Structure–performance correlations of Mg–Al hydrotalcite catalysts for the isomerization of glucose into fructose

Journal of Catalysis 327 (2015) 1–9

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Structure–performance correlations of Mg–Al hydrotalcite catalysts for the isomerization of glucose into fructose Irina Delidovich 1, Regina Palkovits ⇑ Chair of Heterogeneous Catalysis and Chemical Technology, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany

a r t i c l e

i n f o

Article history: Received 11 February 2015 Revised 2 April 2015 Accepted 8 April 2015

Keywords: Hydrotalcite Glucose Fructose Isomerization Solid base

a b s t r a c t Mg–Al hydrotalcites with different Mg-to-Al molar ratio, texture, and morphology were studied as catalysts for the isomerization of glucose into fructose. The properties of the hydrotalcites were tuned by varying the preparation procedure including pH during co-precipitation, ageing temperature, and solvent. The catalysts were characterized by ICP-OES, N2 sorption, sorption of acrylic acid, XRD, TG-DSC, and SEM. Tuning the conditions of synthesis enables controlling properties such as crystallite size, dispersion of primary particles, and morphology of the agglomerates. These structural parameters of the hydrotalcites critically influence their basicity, which in turn predominantly determines the catalytic performance. The best fructose yields of up to 30% with 89% selectivity were obtained over hydrotalcites synthesized by co-precipitation in (i) an aqueous medium at pH 10 and (ii) an aqueous-ethanol medium at pH 9.5. These synthesis conditions enabled materials with optimal textural properties and the highest amount of accessible basic centres. Although some leaching of magnesium occurred caused by lactic acid formed as by-product of carbohydrate degradation, the hydrotalcites could be recycled without loss of activity and selectivity. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction A rational utilization of renewable resources for production of fuels and chemicals is a challenge of global interest owing to gradual depletion of fossil feedstocks. Lignocellulosic biomass outstands other renewables, since it is cheap, abundant, and does not compete with food production. The strategy of ‘‘platform molecules’’ comprehends a controlled depolymerization of the biopolymers constructing lignocellulose, including, cellulose, hemicelluloses, and lignin, with further upgrading of the monomers into value-added chemicals [1–3]. This approach enables to conserve the high degree of functionality available in renewable feedstock. However, optimization of the catalytic processes necessary for valorization of lignocellulose is still required. In this study, we consider a crucial step of cellulosic biorefineries, namely the isomerization of glucose into fructose. Glucose can be produced via hydrolysis of cellulose, which is the main part of lignocellulosic biomass. A further transformation of glucose is hampered by the high stability of this compound. Therefore, many proposed ⇑ Corresponding author. E-mail addresses: [email protected] (I. Delidovich), palkovits@ itmc.rwth-aachen.de (R. Palkovits). 1 Fax: +49 241 8022185. http://dx.doi.org/10.1016/j.jcat.2015.04.012 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

strategies of cellulose valorization regard not glucose but fructose as a substrate for synthesis of fuels and chemicals [1–5]. Fructose is currently produced biotechnologically from glucose and starch as a component of sweet syrups [6,7]. Nevertheless, development of a solid catalyst for this process will clearly facilitate a large-scale industrial operation. A number of efforts have been made investigating potential solid catalysts for fructose production. Solid bases [8–14] and solid Lewis acids [15–19] are currently the most suggested catalysts for isomerization. The yields of fructose obtained in aqueous solution in the presence of a solid catalyst are ca. 30%. The yield of fructose is restricted owing to reversibility of the isomerization (Keq  1.45 at 110 °C) [15] as well as high reactivity of fructose that undergoes subsequent transformations in the presence of the catalysts [14]. However, the successive destruction of fructose is significantly suppressed under continuous operation conditions [14]. The highest yield of fructose reported to date (55%) is obtained using zeolites as catalysts and a two-step synthesis procedure utilizing methanol and water as solvents [20]. Magnesium–aluminium hydrotalcites (HTs) are solid bases, which exhibit very promising catalytic activity as well as good stability under the required reaction conditions [8–14]. High yields of fructose up to 35–40% can be obtained in the presence of HTs using DMF as solvent [11,12,21]. However, an isomerization

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of glucose in aqueous solution appears more economically feasible and environmentally benign. Fructose yields in aqueous phase over HTs can reach up to 40% when based on 1 wt.% solution of glucose [14], but decrease to ca. 25% if a more concentrated 10 wt.% solution of glucose is applied [8,14]. HTs are layered double hydroxides with the general formula [Mg1xAlx(OH)2]x+(Ax/nn)mH2O where Mg2+ ions are partially substituted by Al3+ in the brucite-type layers. An is an interlayer anion, x (0.17 < x < 0.34) refers to the fraction of aluminium, and m denotes water of crystallization [22]. Recent advances in the synthesis of HTs demonstrated that the chemical composition as well as textural and morphological properties might be fine-tuned [23]. Nevertheless, the current literature data contain only scarce information on design of HT catalysts for isomerization of glucose into fructose. For example, a detrimental effect of precalcination of HTs on fructose selectivity has been shown [9,14]. It is known that hydrated HTs possess mainly weak basic cites, that is, via the interlayer anions of HTs [24–26]. Upon calcination, HTs transform into mixed oxides MgO–Al2O3 with medium–strong Lewis basic O2—Mn+ pairs and isolated O2 as strong basic sites [26]. In fact, weak basic sites of hydrated HTs are significantly more selective for synthesis of fructose than medium–strong and strong basic sites of the calcined materials. Additionally, studies confirmed that the catalytic activity for the isomerization of glucose can be  increased by substitution of CO2 3 by OH anions in the interlayer gallery [9,11]. However, catalysts containing OH as a compensation anion are not stable but undergo a rapid consumption of CO2 from air and convert into the carbonate form [9,23,27]. Lee et al. reported on improved catalytic activity of HTs upon exfoliation of HTs rehydrated with assistance of sonication. It was explained by exfoliation of the HTs layers and formation of smaller crystallites upon vertical breaking, which increases the surface concentration of basic sites [12]. Summing up the literature data, HTs in carbonate or rehydrated form are catalytically active for isomerization of glucose into fructose. The interlayer anions of the HT materials demonstrate basic properties and serve as active species for the isomerization [12]. Co-precipitation is very often used for preparation of HTs as a simple and express synthetic technique [22]. It is well known that variation of the precipitation conditions enables tailoring the properties of HTs [28], which potentially allows to control the catalytic performance. However, little is known about structure–activity relationships for glucose isomerization into fructose in the presence of HTs. Therefore, this work aims at a comprehensive analysis of the influence of the preparation procedure of HTs on material properties and the associated catalytic activity and selectivity of these materials in the isomerization of glucose into fructose. For this purpose, we synthesized HTs with different properties such as (i) magnesium-to-aluminium ratio; (ii) crystallinity and structure of primary particles; (iii) morphology; (iv) textural properties; and (v) basicity. The influence of these properties on the catalytic performance was investigated.

2.2. Synthesis of the HTs The HTs were prepared by co-precipitation. Generally, two solutions were used for preparation of HTs, namely the solution A containing nitrates of aluminium and magnesium and the solution B containing NaOH and Na2CO3. Solution A was prepared by dissolution of Mg(NO3)26H2O (0.075 mol) and an appropriate amount of Al(NO3)39H2O (0.021–0.037 mol) in 100 mL of water. Solution B was an aqueous solution of NaOH (2 M) and Na2CO3 (1 M). The co-precipitation was performed in a round-bottom flask placed in an oil bath thermostated to 30 °C and equipped with an overhead stirrer. The mixture of solutions A and B was performed at a stirring rate of 440 rpm. During precipitation, the solution A was supplied to the flask with a Ritmo R05 metering pump (Fink Chem + Tec). The solution B was charged in a burette of a Titroline alpha titrator unit (Schott) and was used to keep the pH constant in the course of the precipitation. 2.2.1. Preparation of HTs-150 The series of HTs referred to as HTs-150 were synthesized following a modified protocol reported by Béres et al. [29]. Solution A was supplied into the flask containing 80 mL of water with a rate of 1.5 mL min1 at 30 °C. The pH was kept constant in the range of 8.5–9.0 by addition of solution B. The obtained precipitate was aged in a Hastelloy autoclave at 150 °C for 20 h under stirring, filtered, thoroughly washed with ca. 10L of distilled water, and dried at 80 °C overnight. 2.2.2. Preparation of HTs-9.5-RT The HTs-RT denote a set of HTs synthesized according to the method published by Yu et al. [11]. Solution A was supplied into a flask containing 80 mL of water with a rate of 1.5 mL min1 at 30 °C. The pH was kept constant in the range of 9.0–9.5 by periodic addition of solution B. The obtained precipitate was aged at room temperature for 20 h under stirring with an overhead stirrer at 440 rpm, filtered, thoroughly washed with ca. 10L of distilled water, and dried at 80 °C overnight. 2.2.3. Preparation of HTs-10-RT The HTs-10-RT were prepared according to the procedure for the HTs-9.5-RT synthesis, but the co-precipitation and ageing were performed at a pH range of 10.0–10.2.

2. Experimental

2.2.4. Preparation of EHTs-RT The EHTs-RT were prepared in an aqueous-ethanol medium. Solution A was prepared for this synthesis using ethanol and water in 1:1 volume ratio as solvent. The co-precipitation was performed in a flask thermostated at 30 °C and containing 40 mL of ethanol. Solution A was supplied to the flask with a rate of 1.5 mL min1, and solution B was used to maintain the pH in the range of 9.0– 9.5. In the end of the co-precipitation, the volume ratio of solvents VEtOH:Vwater was ca. 1.7. The precipitate was aged at room temperature for 20 h under stirring with an overhead stirrer at 440 rpm, filtered, thoroughly washed with ca. 10 L of distilled water, and dried at 80 °C overnight.

2.1. Chemicals and materials

2.3. Characterization

Glucose anhydrous for biochemistry (Reag. Ph. Eur.) and 98% sulphuric acid were provided by Merck. Fructose (Ph. Eur., BP) and Na2CO3 of >99.5% purity were supplied by Sigma Aldrich. Ethanol absolute for HPLC and NaOH of P98.8% purity were purchased from Geyer Chemsolute. Acrylic acid 99% stabilized with ca. 200 ppm 4-methoxyphenol from Alfa Aesar was used. Cyclohexane of p. A. purity was manufactured by Applichem. All the solutions were prepared in distilled water.

The content of Mg, Al, and Na in the HTs was determined by means of inductively coupled plasma-optical emission spectroscopy (ICP-OES; Spectro Analytical Instruments). Prior to the analysis of Mg and Al, 40 mg HTs were dissolved in 100 mL 1% H2SO4. 20 mg HTs were dissolved in 10 mL 1% H2SO4 prior to the determination of the sodium content. The HTs were characterized by low-temperature sorption of N2 using a Quadrasorb SI Automated Surface Area & Pore Size Analyzer after preliminary

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outgassing under vacuum at 110 °C for 20 h. Brunauer–Emmett– Teller (BET) theory was used for calculation of the specific surface area (SBET). The Barrett–Joyner–Halenda (BJH) model was applied to the desorption curve of the isotherms for estimation of the average pore size (dpore). The pore size distribution was obtained by using a DFT method. X-ray diffraction analysis (XRD) was performed on a D5000 Siemens XRD diffractometer with a Cu Ka Xray tube (k = 1.54056 Å). The tube voltage and current were 45 kV and 40 mA, respectively. Diffraction patterns were collected in the 3–90° 2H range, with 0.02° intervals, and a step time of 1 s. The unit cell parameters were estimated from the position of (0 0 3) and (1 1 0) reflections according to the Braggs law. The lengths of crystallographic coherent domain in the c- and a-directions were calculated using the Scherrer equation based on HT (0 0 3) and HT (1 1 0) diffraction reflections. In case of an overlapping of the reflections, deconvolution of the peaks was performed. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) of the HTs were carried out in air using a ‘‘Netzsch STA 409’’ apparatus. Samples of the HTs of 2–11 mg were heated from 30 to 900 °C with a heating rate of 5 K min1. Scanning electronic microscopy (SEM) images were acquired using a DSM 982 Gemini (Zeiss) microscope with accelerating energy of 3 kV. The samples were coated with carbon prior to SEM investigation. Basicity of the HTs was examined by means of irreversible adsorption of acrylic acid. 20–40 mg of the HTs was added to 10–20 mL of ca. 10 mM acrylic acid solution in cyclohexane. The sorption was performed for 4 h in closed vessels, after that the material was filtered off using syringe filters (Chromafil, PTFE, pore size 0.2 lm). Concentration of acrylic acid in the filtrate was determined spectrophotometrically (StellarNet Inc.) at k 240 nm. 2.4. Catalytic test The catalysts were ground in a mortar prior to catalytic tests. The experiments were performed in a 20 mL autoclave equipped with a glass inlet. The catalyst and 5 mL of 10 wt.% aqueous solution of glucose were charged in the autoclave, sealed, and pressurized with ca. 30 bar of nitrogen. The reactions were carried out under stirring at 750 rpm. After the experiments, the reaction mixture was cooled down in an ice bath and filtered through a polyamide syringe filter (Chromafil, polyamide, pore size 0.2 lm). The pH was measured with a Titroline alpha titrator unit. The amount of leached Mg2+ was determined by ICP-OES analysis of the filtered reaction mixtures.

For recycling, the catalysts were separated from the reaction mixture by centrifugation. The catalysts were rinsed with water, dried at 80 °C, and calcined in air at 500 °C for 5 h. After calcination, the HTs were rehydrated in 0.5 M solution of Na2CO3 at room temperature for 3 h. The materials were thoroughly washed with water, dried at 80 °C, and reused. The mass of catalyst in each run was 100 mg. The products were analysed by means of high-pressure liquid chromatography (HPLC) using a Shimadzu Prominence LC-20 system. The samples were injected into a Multospher APS HP-5l Hilic column (250 mm  4.6 mm, Chromatographie-Service) and eluted at 50 °C with an eluent (90 vol.% acetonitrile + 10 vol.% water) supplied at a rate of 1 mL min1. The system was equipped with an RI detector. The concentrations of by-products, namely glycolaldehyde, glyceraldehyde, dihydroxyacetone, and lactic acid, were determined by HPLC separation on two successively connected organic acid resin columns (CS-Chromatographie, 100 mm  8.0 mm and 300 mm  8.0 mm) as described earlier [14]. 3. Results and discussion Structural and chemical properties of the prepared HTs are summarized in Table 1. The series of HTs aged in aqueous solution at 150 °C and pH ca. 9 are referred to as HTs-150. The abbreviations HTs-9.5-RT and HTs-10-RT denote materials prepared in aqueous medium and aged at room temperature at pH 9.5 and 10, respectively. The catalysts referred to as EHTs-RT were precipitated in ethanol–water medium at pH ca. 9.5 and aged at room temperature. The content of aluminium and magnesium was determined based on ICP-OES data. The amount of water of crystallization was estimated from thermogravimetric data. The sodium content was below 0.05 wt.% for all materials emphasizing efficient washing of the materials. The crystalline structure of the synthesized HTs was confirmed by XRD analysis. The XRD patterns of the materials exhibit the characteristic reflections at 11.4°, 22.8°, and 34.8° 2H corresponding to (0 0 3), (0 0 6), and (0 0 9) basal planes. The reflections found at 39.2°, 46.7°, 60.6°, and 61.9° 2H were assigned to the non-basal planes (0 1 5), (0 1 8), (1 1 0), and (1 1 3), respectively. Representative XRD spectra of HTs are provided in the Supplementary Information (Fig. 1SI). The cell parameters of the HTs estimated from the positions of (0 0 3) and (1 1 0) reflections are listed in Table 1SI. The obtained c and a and cell parameters equal ca. 23 and 3 Å

Table 1 Characteristics of the prepared hydrotalcites.

a b c

Entry

Catalyst

Composition

SBET (m2 g1)

Vpore (cm3 g1)

dpore (nm)

Crystallitea (nm) (0 0 3)

(1 1 0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

HT1-150 HT2-150 HT3-150 HT4-150 HT1-9.5-RT HT2-9.5-RT HT3-9.5-RT HT4-9.5-RT HT1-10-RT HT2-10-RT HT3-10-RT EHT1-RT EHT2-RT EHT3-RT

Mg0.78Al0.22(OH)2(CO3)0.110.57H2O Mg0.76Al0.24(OH)2(CO3)0.120.54H2O Mg0.73Al0.27(OH)2(CO3)0.1350.47H2O Mg0.67Al0.33(OH)2(CO3)0.1650.62H2O Mg0.77Al0.23(OH)2(CO3)0.1150.82H2O Mg0.76Al0.24(OH)2(CO3)0.120.85H2O Mg0.73Al0.27(OH)2(CO3)0.1350.76H2O Mg0.66Al0.34(OH)2(CO3)0.170.85H2O Mg0.75Al0.25(OH)2(CO3)0.1250.71H2O Mg0.71Al0.29(OH)2(CO3)0.1450.85H2O Mg0.66Al0.34(OH)2(CO3)0.171.01H2O Mg0.74Al0.26(OH)2(CO3)0.130.91H2O Mg0.71Al0.29(OH)2(CO3)0.1450.86H2O Mg0.69Al0.31(OH)2(CO3)0.1550.79H2O

18 17 59 48 117 160 167 140 118 160 178 86 145 72

0.05 0.03 0.15 0.12 0.46 0.44 0.61 0.50 0.40 0.52 0.55 0.10 0.29 0.17

5.0 5.0 6.6 5.0 7.2 5.6 11.0 11.2 7.4 6.0 5.4 4.4 4.5 7.2

25.1 32.5 17.3 18.5 5.5 4.4 5.5 5.2 5.8 4.9 7.9 3.0 2.5 6.2

18.5 19.9 14.4 15.1 9.9 9.6 9.6 10.4 7.8 12.2 13.2 4.9 5.5 10.4

Crystallographic coherent domain. Total content of carbonate anions calculated from the chemical formula of the HTs. Basicity was defined as amount of accessible carbonate ions and calculated according to formula (1).

Adsorbed acrylic acid (mmol g1)

Amount of CO2b 3 (mmol g1)

Basicityc (mmol g1)

0.4 0.2 1.0 0.5 4.2 3.6 5.0 2.3 7.0 5.2 3.2 6.0 6.4 3.7

1.5 1.5 1.8 2.0 1.5 1.5 1.7 2.0 1.6 1.7 1.9 1.4 1.6 1.8

0.02 0.01 0.06 0.05 0.22 0.20 0.26 0.18 0.41 0.34 0.24 0.33 0.40 0.27

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considering crystallization of the HTs in the 3R space group. These values are in accordance with previously reported characteristics of HTs [22]. The XRD data were used for calculation of the crystallographic coherent domains (Table 1). As expected, a high ageing temperature leads to formation of large crystallites of the HTs. In case of the well-crystallized HT1-150 material, the XRD pattern suggests formation of a MgCO3 phase in trace amounts since a reflection at 32.5° 2h was detected (Fig. 1SI). Therefore, the presence of this impurity in samples with lower crystallinity but the same Mg-to-Al molar ratio cannot be excluded, though the hydrotalcite phase clearly predominates. TG-DSC characterization was used to confirm the structure of the HTs. Representative TG curves of the HTs are shown in Fig. 2SI. The DSC curves (Fig. 1) of the HTs contain an endothermic peak corresponding to the release of interlayer water in the temperature region of 140–260 °C [30,31]. Interestingly, the form of the peak differs in case of EHT1-RT, which probably results from incorporation of ethanol molecules along with water molecules into the interlayer gallery upon preparation of the material. The splitting of this peak for HT1-150 is caused by different coordination environments of water molecules in the HT crystals; for example, either elimination of water surrounded by H2O and CO2 3 occurs or water is environed predominantly by other water molecules [32,33]. The dehydroxylation, that is, elimination of hydroxyl groups, takes place stepwise as reflected by two endothermic peaks on the DSC curves in the temperature range of 250–450 °C. The first peak with a maximum at 320–350 °C corresponds to the decomposition of OH groups, and the second peak at 390– 420 °C can be assigned to simultaneous dehydroxylation and CO2 release [30,31]. The two peaks are clearly separated for HTs-150 but merge into one peak for materials aged at room temperature probably owing to smaller crystallite sizes of the latter. The preparation procedure of HTs also significantly impacts the textural properties of the materials. For instance, the specific surface area (SBET) decreases for materials aged in hot water (entries 1–4 vs. 5–14, Table 1). All HTs are mesoporous materials possessing pores larger than 2 nm in diameter. Interestingly, HTs

Fig. 1. Representative DSC curves of hydrotalcites from each series.

synthesized in water (entries 1–11, Table 1) exhibit a broad pore size distribution with pore diameters in the range of 4–20 nm. The EHTs prepared in an ethanol–water medium feature a narrower pore size distribution in the range of 6–14 nm (Fig. 3SI). Thus, variation of the precipitation medium enables tuning the textural properties of HTs. Additionally, ageing at 150 °C results in a decrease of the average pore size as well as the total pore volume compared to materials aged at room temperature (entries 1–4 vs. 5–14, Table 1). The morphology of the materials was examined by SEM analysis (Figs. 2 and 3). In all cases the HTs are composed of platelet-like particles that are agglomerated with each other in different ways depending on the preparation conditions. The platelet-like shape of primary particles of HTs is encountered very often [24,25,28,34,35]. The diameter and thickness of the platelet correspond to the non-basal (1 1 0) and basal (0 0 3) planes, respectively [36]. Remarkably, inspection of the SEM images gives nearly the same thickness of the HT particles equal to ca. 20 nm regardless of the preparation technique of the materials. This value is rather close to the lengths of the crystallographic coherent domain calculated based on XRD (Table 1), especially for well-crystallized HTs aged at 150 °C (entries 1–4, Table 1). The diameter of the primary particles of HTs significantly depends on the preparation technique (detailed discussion see below) and varies in the range of 30 to 200 nm. These dimensions observed by SEM significantly exceed the size of crystallites evaluated for the (1 1 0) planes (Table 1). Therefore, the primary particles of HTs are formed by non-coherent coalescence of domains during the crystal growth, which is most pronounced for the (1 1 0) crystallographic direction. These results are consistent with the observations previously reported for Ni-Al HTs [36]. Considering the HT materials as basic catalysts, Roelofs et al. proposed that the interlayer anions located at the edges of the primary HT particles are the most accessible sites for a substrate, and therefore, they play the main role in catalysis [24]. Obviously, highly dispersed HT particles are beneficial for catalysis, as they possess the highest amounts of edge-located interlayer anions. Additionally, defects in the lamellar structure are also responsible for the presence of accessible basic sites [25]. Hence, HT plates consisting of small crystallites are also expected to be catalytically active, because such HT plates bear a high number of defects in places of coalescence of the domains [12]. However, the primary particles of HTs tend to agglomerate, and the architecture of the agglomerates should be taken into account when considering the accessibility of active cites. The aggregation of particles was examined using the micrographs illustrated in Fig. 2. The HTs-150 (Fig. 2, HT1-150) consist of platelet-like particles with 80–150 nm in diameter. The round shape of the platelets is typical of hydrothermally treated HTs [37]. At high ageing temperatures, some particles undergo coalescence so that the borders between the primary particles are predominantly erased. The HTs-9.5-RT and EHTs-RT were precipitated and aged at room temperature and pH ca. 9.5 using different solvents (Figs. 2 and 3), that is, water or ethanol–water, respectively. For synthesis in aqueous phase, the morphology of HTs dramatically depends on the pH of preparation. For instance, Wang et al. reported ‘‘stonelike’’ and ‘‘rosette’’ structures for HTs prepared at pH 9 and 10, respectively [28]. Working at a pH between 9 and 9.5, we observed an arrangement of HTs-9.5-RT particles into globules of different structures. For HT2-9.5-RT, Fig. 2 illustrates two types of structures: (i) stacking of the platelets one above another with the basal planes, and (ii) formation of the ‘‘rosette’’ morphology with nonbasal orientation of the HT particles (Fig. 2, HT2-9.5-RT). Examination of the SEM micrographs obtained for the materials precipitated in water–ethanol demonstrates that the primary particles are organized into flake-like formations that are

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Fig. 2. SEM micrographs of selected hydrotalcites.

Fig. 3. SEM images of the HTs synthesized by co-precipitation at pH 9.5 in aqueous (HT1-9.5-RT) and in aqueous-ethanol (EHT3-RT) media. Diameters of the primary particles are shown by the yellow bars. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

characterized by high aspect ratio, that is, the ratio of particle diameter to its thickness (Fig. 2 EHT3-RT). Moreover, these flakelike sheets stack with each other into a lamellar structure as shown in Fig. 2 for EHT3-RT. Most probably, the narrower pore size distributions of EHT originate from slit pores between the flake-like sheets. Closer investigation of these sheets demonstrates that they consist of primary HT particles stacked by basal (0 0 3) planes, similar to HTs-9.5-RT. Measurement of the particle diameters shows that the size of primary particles of HTs-9.5-RT with 40–120 nm surpasses that of EHTs-RT with 40–70 nm (Fig. 3). Although only a few studies reported utilization of alcohols as co-solvents for synthesis of HTs [38–41], it was previously shown that adsorption of alcohols on the interface of HTs strongly suppresses the particle growth during the preparation of HTs [38].

The HTs-10-RT were precipitated in water at pH 10 and aged at room temperature. Precipitation of HTs at pH 10 that presents their isoelectric point favours formation of ‘‘sand rose’’ morphology [28], for example, intergrown particles of HTs shaping the ‘‘rosette’’ structure [42]. Owing to the intergrowth, it was impossible to measure the diameter of the primary particle of HTs-10-RT, but the length of the particle edge is ca. 150–250 nm. These spheroidal ‘‘sand roses’’ are interconnected with each other forming agglomerates of micrometric size [28,34,35]. The interspaces between the HT agglomerates are of irregular size (Fig. 2 HT1-150, HT29.5-RT, and HT1-10-RT). Therefore, the pore networks of HTs prepared in an aqueous medium have wide pore size distributions. Microphotographs of selected catalysts with low magnification allow observing the structure of agglomerates (Fig. 4SI).

