Isomerization of glucose into fructose over natural and synthetic MgO

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Isomerization of glucose into fructose over natural and synthetic MgO catalysts Asimina Marianou, Chrysoula Michailof, Stamatia Karakoulia, Dimitrios Ipsakis, Konstantinos Kalogiannis, Haris Yiannoulakis, Konstantinos S. Triantafyllidis, and Angelos A. Lappas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03570 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Sustainable Chemistry & Engineering

Isomerization of glucose into fructose over natural and synthetic MgO catalysts

Asimina A. Marianou†,‡, Chrysoyla M. Michailof†,*, Dimitrios K. Ipsakis†, Stamatia A. Karakoulia†, Konstantinos G. Kalogiannis†, Haris Yiannoulakisǂ, Konstantinos S. Triantafyllidis‡,*, Angelos A. Lappas†

†Chemical Process & Energy Resources Institute, 6th km Harilaou-Thermi Road, 57001, Thessaloniki, Greece ‡Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece ǂ Grecian Magnesite S.A., Research Center, Vasilika, 570 06, Thessaloniki, Greece

Corresponding authors emails: [email protected], [email protected]

ABSTRACT: Fructose is one of the most important aldoses and has been gaining attention as the starting material for the synthesis of bio-based platform and high-added value chemicals such as 5-hydroxymethylfurfural (5-HMF), levulinic acid and lactic acid. However, due to its low natural occurrence, fructose is produced from glucose, an abundant hexose, via isomerization. Currently, the conventional industrial process utilizes glucose isomerase as a catalyst and is therefore subjected to the limitations of enzymatic reactions. Consequently, an alternative efficient solid catalyst is required that will exhibit high activity, selectivity and stability/reusability. Towards this end, we have demonstrated the effectiveness of using natural MgO, derived from simple calcination of magnesite ores, as a low cost catalyst with increased basicity. A series of industrial and laboratory prepared natural MgO materials with different morphology, porosity and basicity were investigated and the optimum catalyst afforded 44.1 wt. % glucose conversion and 75.8 wt. % fructose selectivity (33.4 wt.% fructose yield), at 90 °C for a 45 min reaction in aqueous solution. The activity of the MgO catalysts was directly 1 ACS Paragon Plus Environment

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correlated with their basicity, which in turn depended on their crystal size, surface area and composition. CaO impurities of the natural MgO materials generated strong basic sites that enhanced glucose conversion but at the expense of fructose selectivity. The stability and reuse of the optimum catalyst was confirmed for at least 4 cycles of reaction-regeneration, whereas the mechanism of glucose isomerization was validated via a 1st order kinetic modeling set.

Keywords: glucose, fructose, isomerization, MgO, basicity, kinetic modeling study

INTRODUCTION Lignocellulosic biomass has been identified as one of the most promising renewable sources for the production of chemicals, fuels and energy. Particular emphasis is being placed on the valorization of hemicellulose and cellulose, as they constitute the highest percentage of biomass. Both are sugar based biopolymers, composed of aldoses and hexoses which are highly functionalized and versatile molecules that may be converted to a variety of valuable intermediates or products such as 5-hydroxymethylfufrural (5-HMF), levulinic acid, lactic acid, etc. 1, 2, 3. Glucose is the most abundant of the hexoses occurring both in cellulose and hemicellulose, therefore increasing effort is being made towards the development of scalable processes for its valorization, in addition to the already mature process of bioethanol production from cellulose-derived glucose 4. Thus, the valorization of glucose existing in side-streams such as solubilized/hydrolyzed hemicellulose or even in biomass/cellulose-derived hydrolysis streams, which are not suitable for subsequent fermentation due to high levels of inhibitors being toxic to bacteria and yeasts (i.e. furfural, HMF, organic acids, etc.) 5, 6, is becoming increasingly important towards the “whole biomass” valorization concept. In integrated biorefinery schemes, glucose interconversion to fructose has been identified as a key intermediate step for the production of bio-fuels and bio-chemicals. Currently, isomerization of glucose into fructose is one of the most important reactions both in the field of food industry and pharmaceuticals 7,8. In particular, glucose isomerase is applied for the production of high-fructose corn syrup (HFCS), the largest immobilized biocatalytic 2 ACS Paragon Plus Environment

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process worldwide 9. Although the enzymatic processes are highly selective, they are accompanied by several drawbacks such as low tolerance in variations of feedstock, the use of buffer solutions, slow reaction times and narrow operating temperatures

10.

