Glucose isomerisation into fructose over Mg-impregnated Na-zeolites: Influence of zeolite structure

Accepted Manuscript Glucose isomerisation into fructose over Mg-impregnated Na-zeolites: Influence of zeolite structure I. Graça, M.C. Bacariza, D. Chadwick PII:

S1387-1811(17)30487-0

DOI:

10.1016/j.micromeso.2017.07.015

Reference:

MICMAT 8444

To appear in:

Microporous and Mesoporous Materials

Received Date: 8 May 2017 Revised Date:

5 July 2017

Accepted Date: 7 July 2017

Please cite this article as: I. Graça, M.C. Bacariza, D. Chadwick, Glucose isomerisation into fructose over Mg-impregnated Na-zeolites: Influence of zeolite structure, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2017.07.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Glucose isomerisation into fructose over Mg-impregnated Na-zeolites: Influence of zeolite structure I. Graça1,*, M.C Bacariza2, D. Chadwick1 1

2

Department of Chemical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001

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Lisboa, Portugal

*Corresponding author: [email protected]

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Abstract

Magnesium-impregnated NaY, NaMOR, NaBEA, NaZSM-5 and NaFER zeolites have been prepared and investigated for glucose isomerisation into fructose. It was shown that better

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magnesium dispersion and smaller reduction of textural properties were obtained with threedimensional rather than with mono- and two-dimensional zeolites. MgO particle size was also observed to be dependent on the zeolite structure. Various contributions were found to affect the final catalyst performances: availability of MgO, the strength of basic sites, location where the reaction takes place, and the extent of homogeneous reaction due to Na

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and Mg leaching. Higher glucose conversions were achieved over the MOR, BEA and ZSM5 zeolites (37-39%), while Y and FER zeolites presented a relatively moderate performance (28 and 27%). In general, lower fructose selectivities were reached for the most active samples, except for the ZSM-5 zeolite. For this catalyst, the reaction appeared to take place

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mostly on the external surface due to the smaller pore size. Among the various structures investigated, 5%MgNaY zeolite revealed the most resistance to MgO particle size

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agglomeration during consecutive reaction runs. In addition, 5%MgNaY was found to be the only catalyst capable of recovering its initial activity when regenerated at high temperature. Thus, the type of zeolite structure selected as support for MgO appears to have a significant effect on the catalyst performance for the glucose isomerisation into fructose, with Y zeolite being the most attractive choice for this application.

Keywords: Glucose, isomerisation, fructose, zeolite structures, magnesium

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ACCEPTED MANUSCRIPT 1. Introduction

Valorisation of lignocellulosic biomass has been revealed as one of the most promising options for the sustainable production of chemicals and fuels by replacing the traditional

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fossil resources [1-3]. Lignocellulosic materials are composed of several polysaccharides, with glucose being the main building block, and so is the most available carbon renewable source [1,3-5]. Besides the common applications of glucose in the food industry and medicine [6-8], this sugar monomer can also be used in the synthesis of fuels, important platform chemicals and polymeric materials. One potential important glucose reaction is the

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production of fructose through isomerisation, since fructose is an attractive intermediate in the production of platform chemicals such as 5-hydroxymethylfurfural (HMF), a possible

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precursor for the production of caprolactam, the monomer for nylon-6 [9-15]. The isomerisation of glucose into fructose is nowadays industrially carried out using immobilised enzymes as catalysts (D-glucose or xylose isomerase) [16,17]. However, in the context of biomass valorisation, chemical catalysts would be economically more advantageous, especially heterogeneous catalysts that are more likely to be capable of being

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regenerated and recycled. Among the heterogeneous catalysts reported in the literature (hydrotalcites, basic zeolites, anion-exchanged resins, mesoporous ordered molecular sieves of the M41S family, magnesium oxide, metallosilicate solid bases, Sn- and Ti-promoted microporous and mesoporous materials), zeolites either presenting basic or Lewis acid

[18-27].

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properties have shown significant performance for this reaction at mild operating conditions

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Basic alkaline-exchanged (Li+, Na+, K+ and Cs) 4A, 13X and Y zeolites have been studied for the glucose isomerisation into fructose [18,20,22]. 4A zeolite showed the best glucose conversions (24-34%), whereas the highest fructose selectivities (77-86%) were found for the 13X zeolite. The superior performance for the 4A and 13X zeolites was attributed only to their lower Si/Al ratio that allowed a higher degree of ion-exchange, but no activity-structure correlation was established, even though 4A is a small pore and 13X a large pore zeolite. On the other hand, Moliner et al. [27] analysed the reactivity of different catalysts presenting enhanced Lewis acidity, concluding that the intermediate pore size Ti-containing zeolites (5 to 6Å) have practically no activity when compared to the large pore Ti-BEA zeolite (8Å). A similar observation was made when using Sn-BEA and Sn-MFI zeolites, which reached 65 2

ACCEPTED MANUSCRIPT and 9% glucose conversion respectively after 210 min at 90ºC [28]. Differences in the results for both zeolites have been assigned to the lower diffusivity of the bulky glucose molecule (8.6Å) into smaller pore zeolites. This conclusion was confirmed by Li et al. [29] by periodic DFT calculation, simulating glucose isomerisation into fructose over Sn-containing BEA, MOR, MFI and MWW zeolites. Indeed, the stability of adsorbed sugars seems to be higher in

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narrow pore zeolites, limiting their diffusion into the structure and accessibility to the lattice Sn sites. Nevertheless, Sn-MWW zeolite, having pores of similar size than the Sn-MFI, have been demonstrated to be much more active as a result of the high reactivity of a small fraction of accessible sites. Several zeolite structures in the protonic-form (Y, BEA, MOR, ZSM-5)

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were also tested for glucose isomerisation into fructose in consecutive reactions using alcohol and aqueous media [30]. The larger pore zeolites, Y and BEA, were found to more active than smaller pore zeolites, with the Y zeolite presenting the best reaction results. Therefore, it

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appears that while larger pore zeolites seem to be more favourable there is currently no consensus concerning the best zeolite structure to be used for the glucose isomerisation into fructose. It might depend on whether the reaction is performed using basic or acid zeolites. In this paper, the effect of the zeolite structure on glucose isomerisation into fructose over basic zeolites has been investigated by using different Na-exchanged zeolite structures (NaY,

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NaBEA, NaMOR, NaZSM-5 and NaFER) impregnated with 5 wt.% of magnesium. Magnesium-impregnated NaY zeolites have recently been demonstrated to be very promising catalysts for the glucose isomerisation into fructose [31]. As previously, the Na-forms of the zeolites were used to promote framework basicity. Large and intermediate pore zeolites have

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been selected due to their different pore architecture and their ability for the glucose diffusion. Inside each category, typical zeolite structures have been studied. Besides X-Ray

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diffraction (XRD), N2 adsorption and temperature programmed desorption of CO2 (CO2TPD) characterisation used in the previous paper [31], transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (TEM-EDS) and diffuse reflectance ultraviolet-visible (UV-Vis) spectroscopy were also applied to evaluate magnesium oxide dispersion and relative particle size respectively. The deactivation and regeneration capacity of the catalysts have also been investigated, where it was found that only the 5%MgNaY zeolite is able to be regenerated to close to its original activity.