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We investigated the basicity of the prepared HTs by irreversible adsorption of acrylic acid (pKa = 4.3) [12,26,43,44]. This method provides information on the concentration of both strong and weak basic sites. However, it is known that not-calcined HTs possess predominantly weak basic sites that are beneficial for isomerization of glucose into fructose [11,14]. The amounts of acrylic acid adsorbed by HTs aged at room temperature exceed the content of carbonate anions (Table 1). This finding suggests that the acid is probably adsorbed on both accessible carbonate and hydroxyl groups of HTs. Basicity of the HTs was estimated assuming that an accessible carbonate anion adjoins a number of accessible hydroxyl groups, and the amount of the latter was calculated based on stoichiometry (Table 1). We assume 1:1 stoichiometry for the interaction of both hydroxyl group and carbonate anion with acrylic acid. Considering the composition of hydrotalcites as Mg1xAlx(OH)2(CO3)x/2mH2O, 1 mol of a hydrotalcite contains 2 mols of hydroxyl anions and x/2 mols of carbonate anions. As basicity corresponds to the amount of accessible carbonate anions, it can be calculated from the data on sorption of acrylic acid according to the following formula:

Basicity ðmmol g1 Þ ¼ nAcrylic acid ðmmol g1 Þ

nCO2 3

nCO2 þ nOH 3

1

¼ nAcrylic acid ðmmol g Þ x 2

x 2

þ2

;

ð1Þ

where nAcrylic acid denotes the amount of adsorbed acrylic acid and x is the fraction of aluminium in the common formula Mg1xAlx(OH)2(CO3)x/2mH2O. The obtained values of basicity are in good agreement with previously published data obtained by temperature-programmed desorption of CO2 (0.4–0.8 mmol g1) [35] or sorption of acrylic acid (0.3–0.7 mmol g1) [12]. It should be noted that the literature data are provided for meixnerite, that is, a hydrotalcite containing OH as compensating anion. But in this work a divalent carbonate anion is used, therefore half amount of the latter is required for charge balancing of the hydrotalcite. The preparation method clearly influences the basicity of the samples, which increases as follows: HTs-150 < HTs-9.5-RT < HTs-10-RT  EHTs-RT. The difference in the concentration of basic sites of the HTs might be explained in terms of accessibility of the interlayer anions. This includes concentration and accessibility of edge anions, that is, dispersion of primary particles and their agglomeration. Additionally, the crystallite size can influence basicity, as the concentration of defects is expected to be higher for materials with small crystallographic coherent domains. According to the SEM study, the primary particles of materials aged at 150 °C undergo coalescence suggesting less accessibility of the interlayer anions, which results in low basicity. Moreover, ageing at high temperature leads to crystallite growth (entries 1–4, Table 1). HTs-9.5-RT and EHTs-RT are formed by the agglomeration of primary particles stacked by basal planes. The EHTs-RT are potentially more basic than HTs-9.5-RT since EHTs-RT are composed of more dispersed primary nanoparticles (Fig. 3). Moreover, the EHTs-RT exhibit smaller sizes of crystallographic coherent domains in (1 1 0) direction than HTs-9.5-RT (Table 1). Therefore, the probability to introduce structural defects, which increase basicity, is higher in case of EHTs-RT. Stacking of primary particles into agglomerates leads to more porous materials with higher specific surface area for synthesis in water (SBET = 117–167 m2 g1, Table 1), while more dense flake-like secondary particles (SBET = 72–145 m2 g1, Table 1) are formed in aqueous-alcohol media. Balancing of these factors (size of crystallites, dispersion of primary particles, and the way of their agglomeration) gives rise to the formation of more basic materials in an ethanol–water medium (entries 13–14 vs. 5–8, Table 1). Apparently, even after agglomeration, EHTs-RT possess a large