In addition,

isomerization is a reversible reaction and therefore the maximum conversion of glucose into fructose is limited by the thermodynamic equilibrium between the two sugars at each specific reaction temperature 11, 12. Chemically the reaction is catalyzed effectively by basic catalysts or Lewis acids. Under basic conditions, it follows the Lobry de Bruyn-Alberda van Ekenstein (LdB-AvE) mechanism

13

where the isomerization of the aldose results in the simultaneous

formation of a ketose, as a prevailing product, and an epimeric aldose 14. Homogeneous basic catalysts such as alkali and alkaline earth hydroxides have been widely studied for the isomerization reaction

13

and despite their effectiveness, they result in

the formation of numerous by-products due to the aldolization/retroaldolization, b-elimination, and benzilic rearrangement reactions that take place simultaneously to isomerization 15, 16, 17. Additionally, the highly basic conditions favor further dehydration and condensation reactions of the by-products 18. Recently, organic amines have been reported as effective isomerization catalysts resulting in high fructose yields and selectivities, superior to those obtained with inorganic bases 19. Soluble organometallic complexes 20, as well as, Lewis acid salts 21, 22 have also been studied, while enhanced fructose yield in the presence of NaAlO2 has been reported 23, 24.

Heterogeneous catalytic systems are beneficial compared to homogenous, owing to the facile separation and reuse of the solid catalysts. Therefore, various types of materials with basic or Lewis acid properties have been proposed for glucose isomerization. Regarding the catalysts with Lewis acidity, the efficiency of Ti- and Sn-containing zeolites and porous silicates has been demonstrated by different groups 25, 26, 27, 28. The reaction mechanism in the presence of these catalysts involves the coordination of the acid sites with O-1 of the glucose molecule through its lone electron pair, leading to polarization of the C=O bond. In addition, the same Lewis acid site coordinates with a second O atom, forming a complex that facilitates the intramolecular hydride shift from C-2 to C-1, ultimately leading to the formation of fructose 3 ACS Paragon Plus Environment


29.

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However, in aqueous medium there is an antagonistic strong coordination of Lewis acid sites

with H2O, which inhibits the catalytic activity. This negative effect of water is lowered when the Sn and Ti metals that provide Lewis acidity, are incorporated inside the relatively hydrophobic zeolitic framework

26

with a method reported by Corma et al. 30. In the case of

solid bases, materials such as immobilized amines 31, 32, hydrotalcites with different Mg/Al molar ratios

33, 34, 35,

anion-exchanged resins

36,

microporous and layered metallosilicates

containing framework and non-framework alkali metals 37, 18, mesoporous ordered molecular sieves of the M41S family

38

and zirconium carbonate

39

have been reported as efficient

isomerization catalysts. In addition, cation-exchanged zeolites with basic metals have also been studied

40

and among them Mg impregnated NaY zeolites appeared very promising

41 42.

Recently, attapulgite, as a natural hydrate magnesium silicate material with increased basicity, has also been investigated 43. The effectiveness of MgO, a commercial widely available material with increased basicity, was recently demonstrated by our group

24.

When the reaction was conducted in

aqueous solution, at mild reaction conditions (90 o C, 45min), MgO resulted in high fructose yields (33.5 wt.% on feed), while the stability of the catalyst without loss in isomerization activity was also confirmed. In an effort to further investigate the effect of the texture, morphology and surface properties of MgO, a series of natural MgO materials have been investigated in the present study. Natural MgO is being produced industrially by the thermal decomposition of magnesium carbonate mineral (MgCO3, magnesite). Its chemical composition/purity can be tuned by a process called beneficiation via which, undesirable mineral mixtures can be removed. Furthermore, the severity of the calcination process affects the crystal/particle size and morphology, the porosity and the basicity of the natural MgOs 44. In this framework, the isomerization of glucose to fructose was studied over natural MgO catalysts (synthetic MgO was also tested for direct comparison) with different purity/composition, morphology, texture and basicity, attempting to clarify the correlation between their physicochemical characteristics and catalytic performance. In addition, the

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stability and reuse of natural MgO and the kinetics of glucose isomerization were also investigated.

EXPERIMENTAL SECTION

Materials-catalysts and reagents: D-Glucose was purchased from Sigma–Aldrich and used without further purification. All natural MgO catalysts were prepared either industrially (Ind.MgO series) or in the laboratory (Lab.MgO series) from the calcination of raw magnesite samples of different purity (Table 1), following previously reported methods 44. The magnesite materials originated from Grecian Magnesite’s Yerakini mine in Northern Greece. Depending on the content of impurities (mainly SiO2 and CaO) the catalysts were classified into a) high-purity (SiO2 and CaO up to 2%), b) medium purity (SiO2 5-6% and CaO up to 2.6%) and c) low-purity (SiO2 27% and CaO up to 3.3%) materials. Prior to any characterization and catalytic test, the natural MgOs were re-calcined at 500◦C for 3 h in air. For a comparison with the natural materials, one pure synthetic MgO catalyst (Syn.MgO) was prepared in the laboratory by the precipitation method in alkaline conditions and was studied accordingly. Typically, a specific amount of Mg(NO3)2 • 6H2O was fully dissolved in an aqueous solution followed by a controlled precipitation with specific amount of aqueous solution of NaOH in order to obtain a pH value of 10. The final emulsion was aged at 60°C for 3 h under stirring and then filtered and washed with water. The obtained white powder was then calcined at 500°C with 5°C/min under air flow for 3 h, followed by further calcination at 700°C for 3 h.


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Table 1. Chemical composition, textural and basic properties of MgO catalysts. Calcination Catalyst

Syn.MgO Ind.MgO-1 Ind.MgO-2 Ind.MgO-3 Ind.MgO-4 Lab.MgO-1 Lab.MgO-2 Lab.MgO-3

Temp. (oC)

Time (min)

700(g)

180

Industrial kiln profiles with max temperatures 800-1200 o C 700 800 700 (a)

Basicity (μmol CO2/g) (f)

Chemical composition (ICP-OES)

60 60 60

MgO (%)

SiO2(a) (%)

CaO (%)

Fe2O3 (%)

Al2O3 (%)

LOI (1000oC)

100.00(h) 97.25 95.41 90.08 95.27 92.16 93.78 68.64

0 1.21 1.98 5.93 5.30 5.14 6.66 27.10

0 1.13 1.76 2.48 2.57 1.91 1.68 3.29

0 0.01 0.10 0.08 0.09 0.12 0.11 0.77

0 0.00 0.00 0.07 0.09 0.02 0.03 0.14

4.84 3.36 4.53 1.56 4.62 3.53 5.37

SBET (m2/g)

Pore volume(c) (cm3/g)

Average pore diameter(d) (nm)

Crystal size(e) (nm)

36 63 40 48 20 112 54 65

0.76 0.36 0.26 0.28 0.19 0.39 0.34 0.27

91.0 28.9 32.4 32.3 44.1 10.0 23.4 12.9

36.1 19.5 28.5 24.1 42.9 11.1 18.6 13.9

(b)

Total basic sites

Weak/medium basic sites

Strong basic sites

123 243 144 165 63 236 210 213

123 199 101 108 37 161 146 116

0 44 43 57 26 75 64 98

From XRF analysis, (b) BET area determined by the multipoint BET method of N2 adsorption data at -196 o C; t-plot method analysis revealed the absence of microporosity

in the solids, (c) At P/Po=0.99, (d) From BJH analysis using adsorption data, (e) From Scherrer equation using XRD data of peak 2θ=42.9o, (f) TPD-CO2 experiment, (g) Before calcination at 700°C for 3h, a calcination step at 500°C for 3h was also conducted, (h) Nominal value.


Ratio of Strong toWeak/ Medium basic sites 0.22 0.43 0.53 0.70 0.47 0.44 0.84

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Catalyst characterization: The physicochemical properties of the MgO catalysts were determined by the use of inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence (XRF), N2 adsorption/desorption measurements, X-ray diffraction (XRD), Scanning electron microscopy – energy dispersive spectroscopy (SEM-EDS), Transmission electron microscopy (TEM) and CO2-temperature programmed desorption (TPD-CO2). More specifically, the metal content (Mg, Ca, Fe, Al) of the catalysts was determined by ICP-OES analysis by using an Optima 4300 DV PerkinElmer spectrometer (USA). The solid samples were dissolved in H2SO4 and/or HF aqueous solutions, while external standards of the respective metals were used for their quantification in the catalysts. Silicon in the samples was determined by the use of XRF in a Spectro X-Lab 2000 (pressed tablets of samples). Loss on ignition (LOI) was determined by igniting 2g samples in a 1.5 kW laboratory furnace for 1 h at 1000 o C. N2 adsorption/desorption measurements at -196 o C were conducted on an Automatic Volumetric Sorption Analyzer (Autosorb-1MP, Quantachrome). Prior to the analysis, the catalysts were outgassed overnight at 250 o C under 5x10-9 Torr vacuum. The BET area (i.e. total surface area) of the catalysts was determined by the multi-point BET (Brunauer–Emmett– Teller) method, the total pore volume was estimated by the adsorbed N2 at P/Po=0.99 and the pore size distribution and average mesopore size were determined by the Barrett-JoynerHalenda (BJH) analysis of the adsorption data. The crystallinity of the samples was evaluated by powder XRD measurements on a SIEMENS D-500 diffractometer equipped with Cu Kα X-ray radiation and a curved crystal graphite monochromator operating at 40kV and 30 mA; counts were accumulated in the range of 5-75 o 2θ every 0.02 o (2θ) with a counting time 2 sec per step. The crystal size of MgO catalysts was calculated by Scherrer’s equation using the data of the XRD main peak at 2θ=42.9 o.

Scanning Electron Microscopy (SEM) images were obtained on a JEOL 6300 microscope equipped with X-ray energy dispersive spectroscopy (EDS) system for X-ray microanalysis (OXFORD ISIS 2000). Elemental mapping experiments were conducted on flat 7 ACS Paragon Plus Environment


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cross-section surfaces of MgO particles, which were embedded in epoxy resin, ground, polished and coated with gold to avoid charging effects. Transmission electron microscopy (TEM) experiments were carried out on a JEOL 2011 high resolution transmission electron microscope operating at 200 kV, with a point resolution of 0.23 nm and Cs=1.0 mm. Samples were prepared by gently grinding MgO solids in high-purity ethanol using an agate pestle and mortar. A drop of the suspension was subsequently deposited onto a lacey carbon-film supported on a Cu grid and allowed to evaporate under ambient conditions. The total basicity and the strength of basic sites were determined by the use of TPDCO2 measurements. In a typical experiment, 0.2 g of the sample were loaded in a fixed bed quartz reactor and pretreated at 600 o C in He for 1 h, followed by cooling at 80 o C under He flow and subsequent treatment with a flow of 40% CO2/He for 1 h at 80 o C. Flushing with pure He at 80 o C for 3 h was then applied to remove the physisorbed CO2. TPD analysis was carried out from 80 to 600 o C at a heating rate of 10 o C/min and under a He flow rate of 50 ml/min. The composition of the exit gas was monitored online by a quadrupole mass analyzer (Omnistar, Balzer). Quantitative analysis of the desorbed CO2 was based on the fragment m/z = 44.

Glucose isomerization and analysis of the products: Isomerization of glucose into fructose was carried out in a batch, stirred, autoclave reactor (C-276 Parr Inst., USA). A solution of glucose (4%w/w) in water (50.0g) was charged into the reactor and heated to the desired temperature. The catalyst (0.5%w/w) was added to the reactor when the required temperature was reached and zero time was then recorded. After completion of the reaction, the reactor was cooled rapidly and the liquid product was separated by vacuum filtration. The reaction products were analyzed by Ion Chromatography on an ICS5000 (Dionex, USA). The quantification was based on external calibration, using standard solutions of sugars (rhamnose, arabinose, galactose, glucose, mannose, fructose and xylose), sugar alcohols (sorbitol, mannitol), 5-hydroxylmethylfurfural (5-HMF) and organic acids (glycolic, acetic, lactic, formic, propionic, levulinic and butyric acid). The analysis of sugars, sugar alcohols and 8 ACS Paragon Plus Environment

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5-HMF was performed using a CarboPac PA1 (5 μm, 4 x 250 mm) column and precolumn (5 μm, 4 x 30 mm) connected to a pulsed amperometric detector (PAD). The eluent was NaOH 20mM at 0.6ml/min flow rate and the total analysis time was 75 min. The analysis of the organic acids was performed on a AS-15 (5 μm, 4 x 250 mm) column and pre-column (5 μm, 4 x 30 mm) connected to a conductivity detector (CD). The eluent was NaOH 8 mM at 1 ml/min flow rate and the total analysis time was 75 min. The conversion and yields of the products (mole based) were calculated according to the following equations (1), (2): 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛𝑑 ― 𝑔𝑙𝑢𝑐𝑜𝑠𝑒(%) = 100 × 𝑌𝑖𝑒𝑙𝑑𝑠𝑢𝑔𝑎𝑟(%) = 100 ×

𝑔𝑙𝑢𝑐𝑜𝑠𝑒 𝑚𝑜𝑙𝑒𝑠 𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑔𝑙𝑢𝑐𝑜𝑠𝑒 𝑚𝑜𝑙𝑒𝑠 𝑖𝑛𝑖𝑡𝑖𝑎𝑙

𝑠𝑢𝑔𝑎𝑟 𝑚𝑜𝑙𝑒𝑠 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑔𝑙𝑢𝑐𝑜𝑠𝑒 𝑚𝑜𝑙𝑒𝑠 𝑖𝑛𝑖𝑡𝑖𝑎𝑙

(1)

(2)

The stability of the catalysts in terms of metal leaching was studied by analyzing the liquid products for the presence of Mg, Ca, etc. by means of ICP-OES analysis. Furthermore, the changes in the crystal structure of MgO (i.e. partial shift to Mg(OH)2 under the reaction conditions) was investigated by performing XRD measurements on the recovered solids after washing with water and acetone and drying at 80oC overnight. Prior to their re-use in the glucose isomerization reaction, the catalysts were calcined at 500oC for 3 h in air.

RESULTS AND DISCUSSION Characterization of MgO catalysts: The chemical composition data of the MgO catalysts are summarized in Table 1. The MgO samples varied in chemical purity, expressed as the percentage content of MgO, which was between 69-97%. The main impurities were silicon (as SiO2, 1.2-27 wt.%) and calcium (as CaO, 1.1-3.3 wt.%), while LOI (1.6-5.4 wt.%) represents the carbonate and moisture/hydroxides content of the samples (i.e. CO2 and H2O). The purity and composition of the natural MgO catalysts can be tuned by selecting raw magnesite minerals of different purity and by applying appropriate beneficiation and calcination procedures, as described previously

44.

In the present work, a series of MgOs,

industrially or lab prepared, with systematically varied composition were used, in order to 9 ACS Paragon Plus Environment


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determine the effect of MgO and impurities (mainly SiO2 and CaO) content on their catalytic performance. The industrial samples were of high (Ind.MgO-1 and Ind.MgO-2) and medium (Ind.MgO-3 and Ind.MgO-4) purity, with Ind.MgO-1 having the lowest amount of SiO2 (1.21 wt.%) and CaO (1.13 wt.%) impurities. Laboratory prepared samples Lab.MgO-1 and Lab.MgO-2 were of medium purity, while Lab.MgO-3 was the only one of low purity, having the highest content of SiO2 (27.1 wt.%) and CaO (3.29 wt.%). Synthetic sample Syn.MgO had no impurities as it was synthesized in the laboratory by precipitation of Mg(OH)2. In order to obtain an idea of the distribution of impurities, mainly Si and Ca, within the MgO particles, SEM-EDS mapping experiments were performed (Figures S1 – S3, Supporting Information). As it can be seen from the corresponding images of sample Ind.MgO-1, Si and Ca could not be detected due to their relatively low content and/or high degree of homogeneous dispersion, this sample being a “high purity” sample. On the other hand, Si and Ca could be observed in the mapping images of Ind.MgO-3, as this sample has higher content of these metals. A slight preference of Si and Ca aggregation in the periphery (outer surface) of the MgO particles could also be observed. However, due to partial leaching of Ca after one cycle of reaction, as discussed later, Ca could not be detected in the images of the used Ind.MgO-3 sample while Si was still observable. The N2 adsorption/desorption isotherms (at -196oC) with the corresponding pore size distribution curves (BJH analysis of adsorption data) of representative natural MgOs (industrial and laboratory) and the synthetic MgO are shown in Figure 1a, b (the isotherms and pore size distribution curves of the rest catalysts are provided in Figure S4a, b of the Supporting Information), while the corresponding data (BET area, total pore volume, and average pore diameter) are given in Table 1.


Lab.MgO-1

Syn.MgO

300

900

250

750

200

600

150

450

100

300

50

150

0

0 0

0.2

0.4

0.6

0.8

b

1

Ind.MgO-1

Lab.MgO-1

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00

Syn.MgO 2.00

1.00

Dv(logd) (cc/g)

Ind.MgO-1

Dv(log d) (cc/g)

a N2 adsorbed (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N2 adsorbed (cm3/g)

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0.00 10

P/Po

100

1000

Pore Diameter (Å)

Figure 1. (a) N2 adsorption/desorption isotherms and (b) respective pore size distribution curves (BJH analysis of adsorption data) of representative natural MgOs and synthetic MgO (primary Y axis: Ind.MgO-1, Lab.MgO-1, secondary Y axis: Syn.MgO).

The adsorption isotherms of the industrial MgOs (Ind.MgO) were typical of non-porous materials or materials with high textural porosity originating from inter-crystal/particle voids at the meso/macroporous scale (type II in the IUPAC classification), 44,

45

exhibiting a steep

increase of N2 sorption at P/Po ≥ 0.8 and a narrow hysteresis loop of H3 type (Figures 1a and S4a). For the laboratory MgOs (Lab.MgO), which have been calcined at milder conditions (ca. 700 oC-800 oC for 60 min) compared to the industrial samples, the adsorption isotherms were of type IV, which is typical of mesoporous materials with relatively narrow pore size distribution (Figures 1a,b and S4a,b). The adsorption isotherm of the Syn.MgO was of type II, with a very steep increase of sorbed N2 at P/Po ≥ 0.9 which corresponds to meso/macropores with an average size of ~91 nm (BJH curves in Figure 1b, data in Table 1). The t-plot analysis of the adsorption data did not reveal the presence of microporosity, showing that the measured surface area (20-112 m2/g) of all MgO catalysts was solely attributed to inter crystal/particle meso/macropores, with average diameters of 29-44 and 10-23 nm (Table 1) for industrial and laboratory samples respectively. The effect of increasing calcination severity on the morphology/size of the crystals and particles, which is depicted on their textural properties, can be also revealed by the TEM images of the samples Ind.MgO-1 (milder conditions) and Ind.MgO-4 (harsher conditions), shown in Figure S5 (Supporting Information). The difference in the size and morphology of primary nanocrystals as well as of the larger polycrystalline 11 ACS Paragon Plus Environment


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aggregates between the two samples can be clearly identified. A more sponge-like morphology of aggregated small nanocrystals (the majority being 20-30 nm in size) can be observed for Ind.MgO-1, in contrast to the bigger well-formed rectangular crystals of Ind.MgO-4, being mostly in the range of 30-80 nm and forming larger agglomerates of up to ca. 500 nm. The crystal structure of the natural (industrial and laboratory) and synthetic MgO catalysts was investigated by XRD measurements (Figures 2 and S6a of Supporting Information). All materials were highly crystalline and exhibited the typical MgO periclase (cubic) structure 46. In the case of natural MgOs, there were some additional peaks at 2θ 20.6 and 26.6 attributed to SiO2 impurities (quartz hexagonal phase) which were not observed in the case of pure synthetic Syn.MgO-1 sample. The crystal size of the samples was also determined by the Scherrer equation using the data of the peak at 42.9 o and the results are presented in Table 1.

Figure 2. XRD patterns of synthetic and representative natural (industrial and laboratory) MgO catalysts: (a) Syn.MgO, (b) Ind.MgO-1, (c) Lab.MgO-1.