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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Catalyst preparation

Commercial NaY(2.6), NH3-MOR(10), NH3-BEA(12.5), NH3-ZSM-5(11.5) and NH3FER(10) zeolites supplied by Zeolyst were used as supports. For the NH3-forms, Na was

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firstly introduced by ion-exchange (NaNO3, 2M, 25ºC, 4h, 3 times). Afterwards, 5 wt.% magnesium was added by incipient wetness impregnation, using magnesium nitrate hexahydrate (Sigma-Aldrich, 99% purity). After ion-exchange or impregnation, samples were dried in an oven overnight at 100ºC and calcined at 500ºC under air flow. Calcined samples

catalysts.

Samples

are

identified

as

5%MgNaY,

2.2 Catalyst characterisation

5%MgNaMOR,

5%MgNaBEA,

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5%MgNaZSM-5 and 5%MgNaFER.

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have magnesium mostly as MgO, which as such can contribute up to about 8 wt.% of the

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Mg and Na contents in the magnesium-impregnated zeolites were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian Vista MPX ICPOES system (see Table 1).

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Powder XRD patterns were obtained in a PANalytical X’Pert Pro diffractometer, using Cu Kα radiation and operating at 40 kV and 40 mA. The scanning range was set from 5° to 80°

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(2θ), with a step size of 0.0017º/s.

Transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDS) was performed by using a JEOL2010 Transmission Electron Microscope operating at 200 kV coupled with a X-MaxN 80T Silicon Drift Detector from Oxford Instruments. The samples were prepared by dispersing a small amount of solid in ethanol and allowing a drop contacting with a copper grid coated with holey carbon film and the ethanol evaporating. N2 adsorption measurements were carried out at -196ºC on a Micrometrics 3Flex apparatus. Before adsorption, zeolite samples were degassed under vacuum at 90ºC for 1h and then at 350ºC overnight. Spent samples were degassed at 120ºC for 1h. The total pore volume (Vtotal) was calculated from the adsorbed volume of nitrogen for a relative pressure P/P0 of 0.97, 4

ACCEPTED MANUSCRIPT whereas the micropore volume (Vmicro) and the external surface area (Sext) were determined using the t-plot method. The mesopore volume (Vmeso) was given by the difference Vtotal Vmicro. DRS spectra in the UV-Vis region were obtained in a Varian Cary 5000 UV-Vis-NIR spectrophotometer, equipped with a Praying Mantis™ Diffuse Reflection Accessory, in the

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200-800 nm range. Catalyst spectra were obtained using the Na-exchanged zeolites as reference. The reflectance spectra were converted into the Schuster-Kubelka-Munk (SKM) function, F(R), and presented versus wavelength. The SKM function, F(R), was calculated from the reflectance at each wavelength, using the expression: F(R) = (1 − R)2/2R, where R is

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the ratio of the intensity of the light reflected by the sample to the one reflected by a standard. Temperature programmed desorption (TPD) of CO2 was carried out in a fixed-bed flow

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reactor. The sample (100 mg) was pre-treated in-situ at 580°C in flowing 100 mL/min of N2 for 1h. Justification for the selection of this temperature can be found in the Supporting Information S.1. After cooling to 50°C, the sample was exposed to a 20% CO2/N2 mixture for 1 hour. The system was then purged in flowing N2 for 1 hour to remove physisorbed CO2, and the temperature was then increased to 800°C at 10°C/min. Desorbed CO2 was analysed

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using a COx Siemens Ultramat 23 infrared detector.

Spent catalysts were analysed for carbonaceous materials deposition after reaction using thermogravimetric analysis (TGA), which was carried out in a TA Instruments TGA Q500. The samples were heated up from room temperature to 900°C, at 10°C/min, under air flow

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2.3 Catalytic test

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(60mL/min). The weight loss was recorded as a function of the temperature.

Glucose isomerisation reaction was performed in a 25 mL Büchi AG autoclave at 100°C under an inert nitrogen atmosphere of 3 bar to avoid air entering the system. Before the reaction, a solution containing 0.5g of D-glucose (Sigma, ≥99.5% purity) in 5 mL of deionised water was prepared and placed in to the reactor, which already contained the catalyst (100mg). The reactor was sealed, purged with nitrogen and the initial working pressure was set. An oil bath was used to heat up the reactor, which was only introduced after the desired temperature was reached. The reaction was carried out for 2h, under continuous 5

ACCEPTED MANUSCRIPT stirring (1000 rpm) to have a well-mixed condition and avoid diffusional limitations. After the reaction, the reactor was cooled down, the nitrogen was released, and the liquid productcatalyst suspension collected. In order to analyse the liquid product, the catalyst was first separated from the mixture by centrifugation at 5000 rpm for 5 min. All the liquid samples were diluted in water and

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analysed by high-performance liquid chromatography (HPLC) using a Shimadzu Prominence UFLC system equipped with a refractive index (RID-10A) detector. The analysis was carried out with a SupelcogelTM C-610H HPLC column, thermostated at 30°C. The mobile phase was a 0.1% (v/v) H3PO4 aqueous solution at a flow rate of 0.3 mL/min. Citric acid was added as

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standard. The experimental errors in the conversion and yield are on average 2% and in the selectivity 7%.

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Leaching of cations from the catalysts to the reaction mixture was analysed by inductively coupled plasma (ICP), using a Perkin-Elmer Optima 2000 DV ICP system.

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3. Results and discussion

3.1 Catalyst characterisation

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Fig. 1 shows the XRD diffraction patterns obtained for the Na-exchanged zeolites and these zeolites impregnated with magnesium in which it is possible to see the characteristic peaks of the FAU, MOR, BEA, MFI and FER zeolite structures, confirming their crystalline structure

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[32]. The addition of 5 wt.% of magnesium causes only a slight reduction of the XRD peaks intensity for the zeolite structures, which can be purely related to the decrease of the amount of zeolite in the samples, as crystallinities for the magnesium-impregnated catalysts are not lower than 90% (Table 2). XRD diffraction peaks for MgO were not detected for any of the magnesium-impregnated zeolites (2θ = 37.0, 43.0, 62.4, 74.8 and 78.7° [33,34]), meaning that any formed MgO entities are either amorphous or small enough in size to escape XRD detection. A reduction in intensity of the zeolite diffraction peaks has been noticed by several authors when impregnating Y, ZSM-5 and MOR zeolites with up to 15 wt.% of magnesium [31,35-38]. The same authors also observed that MgO diffraction peaks were absent even at the highest percentages of magnesium. 6

ACCEPTED MANUSCRIPT MgO particles were also not observable by TEM, as the contrast between the elements in the zeolite framework and the metal oxide species was insufficient [36]. However, by coupling EDS analysis to TEM it was possible to investigate how homogeneous was the dispersion of MgO on the different zeolite structures. Thus, for each sample, several spectra were collected at different locations and chemical analyses were carried out (Supporting Information S.2).