amount of accessible active sites located on the planes that do not contribute to the stacking of primary particles. Likewise, relatively high concentrations of basic sites were detected for HTs10-RT (entries 9–11, Table 1). It can be explained by the most regular morphology with the exposed basal and non-basal planes easily accessible for adsorption (Figs. 2 and 3SI). The catalytic activity of HTs for the isomerization of glucose into fructose is summarized in Table 2. All catalysts enabled a rather high selectivity for fructose formation, and only minor amounts of dihydroxyacetone and glycolaldehyde as by-products were detected. Dihydroxyacetone and glycolaldehyde are the retroaldolization products of monosaccharides in the presence of HTs [14]. In the presence of basic catalysts, glucose cleaves into glycolaldehyde and erythrose, whereas fructose gives rise to dihydroxyacetone and glycolaldehyde. Interestingly, only dihydroxyacetone and glycolaldehyde were detected in the reaction mixture, probably owing to strong adsorption of the other by-products. Moreover, dihydroxyacetone is an intermediate in the base-catalysed formation of lactic acid [14,45,46]. This acidic by-product leads to leaching of Mg2+ [14], an effect that was observed in this study as well. The amount of leached magnesium correlates with the conversion of glucose. This suggests a minor but continuous formation of acidic by-products that were, however, not detectable by HPLC analysis of the aqueous phase most probably due to adsorption on the surface of the catalysts. The pH of the reaction mixture was ca. 7–8 (Table 2). Our previous study showed that leaching of magnesium can be suppressed significantly under continuous operation conditions compared to batch experiments [14]. The dependency of the catalytic performance on the Al-to-Mg ratios was investigated. The catalytic activity stays unaltered in the series of HTs-150 (entries 1–4, Table 2) and for catalysts with low and medium content of aluminium (entries 5–8, 9, 10, 12, and 13, Table 2). This observation is typical of catalysis by hydrated HTs when the nature and accessibility of the intercalated anion are the most essential factors and the Mg-to-Al ratio is of less significance [26]. Interestingly, in every set of HTs, the material with the highest content of Al results in a slightly lower conversion compared to the other materials from the same set (entries 8, 11 and 14, Table 2). These materials have a molar content of aluminium close to the maximal, which is restricted by the expression Al/(Mg + Al) 6 0.34. Increasing the Al content above 0.34 results in segregation of Al(OH)3 [22]. We assume that minor amounts of aluminium hydroxide form along with the HTs in case of aluminiumrich samples, although we failed to detect another phase by XRD. Moreover, investigation of the catalytic activity showed that some aluminium hydroxide is formed in-situ on the surface of HTs due to leaching of Mg2+ (detailed discussion is given below). Aluminium hydroxide hinders accessibility of the basic sites of HTs for the substrate, blocking the active centres. Therefore, more intensive surface blockage with aluminium hydroxide during the reaction and probably during the preparation leads to lower catalytic activity of HT4-9.5-RT, HT3-10-RT, and EHT3-RT. Moreover, the conditions of catalyst preparation considerably influence the catalytic activity of the materials. The conversion of glucose mainly correlates with the basicity of the catalysts as shown in Fig. 4. Obviously, the concentration of accessible basic sites is a crucial factor for an efficient HT catalyst. The importance of accessible basic sites for glucose isomerization has recently been highlighted by Lee et al. who proposed to increase the accessibility of the HT basic sites by sonication [12]. Moreover, TOF values were estimated for selected catalysts, namely HT2-9.5-RT, HT1-10-RT, and EHT2-RT. For a moderate conversion of glucose (10–19%), TOF values were found to be nearly equal in the range of 18– 23 h1, independent of the preparation procedure. Details of the TOF calculation are provided in Table 2SI.

7

I. Delidovich, R. Palkovits / Journal of Catalysis 327 (2015) 1–9 Table 2 Results of catalytic tests of HTs. Reaction conditions: 5 mL of 10 wt.% aqueous glucose solution, 100 mg catalyst, 110 °C, 1.5 h, 750 rpm. Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Catalyst

HT1-150 HT2-150 HT3-150 HT4-150 HT1-9.5-RT HT2-9.5-RT HT3-9.5-RT HT4-9.5-RT HT1-10-RT HT2-10-RT HT3-10-RT EHT1-RT EHT2-RT EHT3-RT

Recyclability tests 15 HT1-10-RT 2nd run 16 HT1-9.5-RT 2nd run 17 HT1-9.5-RT 3rd run 18 HT1-9.5-RT 4th run a b c

Glucose conversion (%)

Fructose

Selectivity to byproducts (%)

pH

Mg leachedc (%)

Yield (%)

Selectivity (%)

DHAa

GAb

12 9 11 12 18 20 21 14 29 30 24 30 30 23

9 8 9 10 16 16 18 13 25 25 21 26 26 20

75 82 84 82 89 82 86 93 89 83 86 85 87 85

2 1 1 0 1 1 1 1 2 2 1 2 1 1

2 0 1 1 1 1 1 1 1 1 1 2 2 2

7.8 7.9 7.6 8.0 7.6 7.8 7.6 7.9 7.5 7.3 7.7 7.9 8.0 7.8

1.3 1.3 1.8 1.3 3.4 2.8 4.4 2.4 5.2 2.2 1.5 4.8 3.9 3.5

28 18 20 20

26 16 17 17

94 89 84 84

1 1 2 1

1 2 1 1

7.2 7.6 7.8 7.4

5.0 3.5 3.0 3.3

DHA is dihydroxyacetone. GA is glycolaldehyde. Per cent of total Mg content of the samples.

Fig. 4. Glucose conversion vs. basicity of hydrotalcites. Reaction conditions: 5 mL of 10 wt.% aqueous glucose solution, 100 mg catalyst, 110 °C, 1.5 h, 750 rpm.

The accumulation of fructose in the course of the reaction in the presence of HT2-9.5-RT and HT1-10-RT is shown in Fig. 5. Fructose yields of ca. 22% and 30% can be produced over these catalysts. The selectivity of fructose maintains rather high ranging from 80% to 89%. We observed a gradual decrease of the pH during reaction from ca. 8 to ca. 7 accompanied by leaching of magnesium (Fig. 5SI). A filtration test was carried out to elucidate a possible catalytic activity of the leached Mg2+ ions. The reaction was conducted for 30 min in the presence of HT1-10-RT until the conversion of glucose reached ca. 15%. After that, the catalyst was removed by filtration and the filtrate was held under typical reaction conditions for two more hours. No changes of the concentrations of glucose or fructose could be detected by HPLC supporting that the isomerization is catalysed heterogeneously (Fig. 5). However, after two hours of incubation without the HT, the pH of the filtrate decreases from ca. 8.7 to 4.8 confirming that Mg2+ is not active for the isomerization, but catalyses the decomposition of carbohydrates into acidic by-products. Similar results were obtained earlier when a solution of NaOH was used as homogeneous catalyst [14].

Fig. 5. Time dependence of fructose accumulation in the presence of HT1-10-RT and HT2-9.5-RT. Open symbols and the dashed line show the results of the filtration test: the catalyst was removed from the reaction mixture after 30 min (at a glucose conversion of ca. 15%), and the solution was allowed to react for 2 more hours. ‘‘S’’ denotes selectivity of the isomerization. Reaction conditions: 5 mL of 10 wt.% aqueous glucose solution, 100 mg catalyst, 110 °C, 750 rpm.

Deposition of by-products on the HT surface causes deactivation of the catalysts during the reaction [14]. Importantly, HTs can be regenerated by burning of the organic deposits followed by rehydration. Successful recyclability tests were performed for HT1-10-RT and HT1-9.5-RT, which were tested in two and four consecutive runs, respectively (entries 15–18, Table 2). We obtained a constant glucose conversion and fructose yield upon recycling. After each reaction, the catalysts were regenerated by calcination in air, rehydration in a solution of Na2CO3, and washing with water. After the 4th recycling, HT1-RT was regenerated and characterized by means of elemental analysis, low-temperature sorption of N2, and XRD techniques. After the recycling, the molar ratio of Mg-to-Al decreases from 77:23 to 67:33 reflecting loss of Mg during the recycling tests. As shown in Table 2, some leaching of magnesium takes place during reaction; moreover, it is known that migration of Al3+ takes place to some extent during calcination [47] and rehydration in a Na2CO3 solution [48]. According to Jobbágy and Regazzoni, leaching of Mg2+ from HTs under mild acidic conditions is accompanied by the formation of Al(OH)3.

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I. Delidovich, R. Palkovits / Journal of Catalysis 327 (2015) 1–9

Table 3 Characteristics of the as-prepared (HT1-9.5-RT-fresh) and four times recycled (HT1-9.5-used) hydrotalcite. Conditions of the catalytic tests: 5 mL of 10 wt.% aqueous glucose solution, 100 mg catalyst, 110 °C, 1.5 h, 750 rpm. Entry

1 2

Catalyst

HT1-9.5-RT-fresh HT1-9.5-RT-used

Mg-to-Al molar ratio

77:23 67:33

SBET (m2 g1)

117 97

[49]. Since regeneration of the catalysts included a calcination procedure, the transformation of aluminium hydroxide into alumina Al2O3 occurred. Subsequently, after each recycling, the content of Al2O3 in the catalyst gradually increases. However, even after the 4th run, the catalyst contains ca. 91 wt.% of HT1-9.5-RT and ca. 9 wt.% of Al2O3 (details on calculation of the catalyst composition are provided in ESI). Apparently, the yield of fructose stays unaltered upon recycling as the catalyst still contains hydrotalcite as a major constituent (>91 wt.%). Our future study will focus on suppression of Mg leaching to preserve the initial composition of the material. Nevertheless, the crystallinity and textural properties of HT1-9.5-RT undergo only minor changes under reaction conditions (Table 3, Fig. 6SI). This emphasizes the great potential of hydrotalcites as active, selective, and recyclable catalysts for glucose isomerization into fructose. 4. Conclusions The influence of chemical and structural properties of Mg–Al hydrotalcite catalysts in carbonate form for the isomerization of glucose into fructose has been investigated. This reaction is catalysed by weak basic sites, that is, accessible carbonate anions located on the edges of primary particles and defects on the basal planes of hydrotalcite. In line, high basicity of the catalyst is a prerequisite for good catalytic activity. In turn, basicity is a function of structural and morphological properties, which depend on preparation procedure. Four sets of catalysts with varying Mg-to-Al molar ratios were prepared by co-precipitation with different synthesis parameters such as temperature of ageing, pH, and solvent. With the optimum catalysts, fructose can be produced in up to 30% yield and 89% selectivity at 110 °C based on a 10wt.% aqueous solution of glucose. Although leaching of magnesium was detected, the hydrotalcite catalysts demonstrated excellent recyclability. Moreover, the crystalline structure and textural properties of the hydrotalcites remain intact after catalysis. The following conclusions could be made on structure–performance correlations: (1) The best yields of fructose were obtained over catalysts with a molar content of aluminium 0.23 < Al/(Mg + Al) < 0.30, while catalysts with 0.30 < Al/(Mg + Al) < 0.34 were less active. (2) Accessibility of the active sites is of utmost importance for good catalytic performance. Therefore, the size of crystallographic coherent domains and primary particles as well as the way of their agglomeration significantly influences the catalytic activity of HTs, as shown by XRD, adsorption of acrylic acid, and SEM investigations. HTs with optimum structural properties enabling excellent catalytic performance can be synthesized at these conditions: (2.1) Ageing the catalyst at room temperature is necessary to avoid intensive coalescence of primary particles as well as obtain the material with high specific surface area and small crystallites.

Vpore (cm3 g1)

0.46 0.31

dpore (nm)

7.2 7.1

Crystallite size (nm) (0 0 3)

(1 1 0)

5.5 4.5

9.9 8.2

(2.2) Conducting the co-precipitation at the isoelectric point of HTs (pH 10) results in a catalyst of ‘‘sand rose’’ structure with easily attainable active sites. (2.3) Performing the co-precipitation in aqueous-ethanol medium gives rise to more dispersed primary particles and smaller crystallites compared to the synthesis in aqueous solution.

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