The crystal size of the MgO catalysts varied between 11 and 43 nm and it was clearly dependent on the calcination conditions of the raw magnesites. In particular, the more severe the calcination procedure the larger the crystal size obtained due to enhanced sintering of the crystals as was also shown previously 44. In general, the industrial MgO materials exhibited 12 ACS Paragon Plus Environment

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higher crystal sizes (20 – 43 nm) compared to the laboratory prepared samples (11 – 19 nm). Interestingly, the surface area was in smooth correlation with crystal size (Figure 3a), the MgOs with the smaller crystal size exhibiting higher surface area. This can be attributed to the smaller external surface of larger crystals and to less confined voids formed between them, compared to smaller crystals, especially with sizes below ca. 10 nm (Figure 3a).

Figure 3. Correlation of: crystal size with (a) surface area and (b) basicity, (c) basicity with surface area and (d) Ratio of Strong to Weak/Medium basic sites with CaO content for the MgO (natural and synthetic) catalysts.

With regard to basicity, the number and strength of basic sites of the MgO catalysts was determined by the TPD-CO2 experiments and the corresponding curves are shown in Figures 4 and S7. Additionally, the number of total, weak/medium and strong basic sites as well as the ratio between them are presented in Table 1.



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Figure 4. TPD-CO2 curves of representative natural (industrial and laboratory) MgOs and synthetic MgO.

The TPD-CO2 patterns of all natural (industrial and laboratory) MgO samples presented two main CO2 desorption peaks within the ranges of 100-400 and 450-600 oC, attributed to weak/medium and strong basic sites respectively 47, contrary to synthetic MgO which exhibited only one peak at the lower temperatures with maxima between 200-220 oC. The high temperature desorption peak in the patterns of the natural MgO samples can be attributed to the presence of CaO which is capable of providing basic sites of relatively high strength 48, 49. The total basic sites for all the MgOs were from 63 up to 243 μmol CO2/g, which are in the range of values reported previously for natural and synthetic MgOs 47, 48. Furthermore, as can be seen by the results of the present study, the basicity of MgOs is strongly affected by the morphology, texture and chemical composition of the materials. Particularly, samples with smaller crystal size and higher surface area exhibited higher total number of basic sites (Figures 3b and 3c), while samples with high content of impurities exhibited higher portion of strong basic sites, which probably originated from the interaction of CO2 with CaO

48, 49.

In fact, by

plotting the ratio of strong to weak/medium basic sites versus the CaO content (Figure 3d) an almost linear relationship between them is observed. This observation also justifies the absence of strong basic sites in the case of pure synthetic Syn.MgO material.


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The observed differences in the number and the strength of basic sites play a key role in the catalytic performance.

Isomerization of glucose into fructose over MgO catalysts: The performance of the various MgO catalysts was evaluated at 90 oC for 45 min in aqueous medium. The reaction conditions were selected according to a previous study by our group

24.

Based on the

experimental results (Figure 5), all tested MgOs catalyzed effectively the isomerization reaction, resulting in high glucose conversion and fructose selectivity. The main byproducts detected were mannose, formed through the epimerization reaction, and organic acids (glycolic, acetic, lactic and formic acid) as degradation products.. However, in all cases the selectivity of mannose and acids remained below 15% (yield below 8%) and 6% (cumulative yield below 3%) respectively, indicating that the reaction is highly selective towards fructose.

Figure 5. Screening of MgO catalysts for glucose isomerization to fructose in H2O (90 oC, 45 min, glucose 4 wt. %, catalyst/glucose 1/8).

Effect of Basicity: The reaction mechanism in the presence of MgO follows the Lobry de Bruyn–Alberda van Ekenstein (LdB–AvE) pathway 24. The MgO surface is hydroxylated and negatively charged due to water dissociation 50, therefore when glucose comes in contact with the catalyst surface, the C-2 is deprotonated, resulting in the formation of the 1,2-enediol 15 ACS Paragon Plus Environment


Page 16 of 37

intermediate followed by an electron pair movement through the carbon skeleton towards the formation of fructose (Scheme 1) 13.

+ ― 𝑀𝑔𝑂𝑠𝑜𝑙𝑖𝑑 + 𝐻2𝑂→𝑀𝑔𝑂𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑂𝐻𝑎𝑞𝑢𝑒𝑜𝑢𝑠 + ― ― 𝑀𝑔𝑂𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑂𝐻𝑎𝑞𝑢𝑒𝑜𝑢𝑠 →𝑀𝑔𝑂𝐻 + . 𝑂𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒

Scheme 1. Schematic representation of proposed mechanism for glucose to fructose isomerization on MgO.

Based on this proposed mechanism (Scheme 1), increased basicity of the MgO catalyst should be positively correlated with high activity. However, isomerization is a reversible reaction and therefore the maximum conversion of glucose to fructose is limited thermodynamically. Therefore, excessive basicity of the catalyst may favor the formation of byproducts and boost glucose conversion beyond the thermodynamic equilibrium limit, at the expense of fructose selectivity. Consequently, the clarification of the correlation between catalyst basicity and activity is crucial. To this end, glucose conversion as well as fructose selectivity was plotted against catalysts’ total basicity (Figures 6a, b). It is interesting to note that all studied MgO catalysts fall on a single trend, demonstrating that fructose selectivity is favored by the increase of catalyst total basicity, while glucose conversion is decreased. However, total basicity consists of weak/medium and strong basic sites provided mainly by MgO and CaO respectively, as discussed above (paragraph 3.1). Further information are revealed by plotting weak/medium basicity as well as the ratio of strong to weak/medium basic sites versus glucose conversion (Figures 6c and 6e) and fructose selectivity (Figures 6d and 6f) respectively. Based on these


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graphs, it can be concluded that the weak/medium basic sites contribute to the increased selectivity towards fructose, while the presence of strong basic sites, attributed to CaO impurities, appears to favor side reactions and by-products (Figure 5) that boost glucose conversion but decrease fructose selectivity. These results are in agreement with previous published works demonstrating that under highly alkaline conditions, secondary reactions leading to the production of organic acids, condensation products and humins, are being favored 15, 18, 38.

Figure 6. Correlation of: total basic sites with (a) glucose conversion and (b) fructose selectivity, weak/medium basic sites with (c) glucose conversion and (d) fructose selectivity and mole ratio of strong to weak/medium basic sites with (e) glucose conversion and (f) fructose selectivity for all MgO catalysts.

Among the various natural industrial MgOs tested, Ind.MgO-4 of medium purity with the highest ratio of strong to weak/medium basic sites (Τable 1) was the most active catalyst 17 ACS Paragon Plus Environment


Page 18 of 37

for glucose conversion reaching a value of 62.38%, but the least selective towards fructose (41.76%). On the other hand, Ind-MgO-1 of high purity with the smallest ratio of strong to weak/medium basic sites was the most selective catalyst for fructose (75.81%) at glucose conversion 44.02% (Figure 5). The same trend was observed for the laboratory-prepared natural MgOs, as among them the low purity Lab.MgO-3 with the highest ratio of strong to weak/medium basicity resulted in the highest glucose conversion of 46.74% but at the lowest fructose selectivity of 50.19%. The catalytic performance of pure synthetic MgO (Syn. MgO), was similar to those of the Ind.MgO-2 and Ind.MgO-3 in terms of fructose selectivity, probably due to the comparable weak/medium basicity, but glucose conversion appeared relatively lower due to the absence of strong basic sites in Syn.MgO.

Catalyst stability: As described in the experimental section, after the reaction, the catalysts were washed with H2O and acetone, dried at 80oC overnight followed by calcination at 500oc for 3h. The crystal structure of both the dried and the calcined samples was investigated by XRD measurements (Figure 7), while the potential leaching of Mg and Ca into the reaction mixture was determined by ICP-OES analysis (Table 2). According to the XRD patterns of the dried samples (representative results shown in Figure 7), all the used catalysts exhibited the crystal phase of Mg(OH)2 (represented by the peaks at 18o and 58o 2θ 51) along with the typical MgO phase, indicating that hydroxylation of MgO indeed occurs in the aqueous medium under the reaction conditions, confirming the above proposed mechanism for the isomerization reaction. It is postulated that this hydroxylation takes place on the catalyst surface 50, 52 and is correlated with the crystal size. Indeed, no clear peaks of the Mg(OH)2 crystals could be identified in the XRD pattern of the washed/dried Ind.MgO-4, which has the highest crystal size and the lowest surface area among the tested catalysts (Figure 7). With regard to the reuse of the catalysts, after calcination of the washed/dried MgOs at 500oC, the only peaks present in the XRD patterns were those of the MgO periclase phase (Figure 7).


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Figure 7. XRD patterns of representative washed, dried and calcined MgO catalysts after isomerization reaction:

(a)

Ind.MgO-1_washed_dried,

(b)

Ind.MgO-1_washed_calcined,

(c)

Ind.MgO-

4_washed_dried, (d) Ind.MgO-4_washed_calcined, (e) Lab.MgO-1_washed_dried, (f) Lab.MgO1_washed_calcined.

Based on the ICP-OES analysis of the liquid products (Table 2), all the MgO catalysts presented leaching of calcium and magnesium to different extent. The results suggest that the catalysts’ textural characteristics have an impact on its stability, as MgOs with increased crystal size and low surface area resulted in higher Mg leaching, however further study on this subject is required. Nonetheless, the leaching of Mg remained relatively low in all cases due to the inherent insolubility of MgO in water. Among the impurities contained in the MgO catalysts, CaO was the component that presented the highest leaching. Furthermore, there was an interdependence between the extent of Mg leaching and the initial content of CaO in the fresh catalysts (higher Mg leaching was favored by increased CaO content) (Figure S8) providing useful guidance about the selection of the more stable catalysts.

Table 2. Calcium and magnesium leaching from the MgO catalysts in the reaction mixture. Catalyst

ICP-OES (ppm)

Leaching (%)(a)

Ca

Mg

Ca

Mg

Syn.MgO

-

230

-

9.65

IndMgO-1

12

205

28.12

6.87



(a) It

Page 20 of 37

IndMgO-2

25

424

38.67

14.38

IndMgO-3

18

435

20.13

15.68

IndMgO-4

31

518

33.69

17.85

LabMgO-1

12

174

17.19

6.17

LabMgO-2

14

200

23.38

6.88

LabMgO-3

16

214

13.40

10.23

was calculated on the basis of the metal content in the initial MgOs prior to reaction.

The leaching of Ca ions into the solution may have a direct impact on the reaction itself, as in the presence of Ca2+ the formation of cation−ketose complexes can be promoted due to the strong basicity of calcium which leads to unwanted retro-aldolization byproducts 14,53 while the epimerization of glucose towards mannose is also favored. As a result, glucose conversion increases but at the expense of fructose selectivity as several by-products are also produced (Figure 5). The results obtained with Ind.MgO-2 and Ind.MgO-4, which presented the highest leaching of Ca (38.67% and 33.69% respectively) accompanied by the highest glucose conversion (60.04 and 62.38%) and the lowest fructose selectivity (44.87 and 41.76%) could be in accordance with the above discussed effect of leached Ca.. The Mg ions leached out in the solution may also affect conversion and selectivity41. However, when performing additional experiments by using the solution after the first cycle of reaction with the fresh MgO catalysts, which contained the leached Mg and Ca, the glucose conversion obtained was below 4% and the fructose yield was less than 1.5%, confirming the minimum contribution of the leached species in the overall reaction.

Catalyst regeneration and reuse: The reusability of Ind.MgO-1 has been already examined in a previous published work by our group 24, where it was found that this catalyst can be regenerated and reused without loss of activity for the studied reaction. As Ind.MgO-1 was proven in the present study to be the most efficient and selective amongst the different variants of the natural industrial and laboratory samples tested, its performance was studied further. As can be seen in Table 3, glucose conversion and selectivity to fructose remain similar


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within experimental error (ca. 2.5%) for the 4 cycles of reaction-regeneration. The carbonaceous deposits formed during the reaction were about 2.5% after each cycle, but complete regeneration of the catalyst was achieved after washing, drying and calcination (500oC, 3 h). In order to further elucidate this stable performance of Ind.MgO-1, the physicochemical properties of the calcined samples after each cycle were also studied. According to the XRD patterns (Figure S6b), the regenerated catalysts after each cycle exhibited only the typical MgO structure, while the crystal size that was determined by Scherrer equation, was not significantly changed (Table 3) indicating that aggregation/sintering effects due to the hydrothermal conditions of the reaction or the subsequent calcination-regeneration at 500oC were minimal. ICP-OES analysis of the liquid samples showed that Ca and Mg present minor leaching after the first cycle, , while the chemical analysis of the regenerated-calcined samples proved that their respective concentrations were not significantly affected after 4 cycles (Table 3). The more pronounced change (decrease of CaO content) was observed after the 1st cycle while after the 3rd cycle both CaO and MgO contents were not changed further. The change in the Mg and Ca relative content in the regenerated MgO samples is expected to affect its basicity and therefore TPD-CO2 measurements were performed in the calcined samples after each cycle. Based on the results presented in Table 3, it appears that after the 1st cycle the catalyst total basicity was slightly decreased from 243 to 202 μmol CO2/g, with the strong basic sites being reduced by 40%, in accordance with the decrease of CaO content. However, both weak/medium and strong basic sites remained more or less stable in the following cycles, rationalizing its stable catalytic performance. Overall, Ind.MgO-1 which is a natural MgO material of high purity, proved to be the best performing stable catalyst for isomerization of glucose into fructose. It was therefore selected for further study with regard to reaction kinetics.



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Table 3. Ind.MgO-1 characterization after regeneration (500oC/3h) and reuse. Metal content in Cycles

calcined MgO

Glucose

Fructose

conversion

selectivity

(%)

(%)

Fructose

Crystal

yield (%)

size (nm)

Basicity (μmol CO2 /g)

CaO

MgO

(%)

(%)

Fresh

1.13

97.25

-

-

-

19.50

243

199

44

1st

0.91

97.02

44.02

75.81

33.37

20.95

202

175

27

2nd

0.95

97.85

42.30

77.94

32.90

20.73

215

189

26

3rd

0.76

96.36

43.79

76.50

33.50

21.54

200

170

30

4th

0.72

96.36

44.20

75.05

33.17

20.80

209

178

31

Total

Weak/ medium

Reaction kinetics: In order to investigate and delve into the kinetics of glucose isomerization, a 1st order kinetic model was applied and validated for the best performing catalyst (Ind.MgO-1). It should be noted that the model accounts for D-glucose, without discriminating between the corresponding α- and β-anomers, contrary to the common strategy applied for the determination of kinetics in biological systems and enzymatic reactions. Based on the experimental results discussed earlier, glucose conversion leads to fructose (main product) and mannose (main by-product), possibly through a set of reversible reactions. Simultaneously, a group of acids (glycolic, formic, lactic and acetic) is also formed. Due to the alkaline conditions applied, retro-aldolization and benzillic rearrangement reactions of the hexoses are promoted, leading mainly to

Isomerization of glucose into fructose over natural and synthetic MgO

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