Catalytic isomerization of d-glucose to d-fructose over BEA base zeotypes using different energy supply methods

Highlights

• Catalytic isomerization of d-glucose in water, using different energy supply methods.

• Base catalysts prepared via zeolite Beta desilication and solid-state impregnation.

• Types of alkali (Na,K) and alkaline earth (Ca,Mg) metals influence the performances.

• Fructose selectivity of 100 % up to 31 % conversion, over base catalysts (H₂O, 75 °C).

• Type of energy supply method may influence initial activity and catalyst stability.

Abstract

The isomerization of d-glucose (Glu) to d-fructose (Fru) is a key step in the selective conversion of carbohydrate biomass to renewable products and has long been of great importance in the food and beverage industry. Expansion of the Fru market (food, non-food) poses various challenges to enhance productivity, where energy efficiency and use of cheap, robust synthetic catalysts may play important roles. Here, Glu isomerization, using water as the sole solvent, was studied using (i) micro/mesoporous catalysts of different compositions prepared from commercial zeolite Beta, and (ii) different energy supply methods, namely conventional conductive heating, microwave irradiation and ultrasound waves, at 75 °C. The type of energy supply method influenced the initial turnover frequency. Besides catalytic activity, the stability is important, which depended on the catalytic material properties (e.g., composition, textural properties), type of energy supply method and reaction temperature. Favourable results were verified for Glu isomerization using a hierarchical potassium/magnesium-containing Beta catalyst and ultrasound waves, at 75 °C.

Introduction

Lignocellulosic biomass is one of the most attractive, available renewable organic carbon sources, obtainable from agricultural or forestry residues/surpluses, for sustainably complementing or substituting some petrochemicals, with reduced impact on the food chain [[1], [2], [3], [4]]. This may contribute to solve the problems resulting from the growing global population and energy demands, with the concomitant environmental/atmospheric pollution compounded by the strong dependence of modern civilization on fossil fuels [5,6]. Lignocellulosic biomass may be converted to a plethora of bio-products via chemical and/or biological transformations [7,8]. A major component of lignocellulosic biomass is cellulose, a natural organic linear polymer. One of the important selective conversion routes of cellulose to value-added chemicals involves the depolymerization of this polysaccharide to the monosaccharide d-glucose (Glu), followed by the isomerization of (aldose) Glu to its isomer (ketose) d-fructose (Fru) (Scheme 1). This isomerization step is important in food and biomass chemistry. On the one hand, Fru is a low-cost, low-calorie sweetener globally used in food and beverages, and for this purpose it is industrially produced using enzymes [9,10]. On the other hand, Glu-to-Fru is a key step leading to potential giant platform chemicals such as levulinic acid and 5-(hydroxymethyl)furfural [[11], [12], [13], [14], [15], [16]], which are in the Top 12 of promising building block intermediates for producing drugs, polymers, resins, solvents, agrochemicals, biofuels, etc [14,17].

Efforts have been made to develop improved (bio)catalysts for this key step. Glu isomerization to Fru may occur via different mechanisms, such as proton transfer involving deprotonation (with a base) and formation of enolate intermediates [18,19], intramolecular hydride shift (with a Lewis acid) [[20], [21], [22]], or enzymatic routes using the commonly known glucose isomerase that requires an activator such as an alkaline-earth metal, e.g., divalent magnesium, and operates via a hydride shift mechanism [21,23]. Several types of catalysts have been reported for Glu isomerization. Homogeneous catalysts, which may be base (e.g., NaOH) [13,22,24] or Lewis acidic (e.g., soluble metal chlorides) [13,22,24], present drawbacks such as demanding catalyst separation from the products and the need for downstream waste treatment/neutralization stages, compromising the energy efficiency of the overall process. The alternative use of heterogeneous catalysts may allow easier catalyst separation/reuse, enhanced catalyst productivity and product selectivity [25,26]. Solid catalysts reported for Glu isomerization include organic ion-exchange resins [22,24], organic-inorganic hybrids [22], metal(IV) phosphates [22,24], hydrotalcites [13,22,24], transition metal oxides [13,22], and metallosilicates including zeolites [13,22,24,27], with one of the most effective catalysts being tin-containing zeolite Beta (which consists of a stacking-fault intergrowth structure of distinct polymorphs and a 3-dimensional channel system with 12-membered ring pore openings [20]).

In the choice of the catalysts, thermal stability is important to allow catalyst regeneration/reuse (e.g., for removal of adsorbed carbon-containing compounds), and thus catalytic materials with organic components may present limitations. Since water is the most attractive solvent to Glu conversion, hydrothermal stability is desirable. Moreover, versatile families of materials are preferable to allow tuning of the surface chemistry (e.g., amount and type of active sites) and textural properties (pore sizes that minimize internal diffusion limitations) to meet superior catalytic performance. In this sense, hierarchical (micro and mesoporous) zeotypes are promising for Glu isomerization, being versatile and enabling facile internal diffusion. However, very few hierarchical catalysts have been explored for Glu isomerization, namely: Sn-containing zeotypes using water [[28], [29], [30]], an alcohol or water + alcohol as solvents [[31], [32], [33]]; and very mildly desilicated zeolite Y possessing magnesium, using water as solvent [34].

Besides improved catalysts, the use of effective energy supply methods is important. The reaction of Glu over heterogeneous chemo catalysts was mostly studied using conventional conductive (CC) heating (with an external heat source), which depends strongly on the thermal conductivity of the reactor walls and medium. Alternatively, microwave (MW) irradiation produces fast internal heating that may favour the reaction kinetics [35,36]. Yu and coworkers [35] reported Glu isomerization over a silica-supported enzyme under MW irradiation, which was more efficient than CC heating. To the best of our knowledge, there are only two studies of Glu isomerization over heterogeneous chemo catalysts, under MW irradiation, namely that of Watanabe and coworkers [37] using metal (Ti, Zr, Ca) oxide catalysts, and a more recent study by Tsang and coworkers [38] which used graphite oxide- or graphene oxide-supported AlCl₃. Alternatively, ultrasound (US) waves is an interesting energy supply method [36,39]. In contact with a liquid, US waves create positive and negative pressure oscillations, and cavitation bubbles (i.e. vapour filled cavities formed inside a liquid medium) may form when the pressure amplitude exceeds the tensile strength of the liquid during the propagation of US waves. Collapse of these bubbles releases thermal energy [40]. These effects may depend on several factors such as ultrasonic power and frequency, and properties of the liquid medium (e.g., viscosity). US waves may intensify mixing and mass transfer, e.g. by microstreaming, shockwaves etc. [36,41,42], and may increase the activity of metal-catalysts by several orders of magnitude [39]. To the best of our knowledge, no studies have yet been reported for US-assisted Glu isomerization over non-biocatalysts, targeting Fru. An enzymatic process of Glu-to-Fru conversion using US waves and ionic liquids was reported to favour the reaction kinetics compared with the conventional stirred batch reactors [43]. Da Costa and coworkers [44] studied other types of organic reactions catalysed by mixed metal oxides using US waves, which enhanced the reaction kinetics as compared to the reactions without US, possibly due to US favourably influencing the diffusion of the reactants in the vicinity of the active sites. For biomass valorization using base catalysts, Adewuyi and co-workers [41] designed a multifrequency ultrasonic reactor that reduced energy losses (e.g., by scattering, reflection).

In this work, the catalytic isomerization of d-glucose to d-fructose was studied at 75 or 100 °C, in the presence of solid base catalysts, specifically desilicated Beta type materials possessing alkali and alkaline-earth metals (Me = Ca, Mg) prepared from commercial microcrystalline zeolite Beta via sequential alkaline treatment, solid-state impregnation of Me and calcination. The modified materials possessing Me performed superiorly to the respective precursor materials, and the catalytic results were further improved using appropriate energy supply methods.