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MgO appears to be homogeneously dispersed for the Y, BEA and ZSM-5 zeolites, with the chemical composition being similar at all evaluated locations. A homogeneous magnesium distribution verified by EDS was also obtained in the literature when using a BEA zeolite [39]. On the other hand, for both MOR and FER zeolites a very heterogeneous magnesium

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distribution was found, with sites of the structure much richer in magnesium than others. In order to try to compare the relative size of the MgO on the different zeolite structures,

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DRS-UV-Vis was carried out. However, it is important to take into consideration that DRSUV-Vis is a surface technique, meaning that mainly species on the outer surface of the zeolite and closer to the pore entrances are screened. The UV-Vis spectra obtained for the magnesium-impregnated zeolites using their Na-exchanged forms as reference are presented in Fig. 2. All the spectra show a main band with maximum at around 205 nm that is attributed to the presence of MgO [40,41]. Based on the UV-Vis data, the band gap energy for the

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supported MgO (Table 2) can be determined by plotting F(R)2 as a function of the energy (E=hc/λ) and extrapolating the linear region to F(R)2 = 0 (Supporting Information S.3) [4042]. Band gap energies for MgO increase in the following order: FER < Y < BEA < ZSM-5 < MOR, and can be usually correlated with the particle size. For MgO, the smaller the size of

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the particles, the smaller the bang gap energy [43-44], meaning that the particle size follows the same order as the band gap energy.

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In addition, the analysis of the textural properties of the zeolites could give some insights into the MgO particles preferential location in a given structure. Table 2 presents the microporous and mesoporous volumes and external surface areas for all the zeolite structures without and with magnesium. First of all, it can be noticed that in general the addition of magnesium leads to a reduction of textural parameters for all the zeolites, the extent of which depending on the type of structure. For the BEA zeolites, practically no changes can be observed in the microporosity and mesoporosity, which might indicate that MgO is well dispersed all over the zeolite. However, the very slight reduction of the external surface area seen for this zeolite maybe reveals a small preference of the MgO particles for this location. In the case of the Y and ZSM-5 zeolites, MgO seems to be mainly located in the pores, as confirmed by the 7

ACCEPTED MANUSCRIPT reduction of the microporous and mesoporous volumes respectively, whereas the external surface area remains identical after magnesium addition. On the other hand, MOR and FER register a very significant decrease of the external surface area due to magnesium incorporation, which could reveal that a good part of the MgO is deposited at the pore openings. For these zeolites, both microporous and mesoporous volumes are as well reduced

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through magnesium impregnation. This could only be caused by a partial pore blockage due to the presence of MgO at the pore mouth or some particles are also located in the micro- and mesopores of these zeolites. In general, the reduction of the textural properties in the presence of magnesium is higher for MOR and FER than for the other zeolites structures.

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Overall, one can conclude that the impact of the magnesium oxide impregnation on the textural properties of the zeolites is a consequence of the combined effect between the shape

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and size of the zeolite porous system, the magnesium oxide distribution on these samples and the relative size of the MgO particles. MOR zeolite is a large pore zeolite, but mostly with a mono-dimensional structure. Thus, the fact that MOR has the biggest MgO particles among the other zeolite structures (UV-Vis) and these are mainly heterogeneously dispersed on its surface (TEM-EDS), it is enough to cause the observed significant decrease of the textural parameters. It is known that one dimensional channel systems block more easily than three-

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dimensional structures as a result of material deposition [45]. On the other hand, it is interesting that FER zeolite, having the smallest MgO particles and 2D pore system, presents an identical reduction of the textural properties to the MOR zeolite. This can be explained by the fact that the pore apertures on FER zeolite are much smaller in size, leading to a higher

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blockage as a result of particles preferential deposition at certain locations of the structure close to the pore entrances. In the case of Y, BEA and ZSM-5 zeolites, their three-

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dimensional accessibility might facilitate the migration and dispersion of the MgO species into the structure during preparation, which would explain the observed more homogeneous magnesium oxide distribution, the smaller size of the particles, and so the lower reduction of their textural properties. MgO particles deposited on ZSM-5 are bigger than those on Y and BEA zeolites as they are mainly located in the mesopores. In order to assess the total basicity of the magnesium-impregnated zeolites, CO2-TPD technique was applied. Fig. 3 presents the CO2 desorption versus temperature for magnesiumimpregnated zeolites. Y, MOR, ZSM-5 and FER zeolites show a main desorption band between 100 and 300ºC, revealing the relatively weak character of their basic sites. Identical CO2-TPD profiles were obtained in the literature for Al-rich H-ZSM-5 and MgY zeolites 8

ACCEPTED MANUSCRIPT modified with alkaline earth metal oxides of Mg [35,46]. On the other hand, for the BEA zeolite, besides the low basicity band, a more intense CO2 desorption band can also be seen at higher temperatures (550-750 ºC). Two distinct CO2 desorption bands at low and high temperature were also observed for a BEA zeolite promoted with ZnO and SnO2 [47]. Thus, contrary to the other zeolite structures, high basicity strength sites are also formed on the

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BEA zeolite through MgO incorporation. The CO2 adsorption capacity of the magnesiumimpregnated zeolites was estimated from the CO2-TPD (Table 3), which gives an indication of the basic sites density on the catalysts. The amount of basic sites in the samples follows the order: Y > BEA > ZSM-5 > MOR > FER. However, it is necessary to take into consideration

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that the results of the CO2 adsorption measurements incorporate the intrinsic basicity of the zeolite structure and also the level of pore blockage due to magnesium addition. Thus, comparison of basicity through CO2-TPD for different zeolite structures might not be

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straightforward. Intrinsic framework basicity is normally higher for Y zeolites, being followed by BEA, MOR, ZSM-5 and FER zeolites [48]. In this study, Y zeolite is also the sample presenting the highest degree of Na-exchange (Table 1), which explains its higher number of basic sites. MOR and ZSM-5 have a similar level of Na-exchange (Table 1), but the density of basic sites for the ZSM-5 is higher than for MOR. In fact, MOR has a much

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higher pore blockage due to magnesium, which might impede CO2 diffusion in the structure. Despite the lower Na-exchange of BEA when compared to the ZSM-5 (Table 1), the density of basic sites for BEA is higher. This can be explained based on the higher intrinsic basicity of the BEA zeolite [48]. The lowest framework basicity and % of Na-exchange (Table 1)

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sites.

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were found for FER, so that this structure presents the smallest estimated quantity of basic

3.2 Catalytic activity

A comparison of the glucose isomerisation performance, in terms of glucose conversion and fructose yield and selectivity, obtained with the different magnesium-impregnated zeolites is shown in Fig. 4. Although it is not presented in Fig. 4, it is important to note that only low glucose conversions were found for the Na-exchanged forms of the zeolites: 5.6, 4.5, 2.2, 1.2 and 1.6% for NaY, NaMOR, NaBEA, NaZSM-5 and NaFER, respectively. Magnesiumpromoted MOR, BEA and ZSM-5 zeolites show the highest glucose conversions, with values 9

ACCEPTED MANUSCRIPT >35%, whereas for Y and FER zeolites 28 and 27% were achieved respectively. The final activities reached by the magnesium-impregnated zeolites are affected by various factors: (i) the extent of the homogeneous reaction taking place due to Na and Mg leaching [31], (ii) preferential location of the magnesium oxide in the structure, (iii) strength of basic sites and (iv) place where the reaction occurs. The significance of the homogeneous reaction was

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analysed for all the zeolites by performing a pre-leaching of the samples through contact with water at 100ºC for 2h under nitrogen and stirring. The liquid phase was then separated from the catalyst by centrifugation, glucose was added and another 2h of reaction were carried out at 100ºC. Table 4 gives the concentration of Na and Mg found in the recovered liquid phase

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and the homogeneous glucose conversions for the magnesium-promoted zeolites.

Considering large pore zeolites after impregnation with magnesium (Y, MOR, BEA), MOR

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is the zeolite presenting the highest amount of magnesium oxide on its surface, while for BEA and especially for the Y zeolite, magnesium is mainly located inside the pores, as previously discussed above when analysing N2 adsorption data (Table 2). Therefore, this means that magnesium oxide on MOR would be more easily available than for the other large pore zeolites, for which diffusion into the structure firstly needs to occur. Thus, this could be one of the important reasons for the good activity of the MOR zeolite. Nevertheless, a similar

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glucose conversion was obtained with the BEA, which could be explained by the fact this zeolite is the only one presenting strong basic sites (Fig. 3). The higher strength of the basic sites on the BEA zeolite is also responsible for its much lower fructose selectivity compared with the other wide pore structures, as discussed further below. Furthermore, it can be seen

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that higher homogeneous conversions are obtained with the MOR and BEA zeolites than with the Y zeolite (Table 4). Despite both metals contribution for the homogeneous reaction, this

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is possibly the result of the higher Mg leaching for MOR and BEA. The fact that Mg is more easily released from these structures could also possibly contribute for their higher observed activity. In the case of the Y zeolite, MgO entities are mainly located in the microporosity (Table 2), and so they are not as easily extracted as for the other large pore zeolites. For the intermediate pore zeolites (ZSM-5 and FER), reaction should mainly take place on their surface as glucose molecules are too voluminous to diffuse into their porous system. Comparing ZSM-5 and FER external surface areas after magnesium impregnation (Table 2), this is higher for the ZSM-5 zeolite, which signifies that more surface is available for the transformation, consistent with the observed higher activity. In addition, ZSM-5 is the zeolite with the highest total degree of leaching and FER the lowest, the homogeneous contribution 10

ACCEPTED MANUSCRIPT being approximately 47 and 25% of the total conversion, respectively (Table 4). This could be related to the stronger interaction between magnesium oxide and FER due to its smallest relative particle size (Table 2). In general the most active catalysts are the ones presenting the lowest fructose selectivity, except for the ZSM-5 zeolite (Fig. 4). The lowest fructose selectivity was found for the BEA

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zeolite (64%), which is probably due to the presence of stronger basic sites, as mentioned above, since higher basicity is responsible for fructose further transformation [31]. However, besides the basicity, the ability of glucose to penetrate the zeolite structure also has an influence on the fructose selectivity. If glucose molecules are not able to go inside the zeolite

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structure, reactions occur mostly on the external surfaces with the result that fructose would be more easily released from the catalyst. This is the most likely explanation for the high

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fructose selectivity observed for the 5%MgNaZSM-5 zeolite (88%), even if this is one of the most active catalysts. For ZSM-5, it is also necessary to take into account the much higher degree of Mg leaching, which can be expected to impact on the selectivity as homogeneous fructose selectivity is always higher than 94%. In the case of MOR, an intermediate selectivity was reached (76%). Although for MOR the reactions might mainly take place on the surface due to a degree of pore blockage, some glucose molecules probably can still enter

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the structure, causing selectivity to decrease.

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3.3 Deactivation of catalysts

Similarly to what was observed previously for the 5%MgNaY zeolite [31], deactivation over

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the other Mg-impregnated zeolites during glucose isomerisation into fructose was also found to proceed through carbonaceous materials accumulation and Na and Mg removal from the zeolite (Table 5). During a normal reaction run the amounts of Na and Mg removed can additionally be influenced by the chelating effect of glucose and/or fructose [31, 49], so that a significant increase of both Na and Mg leaching for all the zeolites can be seen when compared to a pre-leaching run (Table 4). This means that the homogeneous contribution can be even more pronounced in all the cases. Moreover, concerning carbon accumulation (Table 5), the lower fructose selectivities found for both BEA and MOR zeolites could be consistent with their higher coke retention in comparison with the Y zeolite. Coke accumulation in

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ACCEPTED MANUSCRIPT smaller pore zeolites is not as significantly different, and indeed selectivities are also comparable. With the purpose of further evaluating the impact of deactivation on the activity and selectivity, the catalysts were used in three consecutive runs, which were carried out always at 100ºC for 2h. After each run, the catalyst was recovered from the liquid product by

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centrifugation, washed with deionised water and placed again into the reactor for the following run. Glucose conversion and fructose selectivity for all the zeolite structures in the different reaction runs is given in Table 6. As previously shown for the Y zeolite [31], a decrease of the glucose conversion with the number of runs can be observed. Deactivation is

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slightly different between the structures, decreasing in the following order: ZSM-5 > BEA > Y > MOR > FER. This is in agreement with the total level of Na and Mg leaching, except for

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the Y zeolite. Deactivation could also be affected by the occurrence of some metal oxide particles agglomeration due to the presence of water. According to DRS-UV-Vis spectroscopy, this effect appears to be smaller for the Y zeolite, which could be due to the preferential location of MgO in the micropores (Table 2). The Y zeolite was the only sample for which a huge shift of the MgO UV-Vis band to lower wavelengths (< 200 nm – not visible in the spectra) was not detected.

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The evolution of the fructose selectivity with the number of runs is also different among the zeolite structures (Table 6). A more significant reduction of the fructose selectivity was found for the FER and MOR zeolites, while it only slightly decreases for the BEA zeolite and increases for the ZSM-5 and Y samples. Selectivity to fructose can improve due to an

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enhancement of the textural properties [31]. In general, all the magnesium-impregnated zeolites register an increase of the mesoporous volume and external surface area after three

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reaction runs, when compared to the fresh samples (Table 2), meaning that this cannot be the main reason for the different selectivity trends. Thus, one could conclude that observed selectivity trend might be a result of a balance between the extent of Na and Mg leaching. For instance, FER and ZSM-5 zeolites have the same degree of Mg leaching (about 120 ppm in the first run, Table 6), but selectivity for fructose is improved with the number of runs for the ZSM-5 and reduced for the FER zeolite. This could be the result of the much different Na removals: 403 ppm for ZSM-5 (89.8% of the initial amount) and 45 ppm for FER (19.4% of the initial amount). In fact, the number of acid sites increases with the Na removal, as Na can be replaced by H3O+ in aqueous medium [31]. Moreover, it can also be seen that the zeolites

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ACCEPTED MANUSCRIPT for which selectivity decreases as a function of the number of runs have significantly higher carbon retention after the three reaction runs (Table 6).

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3.4 Regeneration of the catalysts

The ability of the magnesium-impregnated zeolite samples to be re-used was assessed by carrying out a calcination step under air at 600ºC for 1h after the first run. The calcined samples were used in a second isomerisation run at 100ºC for 2h. Contrary to what is

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observed for the Y zeolite [31], none of the other zeolite structures proved capable of recovering as much their activity upon oxidative treatment at high temperature (Table 7),

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even though coke was successfully removed and some magnesium redistribution might have occurred [31]. One possible cause for the observed loss of activity for the MOR, BEA, ZSM5 and FER could be that the fraction of magnesium leached during the first run was so high than even with some likely magnesium redistribution, it was not possible to re-activate the catalyst. This seems to be the case for ZSM-5 zeolite (Table 7), as its textural parameters are not significantly different without and with regeneration (Table 2). On the other hand, a

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significant depletion of the textural properties can be seen for MOR, BEA and FER zeolites after the second run with regeneration (Table 2), which cannot be related only to the coke formation (Table 7). No major differences in the structure were found when comparing the spent samples after 3 runs without regeneration to the regenerated catalysts (Table 2),

EP

meaning that there is no additional damage to the zeolite structures due to the regeneration. In addition, band gap energies for the regenerated samples after catalytic test were also

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calculated, as an additional increase of the size of the Mg particles could block the porosity of the zeolites. However, MgO particles on the regenerated samples are not bigger than after 3 runs without regeneration. Therefore, the decrease of the textural properties for MOR, BEA and FER zeolites could possibly be due to the combination of the increase in the MgO entities size and a possible displacement of the particles in the structure because of the oxidative treatment. This appears to happen mainly for samples with particles located at the external surface, as confirmed by the very pronounced decrease of the external surface area for the MOR, BEA and FER zeolites comparing the results without and with regeneration, causing a total blockage of the microporosity (Table 2).

13

ACCEPTED MANUSCRIPT Concerning fructose selectivity, the only significant difference when comparing with the catalytic tests without regeneration is the enhancement of selectivity seen for the MOR and BEA zeolites. This could be a result of the complete loss of the microporous volume observed for these catalysts after the high temperature treatment (Table 2). In fact, with the microporosity totally blocked, the reaction might mostly take place on the available external

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surface area instead of inside the zeolite structures, as it would be normal for the large pore zeolites. This would decrease the extent of the fructose further transformation and so improve the selectivity.

Thus, overall Y zeolite appears to be the most promising structure for the glucose

SC

isomerisation into fructose, due to its activity, and unique ability to be regenerated, apparent higher resistance for MgO particles agglomeration, and a consistent improvement of fructose

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selectivity after consecutive reaction runs.

4. Conclusion

The use of different zeolite structures impregnated with magnesium for glucose isomerisation

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into fructose has been analysed. It was shown that dispersion, location and size of the MgO particles depend on the type of zeolite used as support. A better MgO dispersion was achieved for the Y, BEA and ZSM-5 zeolites with their textural properties being less affected

EP

by magnesium impregnation. MOR and FER zeolites showed a very heterogeneous magnesium distribution and a much greater reduction of the micro- and mesoporous volumes and external surface area. Based on the UV-Vis spectroscopy, size of MgO entities was

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inferred, increasing in the order: FER < Y < BEA < ZSM-5 < MOR. The performances of the catalysts were affected by the homogeneous contribution due to Na and Mg leaching, and the properties of the zeolites after impregnation with magnesium. MOR, BEA and ZSM-5 zeolites were found to have the best glucose conversions (>35%), while relatively moderate conversions were achieved for the Y and FER zeolites. In general, high activity is associated to lower fructose selectivity, except for the ZSM-5 zeolite. For the smaller pore zeolites, such as ZSM-5, the reaction might occur mainly on the external surface, from where fructose is more easily released to the fluid reducing the possibility of secondary transformations.

14

ACCEPTED MANUSCRIPT Consecutive reaction runs without regeneration showed that all the catalysts lose their activity due to Na and Mg leaching, an increase of MgO particle size and coke formation. Resistance to particle agglomeration was revealed to be greater for the Y zeolite. In addition, the Y zeolite was the only catalysts for which glucose conversion was almost completely recovered through regeneration under air at high temperature with an improvement of selectivity to

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fructose. This study showed clearly that the zeolite structure used as support for the MgO can have a significant impact on the glucose conversion and fructose selectivity achieved, as well as on the degree of deactivation and regeneration. Overall, Y zeolite was demonstrated to be the

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most favourable support for the glucose isomerisation into fructose, as it has a moderate

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activity, good fructose production and ability to be regenerated.

Acknowledgements

This work was performed with financial support from EPSRC(UK) under grant

AC C

EP

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EP/K014749/1. We also thank Chak Kiu Jason Tam for his help with activity measurements.

15

ACCEPTED MANUSCRIPT References

[1]

G.W. Huber, S. Iborra, A. Corma, Synthesis of Transportation Fuels from Biomass:  Chemistry, Catalysts, and Engineering, Chem. Rev. 106 (2006) 4044-4098.

[2]

V. Menon, M. Rao, Trends in bioconversion of lignocellulose: Biofuels, platform

S.K. Maity, Opportunities, recent trends and challenges of integrated biorefinery: Part I, Renew. Sust. Energ. Rev. 43 (2015) 1427-1445.

[4]

D. Mohan, C.U. Pittman Jr., P.H. Steele, Pyrolysis of Wood/Biomass for Bio-oil:  A Critical Review, Energy Fuels 20 (2006) 848-889.

F. Carvalheiro, L. Duarte, F.M. Gírio, Hemicellulose biorefineries: a review on biomass

M AN U

[5]

SC

[3]

RI PT

chemicals & biorefinery concept, Prog. Energ. Comb. Sci. 38 (2012) 522-550.

pretreatments, J. Sci. Ind. Res. 67 (2008) 849-864. [6]

Use of Sugars and Other Carbohydrates in the Food Industry, Vol. 12, American Chemical Society, 1955.

[7]

P. Hull, Glucose Syrops: Technology and Applications, First Edition, Wiley-Blackwell,

[8]

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United Kingdom, 2010.

H.G. Garg, M.K. Cowman, C.A. Hales, Carbohydrate Chemistry, Biology and Medical applications, First Edition, Elsevier, Oxford, 2008.

[9]

M. Aresta, A. Dibenedetto, F. Dumeignil, Biorefinery: Biomass to Chemicals and

EP

Fuels, De Gruyter, Germany, 2012.

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[10] Bio-based Chemicals: Value Added Products from Biorefineries, IEA Bioenergy, 2012. [11] J.N. Chheda, G.W. Huber, J.A. Dumesic, Liquid-Phase Catalytic Processing of Biomass-Derived Oxygenated Hydrocarbons to Fuels and Chemicals, Angew. Chem. Int. Ed. 46 (2007) 7164-7183. [12] X. Qian, Mechanisms and Energetics for Brønsted Acid-Catalyzed Glucose Condensation, Dehydration and Isomerization Reactions, Top. Catal. 55 (2012) 218226. [13] R.-J. van Putten, J.C. van der Waal, E. de Jong, C.B. Rasrendra, H.J. Heeres, J.G. de Vries, Hydroxymethylfurfural, A Versatile Platform Chemical Made from Renewable Resources, Chem. Rev. 113 (2013) 1499-1597. 16

ACCEPTED MANUSCRIPT [14] I. Delidovich, R. Palkovits, Catalytic Isomerization of Biomass-Derived Aldoses: A Review, ChemSusChem 9 (2016) 547-561. [15] T. Buntara, S. Noel, P.H. Phua, I. Melián-Cabrera, J.G. de Vries, H.J. Heeres, Caprolactam

from

Renewable

Resources:

Catalytic

Conversion

of

5-

7087.

RI PT

Hydroxymethylfurfural into Caprolactone, Angew. Chem. Int. Ed. 50 (2011) 7083-

[16] V.J. Jensen, S. Rugh, Industrial-scale production and application of immobilized glucose isomerase, Meth. Enzym. 136 (1987) 356-370.

[17] K. Buchholz, J. Seibel, Industrial carbohydrate biotransformations, Carbohydr. Res.

SC

343 (2008) 1966-1979.

[18] C. Moreau, R. Durand, A. Roux, D. Tichit, Isomerization of glucose into fructose in the

M AN U

presence of cation-exchanged zeolites and hydrotalcites, Appl. Catal. A: Gen. 193 (2000) 257-264.

[19] J. Lecomte, A. Finiels, C. Moreau, Kinetic Study of the Isomerization of Glucose into Fructose in the Presence of Anion-modified Hydrotalcites, Starch/Stärke 54 (2002) 7579.

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[20] C. Moreau, J. Lecomte, A. Roux, Determination of the basic strength of solid catalysts in water by means of a kinetic tracer, Catal. Commun. 7 (2006) 941-944. [21] S. Yu, E. Kim, S. Park, I.K. Song, J.C. Jung, Isomerization of glucose into fructose

EP

over Mg–Al hydrotalcite catalysts, Catal. Commun. 29 (2012) 63-67. [22] R. Shukla, X.E. Verykios, R. Mijtharasan, Isomerization and hydrolysis reactions of

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important disaccharides over inorganic heterogeneous catalysts, Carbohydr. Res. 143 (1985) 97-106.

[23] L. Lv, X. Guo, P. Bai, S. Zhao, Isomerization of Glucose into Fructose and Mannose in Presence of Anion-Exchanged Resins, Asian J. Chem. 27 (2015) 2774-2778. [24] R.O.L. Souza, D.P. Fabiano, C. Feche, F. Rataboul, D. Cardoso, N. Essayem, Glucose– fructose isomerisation promoted by basic hybrid catalysts, Catal. Today 195 (2012) 114-119.

17

ACCEPTED MANUSCRIPT [25] A.A. Marianou, C.M. Michailof, A. Pineda, E.F. Iliopoulou, K. S. Triantafyllidis, A.A. Lappas, Glucose to Fructose Isomerization in Aqueous Media over Homogeneous and Heterogeneous Catalysts, ChemCatChem 8 (2016) 1100-1110. [26] S. Lima, A.S. Dias, Z. Lin, P. Brandão, P. Ferreira, M. Pillinger, J. Rocha, V. Calvino-

solid bases, Appl. Catal. A: Gen. 339 (2008) 21-27.

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Casilda, A.A. Valente, Isomerization of d-glucose to d-fructose over metallosilicate

[27] M. Moliner, Y. Román-Leshkov, M.E. Davis, Tin-containing zeolites are highly active catalysts for the isomerization of glucose in water, PNAS 107 (2010) 6164-6168.

[28] C.M. Lew, N. Rajabbeigi, M. Tsapatsis, Tin-containing zeolite for the isomerization of

SC

cellulosic sugars, Microp. Mesop. Mater. 153 (2012) 55-58.

[29] G. Li, E.A. Pidko, E.J.M. Hensen, Synergy between Lewis acid sites and hydroxyl

M AN U

groups for the isomerization of glucose to fructose over Sn-containing zeolites: a theoretical perspective, Catal. Sci. Technol., 4 (2014) 2241-2250. [30] S. Saravanamurugan, M. Paniagua, J.A. Melero, A. Riisager, Efficient Isomerization of Glucose to Fructose over Zeolites in Consecutive Reactions in Alcohol and Aqueous Media, J. Am. Chem. Soc. 135 (2013) 5246-5249.

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[31] I. Graça, D. Iruretagoyena, D. Chadwick, Glucose isomerisation into fructose over magnesium-impregnated NaY zeolite catalysts, Appl. Catal. B: Environ. 206 (2017) 434-443.

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[32] M.M.J. Treacy, J.B. Higgins, Collection of simulated XRD powder patterns for zeolites, Elsevier, 2001.

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[33] M. A. Aramendía, J. A. Benítez, V. Borau, C. Jiménez, J.M. Marinas, J.R. Ruiz, F. Urbano, Characterization of Various Magnesium Oxides by XRD and 1H MAS NMR Spectroscopy, J. Solid State Chem. 144 (2009) 25-29. [34] A.A. Rownaghi, R.L. Huhnke, Producing Hydrogen-Rich Gases by Steam Reforming of Syngas Tar over CaO/MgO/NiO Catalysts, ACS Sustainable Chem. Eng. 1 (2013) 80-86. [35] I.A. Bakare, O. Muraza, M. Yoshioka, Z.H. Yamania, T. Yokoib, Conversion of methanol to olefins over Al-rich ZSM-5 modified with alkaline earth metal oxides, Catal. Sci. Technol. (2016) 7852-7859.

18

ACCEPTED MANUSCRIPT [36] Y. Sugi, H. Tamada, A. Kuriki, K. Komura, Y. Kubota, S. Joseph, A. Chokkalingam, M. El. Newehy, S.S. Al-Deyab, H.-G. Jang, J.-H Kim, G. Seo, A. Vinu, Alkaline Earth Metal Modified H‑Mordenites. Their Catalytic Properties in the Isopropylation of Biphenyl, Ind. Eng. Chem. Res. 54 (2015) 12283-12292. [37] A. Nemera Emana, S. Chand, Alkylation of benzene with ethanol over modified

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HZSM-5 zeolite catalysts, Appl. Petrochem. Res. 5 (2015) 121-134.

[38] C. Chen, Q. Zhang, Z. Meng, C. Li, H. Shan, Effect of magnesium modification over H-ZSM-5 in methanol to propylene reaction, Appl. Petrochem. Res. 5 (2015) 277-284.

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[39] B. Zeifert, J.C. Villegas, J. Salmonesa, J.L. Contreras, I. Cordova, A. Romero Serrano, T. Vázquez, Synthesis and characteristics of magnesium inserted on porous silica

M AN U

materials by mechanical alloying, Materials Today: Proceedings 3 (2016) 2748-2754. [40] K.R. Nemade, S.A.Waghuley, Synthesis of MgO Nanoparticles by Solvent Mixed Spray Pyrolysis Technique for Optical Investigation, Intern. J. Metals (2014), Article ID 389416.

[41] N.F. Chayeda, N. Badara, R. Rusdia, N. Kamarudin, N. Kamarulzaman, Optical Band Gap Energies of Magnesium Oxide (MgO) Thin Film and Spherical Nanostructures,

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AIP Conf. Proc. 1400 (2011) 328-332.

[42] L.A. Palacio, E.R. Silva, R. Catalão, J.M. Silva, D.A. Hoyos, F.R. Ribeiro, M.F. Ribeiro, Performance of supported catalysts based on a new copper vanadate-type

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precursor for catalytic oxidation of toluene, J. Hazard. Mat. 153 (2008) 628-634. [43] N. Badar, N.F. Chayed, R. Rusdi, N. Kamarudin, N. Kamarulzaman, Band Gap

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Energies of Magnesium Oxide Nanomaterials Synthesized by the Sol-gel Method, Adv. Mat. Res. 545 (2012) 157-160. [44] M.Fernández-Garcia, J.A. Rodriguez, Metal Oxide Nanoparticles, Chemistry Department, Brookhaven National Laboratory, BNL-79479-2007-BC, 2007.

[45] R.M. Barrer, D.A. Harding, A. Sikan, Zeolite Sorbents: Modification by Impregnation with Salt, J.C.S. Faraday I, 76 (1979) 180-195. [46] H. Tsuji, F. Yagi, H. Hattori, H. Kita, Characterization of Basic Sites on Fine Particles of Alkali and Alkaline Earth Metal Oxides in Zeolites, Stud. Surf. Sci. Catal. 75 (1993) 1171-1183. 19

ACCEPTED MANUSCRIPT [47] W. Dong, Z. Shen, B. Peng1, M. Gu, X. Zhou, B. Xiang, Y. Zhang, Selective Chemical Conversion of Sugars in Aqueous Solutions without Alkali to Lactic Acid Over a ZnSn-Beta Lewis Acid-Base Catalyst, Scientific Reports 6, Article number: 26713 (2016). [48] D. Barthomeuf, Framework induced basicity in zeolites, Micropor. Mesopor. Mat. 66 (2003) 1-14.

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[49] Y. Ye, Q. Liu, J. Wang, Influence of saccharides chelating agent on particle size and magnetic properties of Co2Z hexaferrite synthesized by sol–gel method, J. Sol-Gel Sci.

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Technol. 60 (2011) 41-47.

20

ACCEPTED MANUSCRIPT Table 1. Na and Mg contents on the magnesium-impregnated zeolites determined by ICPOES and % of Na exchange. Na content (%)

% Na exchangea

Mg content (%)

5%MgNaY

7.26

77

5.02

5%MgNaMOR

2.40

74

4.90

5%MgNaBEA

1.59

63

4.54

5%MgNaZSM-5

2.24

72

5%MgNaFER

1.14

32

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Catalyst

4.78

AC C

EP

TE D

M AN U

SC

5.11

21

ACCEPTED MANUSCRIPT Table 2. Crystallinity, band gap energy and textural properties of the catalysts. Cryst.

Catalyst

(%)

Band gap energy

Vmicro 3

Vmeso

Sext

(cm /g) (cm /g) (m2/g)

(eV)

3

100

-

0.323

0.043

48

5%MgNaY

92

5.56

0.243

0.045

49

82

5.72

After 2 runs with intermediate regeneration

75

5.68

NaMOR

100

-

5%MgNaMOR

98

5.67

After 3 runs

67

n.v.b

a

a

60

NaBEA

100

109

0.251

0.158

93

0.168

0.155

61

0.056

0.069

32

0.052

0.215

131

5.77

0

0.235

34

-

0.149

0.556

247

M AN U

After 2 runs with intermediate regeneration

0.171

SC

After 3 runs

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NaY

97

5.59

0.140

0.660

208

51

n.v. b

0.083

0.655

358

After 2 runs with intermediate regenerationa

48

5.73

0

0.468

180

NaZSM-5

100

-

0.130

0.152

58

92

5.63

0.119

0.095

60

51

n.v.

b

0.077

0.120

108

After 2 runs with intermediate regenerationa

70

5.70

0.072

0.139

127

NaFER

100

-

0.120

0.178

78

91

5.47

0.082

0.074

32

55

n.v.

b

0.070

0.133

76

n.v.

b

0

0.121

14

5%MgNaBEA After 3 runs

5%MgNaZSM-5

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After 3 runs

5%MgNaFER

EP

After 3 runs

a

After 2 runs with intermediate regeneration b

Spent sample with coke UV-Vis band not visible in the spectral range 200-800 nm

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a

78

22

ACCEPTED MANUSCRIPT Table 3. CO2 adsorption capacity for the magnesium-impregnated zeolites, with activation and adsorption temperatures of 580 and 50ºC respectively. CO2 (µmol/g)a

5%MgNaY

115

5%MgNaMOR

61

5%MgNaBEA

91

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Catalyst

70

5%MgNaZSM-5

37

5%MgNaFER a

Error in the CO2 adsorption capacity is on average ± 6 µmol/g

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Table 4. Sodium and magnesium leaching from the magnesium-impregnated catalysts after

Mg leaching

ppma (%)b

ppma (%)b

5%MgNaY

113 (7.8)

2.0 (0.2)

9.1

5%MgNaMOR

42 (8.8)

30 (3.0)

13.2

5%MgNaBEA

106 (29.6)

43 (4.7)

17.0

5%MgNaZSM-5

200 (44.6)

29 (3.0)

18.2

17 (7.4)

12 (1.2)

6.8

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5%MgNaFER

glucose conversion (%)

Concentration of Na or Mg in the liquid product (5 mL) after reaction. Percentage of Na and Mg removed considering their amount in each sample.

EP

b

Homogeneous

Na leaching

Catalyst

a

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contact with liquid water at 100ºC for 2h and homogeneous glucose conversions.

Table 5. Sodium and magnesium leaching and coke retention on the magnesium-impregnated

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zeolites after 2h of reaction at 100ºC. Na leaching

Mg leaching

Coke

ppma (%)b

ppma (%)b

(wt.%)

5%MgNaY

527 (36.3)

75 (7.5)

3.6

5%MgNaMOR

231 (48.1)

141 (14.4)

6.8

5%MgNaBEA

303 (84.6)

68 (7.5)

10.4

5%MgNaZSM-5

406 (90.4)

199 (20.9)

3.7

5%MgNaFER

28 (12.3)

140 (13.7)

5.0

Catalyst

a b

Concentration of Na or Mg in the liquid product (5 mL) after reaction. Percentage of Na and Mg removed considering their amount in each sample.

23

ACCEPTED MANUSCRIPT Table 6. Glucose conversion, fructose selectivity, sodium and magnesium leaching and coke retention for the magnesium-impregnated zeolites, after consecutive runs at 100°C after 2h, without regeneration of the catalyst.

5%MgNaBEA

5%MgNaZSM-5

5%MgNaFER

selectivity

(%)

(%)

1

28.2

82.1

539 (37.1)

72 (7.2)

-

2

17.3

88.2

102 (44.1)

12 (8.4)

-

3

11.5

97.3

62 (48.4)

14 (9.8)

2.7

1

37.8

68.8

151 (31.6)

116 (11.8)

-

2

21.5

59.1

35 (38.8)

24 (14.2)

-

3

16.7

62.5

11 (41.1)

16 (15.9)

12.1

1

37.5

65.9

310 (86.6)

50 (5.5)

-

2

19.5

3

11.3

1

36.9

2

14.8

3

9.0

1

25.5

2 3

ppma (%)b

ppma (%)b

(wt.%)

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Coke

63.7

46 (99.6)

45 (10.4)

-

62.4

13 (103)

45 (15.3)

16.3

84.1

403 (89.8)

122 (12.8)

-

93.0

32 (97.0)

18 (14.7)

-

96.5

11 (99.5)

16 (16.4)

3.8

87.3

45 (19.4)

119 (11.7)

-

19.2

64.5

16 (26.6)

25 (14.1)

-

13.9

57.5

11 (31.3)

14 (15.5)

9.8

Concentration of Na or Mg in the liquid product (5 mL) after reaction. Cumulative percentage of Na and Mg removed considering their initial amount in each sample.

EP

b

Mg leaching

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a

Na leaching

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5%MgNaMOR

conversion

Run

M AN U

5%MgNaY

Fructose

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Catalyst

Glucose

24

ACCEPTED MANUSCRIPT Table 7. Glucose conversion, fructose selectivity, sodium and magnesium leaching and coke retention for the magnesium-impregnated zeolites, after consecutive runs at 100°C after 2h, with intermediate regeneration of the catalyst.

5%MgNaBEA

5%MgNaZSM-5

5%MgNaFER

(%)

1

28.3

85.5

543 (37.4)

78 (7.8)

-

2

26.9

93.1

154 (48.0)

72 (14.9)

2.3

1

36.6

70.7

229 (47.6)

100 (10.2)

-

2

23.5

75.3

48 (57.6)

39 (14.1)

8.4

1

37.4

66.4

282 (78.8)

52 (5.7)

-

2

17.7

76.0

56 (94.4)

69 (13.2)

8.5

1

36.9

84.9

416 (92.7)

137 (14.3)

-

2

19.6

1

29.5

2

22.0

Mg leaching

Coke

ppma (%)b

ppma (%)b

(wt.%)

91.4

61 (106)

47 (19.2)

5.7

83.1

34 (14.7)

133 (13.0)

-

72.7

35 (29.8)

32 (16.0)

8.4

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Concentration of Na or Mg in the liquid product (5 mL) after reaction. Cumulative percentage of Na and Mg removed considering their initial amount in each sample.

EP

b

(%)

Na leaching

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a

selectivity

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5%MgNaMOR

conversion

Run

SC

5%MgNaY

Fructose

M AN U

Catalyst

Glucose

25

ACCEPTED MANUSCRIPT Figure captions

Fig. 1: XRD diffraction patterns for the Na-exchanged (grey lines) and 5 wt.% magnesiumimpregnated zeolites (black lines). Fig. 2: UV-Vis spectra for the magnesium-impregnated zeolites: 5%MgNaY (black line),

black line) and 5%MgNaFER (dashed grey line).

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5%MgNaMOR (dark grey line), 5%MgNaBEA (light grey line), 5%MgNaZSM-5 (dashed

Fig. 3: CO2-TPD results for the magnesium-impregnated zeolites with activation and adsorption temperatures of 580 and 50ºC respectively: 5%MgNaY (black line),

black line) and 5%MgNaFER (dashed grey line).

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5%MgNaBEA (dark grey line), 5%MgNaZSM-5 (light grey line), 5%MgNaMOR (dashed

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Fig. 4: Glucose conversion (black), fructose yield (grey) and fructose selectivity (light grey)

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for the magnesium-impregnated zeolites obtained at 100°C after 2h.

26

ACCEPTED MANUSCRIPT

Fig. 1

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FER

Intensity (a.u.)

ZSM-5

5

10

15

20

25

30

M AN U

SC

BEA

35

40

45

50

55

60

MOR

Y

65

70

75

80

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EP

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2θ°

27

ACCEPTED MANUSCRIPT

200

210

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SC

F(R) (a.u.)

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Fig. 2

220

230

240

250

260

AC C

EP

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Wavelength (nm)

28

ACCEPTED MANUSCRIPT

50

125

200

275

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SC

CO2 detector signal (a.u.)

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Fig. 3

350

425

500

575

650

725

800

AC C

EP

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Temperature (°C)

29

ACCEPTED MANUSCRIPT

Fig. 4

100 87.5

64.3

%

60 40

38.8

37.3 28.2 23.1

28.4 25.0

38.3 33.6

27.2 23.3

M AN U

20

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76.0

SC

80

85.7

82.1

AC C

EP

TE D

0

30

ACCEPTED MANUSCRIPT Research highlights

- Various Mg-promoted zeolite structures studied for glucose isomerisation to fructose. - MOR, BEA and ZSM-5 achieve higher glucose conversion (>35%) than Y and FER (27-

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28%). - Fructose selectivity is greater than 80% for Y, ZSM-5 and FER zeolites. - Only Y zeolite recovers its initial activity after regeneration under air.

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- Overall Y zeolite revealed to be the best support for the reaction.