Hemicellulose Arabinogalactan Hydrolytic Hydrogenation Over Ru-Modified H-USY Zeolites

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Journal Article

Hemicellulose arabinogalactan hydrolytic hydrogenation over Ru-modified H-USY zeolites

Author(s): Murzin, Dmitry; Kusema, Bright; Murzina, Elena V.; Aho, Atte; Tokarev, Anton; Boymirzaev, Azamat S.; Wärnå, Johan; Dapsens, Pierre Y.; Mondelli, Cecilia; Pérez-Ramírez, Javier; Salmi, Tapio

Publication Date: 2015-10

Permanent Link: https://doi.org/10.3929/ethz-a-010792434

Originally published in: Journal of Catalysis 330, http://doi.org/10.1016/j.jcat.2015.06.022

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ETH Library Hemicellulose arabinogalactan hydrolytic hydrogenation over Ru-modified H-USY zeolites Dmitry Yu. Murzin1*, Bright Kusema2, Elena V. Murzina1, Atte Aho1, Anton Tokarev1, Azamat S. Boymirzaev3, Johan Wärnå1,4, Pierre Y. Dapsens2, Cecilia Mondelli2, Javier Pérez-Ramírez2, Tapio Salmi1 1Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Department of Chemical Engineering, Åbo Akademi University, FI-20500 Åbo/Turku, Finland, E-mail: [email protected] 2Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland 3Namangan Institute of Engineering and Technology, Department of Chemical Technology, Namangan, 160115, Uzbekistan 4University of Umeå, Umeä, Sweden

ABSTRACT

The hydrolytic hydrogenation of hemicellulose arabinogalactan was investigated in the presence of protonic and Ru (1-5 wt.%)-modified USY zeolites (Si/Al ratio = 15 and 30). The use of the purely acidic materials was effective in depolymerizing the macromolecule into free sugars. While the latter partly dehydrated into 5- hydroxymethylfurfural and furfural, the generation of high molecular-weight compounds (aggregates of sugars and humins) was not favored, in contrast to previous evidences over beta zeolites. Application of the bifunctional Ru/USY catalyst, comprising well-dispersed metallic nanoparticles on the aluminosilicate support, resulted in the formation of galactitol and arabitol, in the suppression of dehydration side products, and further inhibition of polymerization reactions, which only yielded low molecular-weight oligomers. Detailed analysis of the reaction pathways as well as kinetic modelling of hydrolytic hydrogenation was performed with an advanced reaction mechanism.

Keywords: Arabinogalactan, bifunctional catalysis, biomass upgrading, Ru/USY zeolites, hydrolysis.

Introduction

In view of enabling a transition towards a biobased ecomomy, the depolymerization of the main chemical components of lignocellulosic biomass [1,2], i.e., cellulose and hemicellulose, to their sugars components by hydrolysis is an essential step which lies at the very beginning of multiple value chains for the sustainable production of fuels and chemicals. Due to the recalcitrant nature of the macromolecules, in particular the crystalline cellulose, this process is conducted under harsh conditions. Therefore, the C6 and C5 monosaccharides formed can easily further convert into hydroxymethylfurfural

(HMF) or furfural and oligomerization reactions leading to various humins are favored.

A promising way of avoiding these undesired transformations of thermally-unstable sugars is to hydrogenate them into the corresponding more robust polyols. this methodology is an attractive means to boost the selectivity of the hydrolysis process and generate valuable chemicals in one pot.

Balandin et al. [4] firstly demonstrated the hydrolytic hydrolysis of cellulose to sorbitol, health care, food, and cosmetic additive, over a Ru/C catalyst in the presence of low- concentrated mineral acids. A revival of the interest in this reaction came only after a work of Fukuoka and Dhepe [5] who reported the use of an alumina-supported platinum catalyst for the production of sorbitol and mannitol with 25% and 6% yields, respectively. Later a range of carbon materials used as such or functionalized with sulfonic groups [3, 6] and other solid acids including heteropolyacids, metal oxides, and zeolites [7] have been evaluated. The latter class of catalysts has received particular attention. Thus, the hydrolytic hydrogenation of unspecified cellulose [8], microcrystalline cellulose in the presence of mineral acid traces [9] and birch pulp mill cellulose with lower molecular weight and presence of xylan [10, 11] has been performed with various metal-modified zeolites, including Ru-containing beta [8] and

USY [9]. 2

In contrast to cellulose, which requires extensive pre-treatment (e.g. ball milling) to reduce its crystallinity and thus enhance its reactivity, hemicelluloses, comprising amorphous branched hetero-polymers with shorter chains than cellulose, have been more readily solubilized and processed. Hemicelluloses comprise a number of polysaccharides, e.g., xylan, glucuronoxylan, arabinoxylan, arabinogalactan, galactoglucomannan, etc., which differ from each other in terms of nature and relative amount of their building blocks (hexoses and pentoses) and degree of polymerization.

For instance, arabinogalactans (AG), appearing in large quantities in larch species such as Larix sibirica, consists of -D-galactopyranose as a backbone with D- galactopyranose and L-arabinofuranose side chains (Scheme 1). The average molar ratio of galactose to arabinose in AG is about 6:1 and the molar mass is 20 000-100 000 g mol−1 [12].

Scheme 1. Applications of the sugar polyols obtained by hydrolytic hydrogenation of arabinogalactan.

The preparation of the free sugars arabinose and galactose has been first attempted using sulfonic acid-functionalized polymers [13]. In comparison with such catalysis better results in terms of catalyst activity and stability were obtained with zeolites in their protonic form in the case of xylan hydrolysis [14]. In view of this result and due to the superior performance of Ru-containing materials for the hydrogenation of arabinose and galactose to their corresponding polyols (arabinitol and galactitol) [15-17], which find application as low caloric, non-carcinogenic sweeteners, flavors, dietary supplements, and pharmaceuticals, it was natural to apply bifunctional Ru-beta zeolites catalyst in hydrolytic hydrogenation of arabinogalactan [18, 19]. While the side reactions to HMF and furfural were minimized, only limited yields to the corresponding polyols have been achieved over these bifunctional catalysts. Furthermore, substantial formation of high molecular weight compounds, that is, aggregates of sugars and humins (polydispersed heterogeneous carbonaceous materials), occurred. Mechanistic aspects of humins formation have recently been addressed [20-25]. Their formation mechanism has been related to polycondensation reactions giving a network of furan rings linked by ether or acetal bonds [20]. At the same time infrared spectroscopy studies [23] did not support the concept of acetal bonds formation advanced by

Sumerski et al. [20]. Patil et al. [21, 22] pointed out the involvement of a 2,5-dioxo-6- hydroxyhexenal intermediate formed by rehydration of HMF in the condensation process. In particular, it has been highlighted that this intermediate undergoes aldol condensation with HMF leading to humins, which cannot be formed directly from hexoses.

In the present work, hydrolysis and hydrolytic hydrogenation of AG were alternatively studied over ultra-stable Y (USY) and Ru/USY zeolites, respectively. Materials featuring distinct acidity and metal loading were compared and a range of methods was applied to investigate the formation of humins both in terms of structural properties and 4 chemical nature of the precursor(s). Based on the experimental observations, an advanced reaction network was proposed which was used as the basis for kinetic modelling of the hydrolytic hydrogenation.

Experimental Section

Catalyst preparation

USY-15 (CBV720, molar Si/Al = 15, protonic form) and USY-30 (CBV760, molar

Si/Al = 30, protonic form) zeolites were purchased from Zeolyst International and used as received. The incorporation of ruthenium (nominal loading 1-5 wt.%) into the zeolites was performed by spray deposition using RuCl3·xH2O (Sigma-Aldrich, 99.9%) as the metal precursor. This technique, pioneered by Yara International [26] enabled the deposition of metal (oxide) particles onto a zeolite support with a high degree of dispersion [27]. The preparation was carried out using a benchtop Büchi Mini Spray

Dryer B 290 equipped with a two-fluid nozzle (diameter = 1.5 mm) and a spray chamber of 10 dm3. Prior to the synthesis, deionized water was passed through the system for 45 min in order to reach thermal equilibrium. RuCl3·xH2O was dissolved in deionized water (5 cm3) under magnetic stirring. Thereafter, the zeolite powder (1.0 g) was added. The obtained slurry, continuously stirred, was pumped to the nozzle of the spray dryer at a rate of 2 cm3 min−1 together with a constant spray air flow of 0.4 m3 h−1, resulting in the formation of fine droplets of 5-20 µm size. The aspiration rate was set at

35 m3 h−1 and the inlet and outlet temperatures of the spray chamber were kept at

493 and 373 K, respectively. The hot drying gas flowed co-currently with the sprayed slurry and its residence time in the spray chamber was 1 s. The dried particles were separated and collected by a cyclone, further dried at 373 K for 12 h, and calcined in static air at 623 K (heating rate = 3 K min−1) for 2 h. The obtained solids were reduced in hydrogen at 573 K for 2 h prior to the catalytic experiments. 5

Catalyst characterization

The Ru content in the catalysts was determined by X-ray fluorescence spectroscopy using an Orbis Micro-XRF analyzer (EDAX) operated with a Rh source at 30 kV. X-ray photoelectron spectroscopy (XPS) was measured using a Perkin-Elmer PHI 5400 spectrometer with a Mg K X-ray source operated at 14 kV and 200 W. The pass energy of the analyzer was set at 17.9 eV and the energy step at 0.05 eV. Nitrogen sorption at 77 K was performed using a Quantachrome Quadrasorb-SI analyzer on degassed samples (10−1 mbar, 573 K, 3 h). Powder X-ray diffraction (XRD) was conducted using a PANalytical X’Pert PRO-MPD diffractometer. Data were recorded in the 5-70° 2θ range with an angular step size of 0.05° and a counting time of 7 s per step.

High-resolution magic angle spinning 27Al nuclear magnetic resonance (MAS NMR) spectroscopy was carried out using a Bruker AVANCE 700 NMR spectrometer equipped with a 2.5-mm probe head and a 2.5-mm ZrO2 rotor at 182.4 MHz. Spectra were acquired using a spinning speed of 20 kHz, 4096 accumulations, and a recycle delay of 1 s. The acid properties of the catalysts were evaluated by Fourier transform infrared spectroscopy (FTIR) using pyridine (Sigma-Aldrich, ≥99.5 %, a.r.) as the probe molecule. The measurements were performed using an ATI Mattson spectrometer equipped with an in situ cell containing ZnSe windows. The samples were pressed into thin self-supporting wafers (20 mg, radius = 0.65 cm). Pyridine was first adsorbed for

30 min at 373 K and then desorbed by evacuation for 20 min at different temperatures

(523, 623, and 723 K). Spectra were recorded by co-addition of 32 scans with a resolution of 2 cm−1. The Brønsted-acid sites (BAS) and Lewis-acid sites (LAS) were quantified based on the intensities of the bands at 1545 and 1450 cm−1, respectively, and using the molar extinction coefficients reported in the literature [28]. Transmission electron microscopy (TEM) imaging was undertaken with a FEI Tecnai F30 microscope 6 operated at 300 kV (field emission gun). The samples were prepared by depositing a few droplets of zeolites suspension in methanol onto a carbon-coated copper grid, followed by evaporation at room temperature.

Catalytic evaluation

Hydrolysis in the presence of hydrogen over protonic zeolites and hydrolytic hydrogenation experiments over metal-modified zeolites were carried out in a 300-cm3

Parr autoclave reactor connected to a 200-cm3 pre-reactor. The autoclave was equipped with sampling outlet featuring a 1-m filter to prevent even very fine catalyst particles from escaping it. The temperature was measured with a thermocouple and controlled automatically (Brooks Instrument). At the beginning of each experiment, 400 mg of AG were dissolved in 90 cm3 of deionized water and loaded into the pre-reactor. Thereafter,

200 mg of catalyst with a particle size below 63 m (to avoid internal diffusion limitations) were loaded into the reactor containing 10 cm3 of deionized water. The reactor was pressurized with hydrogen and heated to 458 K, reaching a total pressure of

31 bar. Based on the vapor pressure of the solvent at this temperature, the partial pressure of hydrogen was 20 bar. The stirring (1000 rpm, to minimize external mass transport limitations) was then started and the reactant solution was fed from the pre- reactor into the reactor. This was considered as the initial reaction time. Liquid samples from the reaction mixture were periodically withdrawn for analysis. The decrease in volume of the reaction mixture was taken into account in the calculations of reactant and products concentrations.

Product analysis

The liquid samples from the reaction mixture were quantitatively analyzed without any pretreatment in a high-performance liquid chromatography (HPLC) system using two different columns and a refractive index (RI) detector. A Bio-Rad Aminex HPX-87C column heated at 353 K was used to analyze AG, sugars, sugar alcohols, and furan

3 −1 compounds. A diluted (1.2 mM) CaSO4 solution flowing at 0.4 cm min was employed as the mobile phase. An Aminex cation H+ column heated at 338 K was used to analyze acidic compounds and other degradation products. In this case, the mobile

3 −1 phase comprised a 0.005 M H2SO4 solution flowing at 0.5 cm min .The individual components were identified by gas chromatography-mass spectrometry (GC-MS) as discussed in detail in [19]. The carbon mass balance was calculated considering the concentration of AG, sugars, polyols, furfurals, and low molecular-weight compounds analyzed by HPLC.

The weight average molar mass (Mw) and number average molar mass (Mn) were determined via gel permeation (size exclusion) chromatography (SEC) using a system equipped with two columns in series (300  7.8 mm Ultrahydrogel liner, Waters,

Milford, USA), a multi-angle laser light scattering MALLS unit (miniDAWN, Wyatt

Technology, USA), and RI and UV detectors. A 0.1 M NaNO3 aqueous solution flowing at 0.5 cm3 min−1 served as the eluent. The samples were filtered with a 0.45-m syringe Acrodisc filter. The injection volume was 100 μl. The Astra software (Wyatt

Technology) was used for data analysis.

Results and Discussion

USY and Ru/USY zeolites

The FAU-type zeolites used in this study feature a bulk Si/Al ratio of 15 (USY-15) and

30 (USY-30). According to the NMR spectroscopic analysis (not shown), both zeolites

8 contain a significant amount of distorted tetrahedral Al species as well as extraframework penta- and, especially, hexacoordinated Al centers, which are generated upon stabilization of the pristine Y zeolite via steaming and acid washing. Both samples

2 −1 feature a mesoporous surface area (Smeso) of 125-128 m g and a micropore volume

3 −1 (Vmicro) of 0.29-0.31 cm g , as typically observed for these materials (Table 1). Their acidity was evaluated via FTIR studies using pyridine as a probe molecule. Upon adsorption and desorption of pyridine at different temperatures, the Brønsted-acid sites

(BAS) and Lewis-acid sites (LAS) could be quantified and classified based on their relative strength. Thus, USY-15 and USY-30 possess a similar concentration of BAS and LAS (335 and 65 μmol g−1 and 310 and 51 μmol g−1, respectively), but the former catalyst contains a slightly higher number of medium and strong sites of either type

(Table 1).

Table 1. Characterization data of the protonic and Ru-modified zeolites.

Catalysts a a Ru BAS LAS b b c d Smeso Vmicro content Cryst. (μmol g−1) (μmol g−1) (m2 g−1) (m3 g−1) (wt.%) (%) 523 K 623 K 723 K 523 K 623 K 723 K USY-15 163 126 46 41 17 7 128 0.29 - 100 Ru(1)/USY-15 233 11 2 32 3 1 117 0.29 1.4 - Ru(2.5)/USY-15 231 7 4 42 3 1 115 0.28 2.3 82 Ru(5)/USY-15 197 0 0 35 0 0 104 0.27 4.8 - USY-30 152 133 25 45 5 1 125 0.31 - 100 Ru(2.5)/USY-30 207 7 0 39 5 0 118 0.31 2.0 78 aDetermined by FTIR of adsorbed pyridine. bDetermined by the t-plot method. cDetermined by XRF. dDerived from XRD.

Ruthenium was deposited onto both zeolites to attain bifunctional catalysts in an amount of 1.4, 2.3, and 4.8 wt.% for USY-15 and 2.0 wt.% for USY-30. The mesoporous surface area and microporous volume of the aluminosilicates were substantially retained upon metal incorporation (Table 1), likely due to the rather low loading, while XRD analysis (not shown) indicated a slight decrease in crystallinity (Table 1). The absence

9 of reflections specific to Ru in the patterns suggested the presence of a well-dispersed, nanostructured metal phase. The structural features of the catalysts were further investigated by TEM (Figure 1). As expected, both protonic zeolites comprised crystals featuring intraparticle mesoporosity. With respect to the Ru-modified samples, the crystallinity of the zeolites appeared hardly modified and the supported metal phase was detected in form of nanostructures with a broad size distribution. Small nanoparticles

(3-4 nm) were visualized along with much larger structures (50-60 nm). Only few of the latter were found in Ru(1)/USY-15, whereas they were more abundant in the catalysts with higher metal loadings. Based on these observations, the moderate decrease in crystallinity upon metal deposition determined by XRD seems to be mainly related to the presence of the secondary metal phase in addition to the aluminosilicate.

Figure 1a. TEM micrographs of protonic (P) and Ru-modified USY-15 zeolites.

Figure 1b. TEM micrographs of protonic (P) and Ru-modified USY-30 zeolites.

The total acidity of the Ru-containing samples was substantially modified compared to the metal-free counterparts. In particular, the BAS of medium and high strength almost vanished, while the amount of weak BAS moderately increased (Table 1). This evidence is in line with previous studies for various zeolite supported metal catalysts, which pointed that introduction of metals onto the zeolite support results in redistribution of acid sites strength [33-35]. The origin of such changes was discussed in detail in the previous work and can be attributed to interactions between the metal crystallites and the support material as well as changes in the support properties during catalyst preparation due to exposure of the zeolite to the metal precursor solution.

Interestingly, the change in acid properties was rather comparable regardless of the ruthenium loading. The absence of strong acid sites is expected to suppress side reactions such as sugar dehydration to HMF and furfural, thus favoring the selectivity to polyols.

XPS analysis of the reduced catalysts indicated that the maximum of the Ru3d5/2 peak is shifted to binding energy less than 280 eV when the charging (ca. 2.7 eV) is taken into account meaning that oxidation state of ruthenium is close to 0. The peak assignment was based on the values reported by Pedersen and Lunsford [36].

Non-catalytic hydrolysis and hydrolytic hydrogenation

The key role of acidity in hydrolysis is in donating a proton to the glycosidic bond between the sugar units in the polysaccharide chain, enabling the liberation of the monosaccharides [19]. Disadvantageously, acid species additionally catalyze sugar dehydration to furfural and HMF under the hydrolysis conditions. This comprises a competitive reaction to the further desired conversion of the sugars to polyols.

Acid solids are expected to influence the hydrolysis of AG in similar manner to how homogeneous mineral acids drive the hydrolysis of (hemi)cellulose provided that the molecules involved in the process can have access to the acid sites. It can be speculated that the external surface acid sites will exclusively provide the Brønsted acidity required for the hydrolysis of the macromolecule, since the latter cannot penetrate inside the pores, but that the cleavage of short-chain depolymerization products could also occur on acid sites situated in the channels.

+ However, as the pKw value of water decreases with increasing temperature, H3O ions generated in situ can also homogenously contribute to the overall hydrolysis process and to sugar dehydration.

Thus, prior to the utilization of parent and metal-modified zeolites, a non-catalytic experiment was conducted for the title reaction at 458 K and a total pressure of 31 bar

(Figure 2) to serve as a reference.

100 AG nd humins 90 Oligomers Galactose 80 Arabinose Ethylene glycol 70 HMF 60 Furfural Unknown 50

productdistribution, % 20

0 0 50 100 150 200 250 time, min

Figure 2. Non-catalytic hydrolysis of AG at 458 K and 31 bar.

Evidently, even in the absence of any acid catalyst, a significant decrease in the concentration of hemicellulose, visible formation of oligomers, arabinose, and galactose monosaccharides as well as furans were detected.

The peculiar shape of the AG-humins curve derives from the fact that upon the course of the reaction polymeric humins were formed while AG was consumed and the two classes of species cannot be separated by HPLC analysis. Polymeric humins derive from acid-catalyzed transformations of HMF [20-23] and aggregation of galactose when arabinose is removed from the side chain of the hemicellulose [19].

It was thus interesting to analyze the molecular mass of the polymers by SEC. Since

SEC of water-soluble polymers is less straightforward than analysis of polymers in organic media, due to interactions between the stationary phase and polar carbohydrates

[29], electrolyte solutions of sufficiently high ionic strength were used as the mobile phase to prevent such secondary effects.

In polymer analysis, besides for the peak apex molecular weight Mp characterizing the sample only in a single point, relevant parameters comprise the number

13 average molecular mass of polymers Mn (more sensitive to molecules of low molecular mass) and the molecular weight average Mw (more sensitive to molecules of high molecular mass), which are determined in the following way:

h(M )M h(M )M 2 M   , M   (1) n h(M ) W h(M )M where h(M) is the slice height at a molecular weight M when the eluted peak is divided into several equidistant volume slices. Another significant parameter is dispersity

(Mw/Mn), which gives an indication about the distribution in the polymer, approaching unity when the polymer chain approaches a uniform chain.

The values of the molecular weight average Mw are displayed for the non-catalytic hydrolysis experiment in Figure 2.

a) 40

, kDa

w 15

M 10

0 50 100 150 200 250 time, min

Figure 3. (a) Molecular weight of polymers/oligomers in the reaction mixture as a function of time. Experimental conditions for this and subsequent figures are the same as for Figure 2.

Dispersity, [-]

0 0 50 100 150 200 250 time, min

Figure 3. (b) Dispersity (Mw/Mn) of polymers/oligomers in the reaction mixture as a function of time. Experimental conditions for this and subsequent figures are the same as for Figure 2.

These data evidence a very clear disaggregation of AG with a relatively high molecular mass (ca. 40 kDa) and a dispersity index of ca. 5 in the first 50-75 min of reaction leading to a polymer of ca. 2.5 kDa molecular weight and ca. 1.5 dispersity. Thereafter, there is a clear increase of the molecular weight and of the dispersity of the resulting polymer in excellent agreement with HPLC data which display a sudden increase in the concentration of the macromolecule and a decrease in that of the oligomers. The UV-vis detector confirmed the presence of aromatic moieties in the newly formed compound, which can thus be ascribed to a humin-type polymer. As already discussed in [30], hydrolysis of AG is, moreover, associated with the removal of arabinose from the side chain, relaxing the steric hindrance in the hemicellulose and allowing galactose molecules to oligomerize resulting in aggregates. This process would inevitably lead to an increase of dispersity. It should be noted that differentiation and quantification of humins and other potential aggregates was not in the main focus of the work, therefore it was not pursued further.

Hydrolysis over acidic zeolites

The results for hydrolysis of AG carried out over USY-15 and USY-30 are presented in

Figure 4 and demonstrate that, similarly to a non-catalytic experiment (Figure 4), AG was hydrolyzed into oligomers and monomers.

AG and humins Oligomers Galactose 100 Arabinose Ethylene glycol HMF 80 Furfural Unknown

Productdistribution, % 20

0 0 50 100 150 200 250 time, min a) Figure 4a. Hydrolysis of AG over USY-15.

AG and humins Oligomers Galactose 100 Arabinose Ethylene glycol HMF 80 Furfural Unknown

Product distribution,Product % 20

0 0 50 100 150 200 250 time, min b) Figure 4b. Hydrolysis of AG over USY-30.

Still, a higher conversion level was attained in the presence of both zeolites compared to the blank run. Thus, application of solid acid catalysts is necessary to enhance the efficiency of the hydrolysis.

An interesting observation besides an increase in the rates was the substantial suppression of humins formation, since the peak of AG constantly decreased with time.

Such evidence with USY catalysts differs not only from the blank experiment but also from the previous data on beta zeolites [19], over which humins were generated to a large extent. At the moment it can be only speculated that the differences in the behavior of beta compared to USY could be related to lower Lewis acidity and moreover additional mesoporosity of the latter allowing easier release of sugars and dehydration products thereby avoiding extensive polymerization.

At the same time, there was a clear formation of galactose aggregates (oligomers) along with the generation of HMF and the unknown compound. The relative concentration of side products, HMF and furfural, was not influenced by the type of acidic catalyst

(Figure 5a) and was higher than in the blank experiment, which can be explained by the lower conversion in the latter case.

The weight ratio between galactose and arabinose was close to two (Figure 5b) in catalytic and non-catalytic experiments. In contrast to HCl-catalyzed hydrolysis, in which the release of sugars follows stoichiometry (6) [13], the substoichiometric formation of galactose in our experiments indicates a preferential cleavage of the side chain. This behavior was already observed upon AG hydrolytic hydrogenation over beta zeolites [19] (ratio close to 1.6) and for AG hydrolysis over a heterogeneous catalyst of an ion-exchange type bearing sulfonic groups [13].

12 HMF - USY-30

10 HMF - USY-15

6 HMF-no catalyst

Relativeconcentration,% 2 Furfural-USY-30 Furfural-USY-15 Furfural-no catalyst 0 0 50 100 150 200 250 time, min a) Figure 5. (a) Concentration of HMF as a function of time over the USY catalysts and in the blank experiment.

USY-15 USY-30 16 blank

Arabonose,%

0 0 2 4 6 8 10 12 14 Galactose, % b) Figure 5. (b) Concentrations of arabinose and galactose attained over the USY catalysts and in the blank experiment.

Hydrolytic hydrogenation over Ru-modified zeolites

The bifunctional approach of converting carbohydrate polymers through combining two functions (redox and acidic) in the same catalyst has been practiced by several groups [3,

5, 37-39] as indicated in the Introduction. Ref. [37] postulates for the case of carbon

18 nanofibers supported nickel catalysts that a proper balance of acid and redox site on the material is essential in order to obtain very high polyol yields starting from cellulose.

In the current work the influence of combining sugar formation with their consecutive hydrogenation was investigated using the bifunctional catalysts, i.e., the Ru-modified

USY zeolites. The catalytic results for Ru(2.5)/USY-15 and Ru(2.5)/USY-30 are displayed in Figure 6. A recycling experiment was performed with Ru(2.5)/USY-30 giving exactly the same concentration profile (not shown) as depicted in Figure 6c.

It should be noted, however, that one- or two recycling experiments might not be sufficiently good basis for assessing the catalyst stability. For example, zeolites Y and

ZSM-5 were treated in liquid water at 150 and 200oC resulting in transformations of the zeolite Y [40]. At the same time a recent work related to stability of commercial USY zeolites in liquid water and utilization of Ru/Y materials in hydrolytic hydrogenation of cellulose showed that these zeolites can be in principle stable in hot liquid water at least on the short term [41]. Due to the possible amorphization of the zeolites in the long term, catalytic behaviour and stability of the catalytic systems studied in the current work should be further accessed in the future by performing multiple recycling experiments or by operation in continuous mode. This was, however, outside of the scope of the present work.

AG and humins Oligomers Galactose 100 Arabinose Ethylene glycol Arabitol 80 Galactitol HMF Furfural 60 Unknown

Product distribution,Product % 20

0 0 50 100 150 200 250 time, min a) Figure 6a. Concentration of all products as a function of time in the hydrolytic hydrogenation of AG over Ru(2.5)/USY-15.

Galactose Arabinose Ethylene glycol 25 HMF Furfural Arabitol 20 Galactitol Unknown

Product distribution,Product% 5

0 0 50 100 150 200 250 time, min b) Figure 6b. Concentration of selected products as a function of time in the hydrolytic hydrogenation of AG over Ru(2.5)/USY-15.

AG and humins 100 Oligomers Galactose Arabinose 80 Ethylene glycol Arabitol Galactitol HMF 60 Furfural Unknown

Product concentration,Product% 20

0 0 50 100 150 200 250 time, min c) Figure 6c. Concentration of all products as a function of time in the hydrolytic hydrogenation of AG over Ru(2.5)/USY-30.

AG and humins Galactose 15 Arabinose Ethylene glycol Arabitol Galactitol 10 HMF Furfural Unknown

Product concentration,Product%

0 0 50 100 150 200 250 300 time, min d) Figure 6d. Concentration of selected products as a function of time in the hydrolytic hydrogenation of AG over Ru(2.5)/USY-15.

During the hydrolytic hydrogenation of AG over these catalysts, in addition to the main products detected also for the metal free pure zeolite, the formation of sugar alcohols was visible, confirming the hydrogenation ability of the catalyst. Comparison of the

21 rates for the parent and metal-modified USY-15 and USY-30 revealed that the initial

AG conversion is practically identical. A somewhat similar behavior has been reported in [31] when the addition of a supported iridium catalyst to the reaction mixture already containing zeolite ZSM-5 did not influence the conversion of cellulose. Very similar behavior in terms of product distribution was observed for ruthenium supported on both

USY-15 and USY-30, which can be explained by similar acidity of the catalysts.

In contrast, the presence of Ru in the USY zeolite resulted in the suppression of the dehydration reaction in analogy to previous results with Ru/beta catalysts [19].

The ratio between galactose and arabinose was initially close to unity but substantially decreased with an increasing conversion. This, along with a clear maximum for the galactose concentration, points to a much faster transformations of the latter compared to arabinose. It is also interesting to note that, contrary to a non-catalytic experiment or hydrolysis in the presence of zeolites, the formation of HMF over Ru(2.5)/USY-15 was less prominent than the generation of furfural at the end of the experiment.

Hydrogenation of glucose and arabinose on Ru in their mixtures typically proceeds with a similar rate [16] at least under milder conditions. Several scenarios were considered to explain the concentration profiles. One option is to assume that the more pronounced consumption of galactose and HMF is related to a reaction between them, which happens even in the absence of Ru but is certainly promoted by the metal. Another possibility is that an isomerization of an aldose (galactose and arabinose) to a ketose

(tagatose and ribulose) may occur with different rates and the unknown compound is an isomer of the sugars. Such reaction was recently shown to take place over Lewis-acid catalysts such as tin-promoted beta zeolites [32]. This second hypothesis was discarded by evaluating the retention times of ketoses, which elute at much longer times than the unknown compounds.

The alternative case of generating di- and tri-saccharides from galactose should be ruled out since the retention time of such oligomers is lower than for sugars, contrary to the observed unknown compound, which elute after sugars and prior to sugar alcohols.

Along the same line, the formation of acids such as arabinoic acid or oxidized forms of galactose (either acids or lactones) should be excluded since such oxidized forms of sugars have different retention times from the one for the unknown species. In addition, other types of transformations such as Cannizaro or cross-Canizzaro reactions of aldehydes (galactose and arabinose) to form the corresponding acids and alcohols or dehydration to anhydrosugars are irrelevant since they typically occur in basic media.

In order to address the issue of potential side reactions identification of the unknown compound was done by the so-called spiking technique, when the retention time of a range of compounds was determined separately and those compounds were added to the reaction samples. Among tested compounds several mono- and dissacharides as well as mentioned above sugar alcohols and acids were applied (fructose, mannose, mannitol, maltose, saccharose, lactose, arabinonic and galacturonic acids), confirming their absence in the reaction mixtures.

Another explanation for the observed kinetic behavior of galactose can be related to its cleavage. Since in the literature is it often reported [3] that hydrolytic hydrogenation can result in cleavage of carbon-carbon bond, retention times for several polyols with a shorter carbon chain were measured, confirming formation of ethylene glycol, observed also in the literature [42, 43] for transformations of cellulose. Interestingly enough no formation of other alcohols and polyols mentioned in the literature [42, 43] such as methanol, glycerol, 1,2-propanediol, meso-erythritol could be confirmed. Moreover there also was no formation of levulinic acid.

In order to address the issue of the hydrogenolysis products with carbon-carbon bond breaking, transformations of a mixture of arabinose and galactose in the presence of 23 hydrogen was studied where the initial ratio between the sugars was the same as obtained during the initial stages of AG hydrolysis (Figure 7).

40 Oligomers, Humins Galactose 30 Arabinose Ethylene glycol 20 Unknown Arabitol Galactitol

Product distribution,Product% 10 HMF Furfural

0 0 50 100 150 200 250 time, min a) Figure 7a. Concentration of products as a function of time for hydrogenation of a galactose and arabinose mixture (50 wt.%/50 wt.%) on Ru(2.5)/USY-15.

50 Oligomers Galactose Arabinose 40 Ethylene glycol Unknown HMF Furfural 30

Product composition,Product % 10

0 0 20 40 60 80 100 120 140 160 180 time, min b) Figure 7b. Concentration of products as a function of time for hydrogenation of a galactose and arabinose mixture (50 wt.%/50 wt.%) in a non-catalytic experiment.

As indicated by the profiles in Figure 7a, the hydrogenation rate of both sugars was similar and after 25 min the reaction was practically complete. Mainly arabitol and galactitol were present in the reaction mixture, besides traces of HMF and furfural and

24 heavier products, which eluted at the same time as oligomers. These latter compounds could be either aggregates of galactose or some sort of humins, formed from HMF and furfural. Another observation was that, besides these heavier compounds, the product where the carbon-carbon bond is broken (ethylene glycol) was observed in larger quantities than the unknown compound. The latter in fact could be could be a precursor of oligomers/humins.

Stability of sugar alcohols was further confirmed by a blank test, where the same mixture of sugars was treated at the same temperature without any catalyst (Figure 7b).

Obviously, no sugar hydrogenation reaction occurred due to the lack of redox metals in the system. Still, although at a lower rate, monosaccharides were transformed into ethylene glycol, an unknown compound, oligomers, as well as HMF and furfural. In fact, generation of the latter components was the most prominent. The product mixture became yellowish, which is an indication of humins formation. Note that in a catalytic experiment the resulting mixture was transparent.

The effect of the ruthenium loading on the hydrolytic hydrogenation of AG was studied testing the different Ru-modified USY-15 catalysts. As shown in Figure 8a, the rate of

AG hydrolysis and the concentration profiles for hemicellulose and oligomers over the various catalysts are essentially the same regardless of the metal content. This can be related to similar acidic properties of all Ru-modified zeolites. The same is valid for the concentration of arabinose (Figure 8b), while a very clear decrease in the concentration of galactose was observed for higher metal loadings (8c). Release profiles are in line with the preferential cleavage of arabinose.

A parabola like behavior of galactose could be a result of its more facile transformation to ethylene glycol on one hand and on another that galactose might react further forming an unknown compound, which can be a precursor for humins.

Formation of sugars was enhanced with a higher metal loading in the case of arabitol

(8d), while for galactitol formation such an increase was not evident (8e). Similar formation rates of sugar alcohols could be due to the fact that a higher metal loading does not correlate to a much higher metal surface because of a larger fraction of big particles. In addition, it can be also argued that the much more pronounced disappearance of galactose is related not only to hydrogenation of the latter but also to other competitive transformations.

1.0 % Ru 100 2.5% Ru 5.0% Ru

80 AG

Oligomers 40

Product distribution,Product % 20

0 0 50 100 150 200 250 time, min a) Figure 8a. Concentration of AG and oligomers as a function of time for the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

20 1% Ru 2.5 % Ru Arabinose 5 % Ru 16

Product distribution,Product% 4

0 0 50 100 150 200 250 time, min b) Figure 8b. Concentration of arabinose as a function of time for the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

Galactose 20 1% Ru 2.5 % Ru 5 % Ru 16

Product distribution,Product% 4

0 0 50 100 150 200 250 time, min c) Figure 8c. Concentration of galactose as a function of time for the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

7 1% Ru 6 2.5% Ru 5 % Ru 5

Arabitol , % Arabitol , 3

0 0 50 100 150 200 250 time, min d) Figure 8d. Concentration of arabitol as a function of time for the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

1,4

1,2

1,0

0,8 1% Ru 2.5% Ru 0,6 5 % Ru

Galactitol, % Galactitol,

0,4

0,2

0,0 0 50 100 150 200 250 time, min e) Figure 8e. Concentration of galactitol a function of time for the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

It was thus instructive to verify the concentration profiles for HMF, furfural, ethylene glycol and the unknown compound (Figure 9). The concentration of HMF was the highest in the case of Ru(1)/USY-15, while it was very low for higher ruthenium

28 contents (Figure 10a). This is a result of not only catalyst acidity per se and could be related to presence of competing pathways in the reaction system involving also metallic sites. Interestingly, the concentration of furfural was the highest for Ru(5)-

USY-15 even if this catalyst does not have strong and medium BAS and overall has the lowest amount of BAS. The trends in generation of ethylene glycol and the unknown compound as a function of the metal loading, were less clear (Figure 9 b, c).

6,4 1% Ru HMF 5,6 1% Ru furfural 2.5 % Ru HMF 4,8 2.5 % Ru furfural 5% Ru HMF 4,0 5% Ru furfural

3,2

2,4

Concentration,% 1,6

0,8

0,0 0 50 100 150 200 250 Time, min a) Figure 9a. Concentration of all HMF and furfural as a function of time in the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

12 1% Ru

Ethyleneglycol 2.5% Ru 8 5 % Ru

0 0 50 100 150 200 250 time, min

Figure 9b. Concentration of ethylene glycol as a function of time in the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

1% Ru 4 2.5% Ru 5 % Ru

Concentrationunknown, of %

0 0 50 100 150 200 250 time, min

c) Figure 9c. Concentration of the unknown product as a function of time in the hydrolytic hydrogenation of AG over Ru/USY-15 catalysts.

To elucidate a role of solid acids in transformations of galactose and arabinose, experiments were performed with these sugars over USY-15. The results in Figure 10 demonstrated the formation of some oligomers (or low molecular-mass humins) of type

II, which eluted at the same time as oligomers formed by hydrolysis of AG, denoted as oligomers of type I. HMF was also formed along with some amounts of hydrogenolysis product (ethylene glycol) and the unknown compound in the case of galactose, while for arabinose only furfural was produced in addition to oligomers and lower amounts of ethylene glycol than for the case of galactose. Generation of oligomers was more prominent than in the case of a galactose - arabinose mixture without any catalyst. An unusual behavior of galactose formation after 2 h of reaction can be related to its back- formation from aggregates.

100 Oligomers Galactose Ethylene glycol 80 Unknown HMF

Product composition,Product % 20

0 0 50 100 150 200 250 time, min

a) Figure 10a. Transformation of galactose over USY-15.

100 Oligomers 90 Arabinose 80 Ethylene glycol Unknown 70 Furfural

Concentration,%

0 0 50 100 150 200 250 time, min

b) Figure 10b. Transformation of arabinose over USY-15.

Additional experiments to elucidate the potential reaction network leading to unknown compounds were performed with USY-15. Transformation of arabinose in a mixture with furfural (Figure 11a) clearly that furfural is generated from arabinose, which can be seen as an increase of the furfural concentration during the reaction in comparison to the initial concentration. Moreover formation of oligomers, ethylene glycol and the unknown compound, the same as in hydrolytic hydrogenation, was observed. The 31 concentration of the sugar substantially declined with time. A similar profile was noticed for a mixture of galactose and HMF with the main difference that the formation of the ‘Unknown’ compound was not noticed.

120

110

100 Oligomers 90 Arabinose Unknown 80 Ethylene glycol 70 Unknown 2 60 Furfural

Concentration,% 30

0 0 50 100 150 200 250 time, min

a) Figure 11a. Transformation of arabinose-furfural mixture over USY-15.

100

80 Oligomers Galactose 70 Ethylene glycol 60 HMF

Product concentration,Product% 20

0 0 50 100 150 200 250 time, min

b) Figure 11b. Transformation of a galactose-HMF mixture over USY-15.

It is apparent from the results presented above that hexoses and pentoses (e.g. galactose and arabinose) degrade under acidic conditions to HMF and furfural forming also some

32 intermediates along this pathway and leading to the generation of some condensation products of different molecular weight. Several recent reports address mechanistic and kinetic aspects of hexoses and pentoses degradation [44-46]. One possibility discussed in the literature is that the unknown products are in fact dimers of HMF and furfural, although HMF is not a candidate for self-aldol condensation as it lacks an -hydrogen atom.

It is clear that the pathways for degradation of hexoses and pentoses are still under debate and more work is needed to establish the kinetic path for the formation of furfural and HMF as well as for the generation of the unknown compound. The latter can be tentatively assumed for the kinetic modelling described in the subsequent section as an intermediate on the path from sugars to their degradation products.

Kinetic modeling

In [19] a reaction network for hydrolytic hydrogenation of AG was proposed which besides hydrolysis and hydrogenation included formation of humins. In light of the information about the reaction pathways obtained in the current work, the scheme should be modified (Scheme 2) to include generation of ethylene glycol and the unknown compound from galactose and disappearance of HMF due to formation of minor amounts of humins.

Scheme 2. Reaction network for the hydrolysis of AG.

Initial modelling revealed that the reaction network can be somewhat simplified and reactions without numbers in Scheme 2 (transformations of arabinose and galactose to aggregates) can be omitted. Moreover reactions 9 and 12 in Scheme 2 were considered irreversible. A set of differential equations considering relevant stoichiometry and for simplicity first-order reactions were thus written based on the reaction network in

Scheme 2.

Numerical data fitting was done for the experimental data generated with Ru(5)/ USY-

15. The backward difference method was used for minimization of the sum of residual squares (SRS) with non-linear regression analysis using the Simplex and Levenberg-

Marquardt optimization algorithms implemented in the parameter estimation software

ModEst [47]. The sum of squares was minimized starting with Simplex and thereafter switching to the Levenberg-Marquardt method.

Estimated parameters and the standard errors are presented in Table 2, while the results are displayed in Figure 12. In general, a good correspondence between the experimental and calculated data was found with a degree of explanation 99.26%. The majority of parameters were rather well identified and the large error related to few of them is understandable taking into account the relatively limited set of experimental data and the fact that some products can be formed through different routes, preventing more reliable identification of the rate constants.

AG and humins 0,25 Oligomers Galactose Arabinose 0,20 Ethylene glycol Arabitol Galactitol 0,15 HMF Furfulal Unknown 0,10

concentration,wt%

0,05

0,00 0 50 100 150 200 250 time, min

Figure 12. Comparison between experimental data for Ru(5)/USY-15 and calculations.

Table 2. Values for the parameters. Kinetic constants correspond to reactions in Scheme 2.

Rate coefficient, Value Error, % min-1

k1 1.53 8.5

k2 0.84 25.3

k3 0.41 >100

k4 1.66 >100

k5 0.52 96

k6 0.68 12

k7 0.14 >100

k8 0.45 >100

k9 5.93 >100

k10 0.55 14.8

k11 1.38 7.7

k12 0.61 >100

k13 1.8 6.7

k14 2.0 31.5

Conclusions

The one-pot hydrolytic hydrogenation of arabinogalactan was herein investigated over bifunctional Ru-modified USY zeolites. Parameters including the Ru content (1 to

5 wt.%) and the acidity of the support were varied in order to maximize the yield of arabinitol and galactitol. Remarkably, yields up to 23% to the desired alditols and a limited amount of high molecular-weight compounds (aggregates of sugars and humins) were attained. Accordingly, Ru/USY zeolites stand as more effective catalyst than

Ru/beta materials previously reported for this application. Monitoring the product distribution obtained from different known reaction intermediates, insights into the hydrolytic hydrogenation mechanism were gathered in terms of the nature of the reactions determining the formation of aggregates, humins, and oligomers. Based on the advanced reaction network derived, a kinetic model closely fitting the experimental data was successfully developed.

AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS

This work is part of the activities at the Åbo Akademi University Process Chemistry

Centre. The Swiss National Science Foundation (Project Number 200021-140496) is acknowledged for financial support. Dr. S. Mitchell is thanked for TEM analyses.

REFERENCES

1. P. Mäki-Arvela, B. Holmbom, T. Salmi, D. Yu. Murzin, Catal. Rev. 49 (2007)

197-340.

2. D. Yu. Murzin, I. L. Simakova, Catalysis in biomass conversion, in

Comprehensive Inorganic Chemistry II, vol. 7, From Elements to Applications,

R. Schlögl and J. W. Niemantsverdriet (Eds.) 2013, Elsevier, vol. 7, pp. 559-

586.

3. M. Yabushita, H. Kobayashi, A. Fukuoka, Appl. Catal., B145 ( 2014) 1-9.

4. A. A. Balandin, N. A. Vasyunina, G. S. Barysheva, S.V. