

On-line mass spectrometric methods for the determination of the primary products of fast pyrolysis of carbohydrates and for their gas-phase manipulation.

Speciation and kinetic study of iron promoted sugar conversion to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA)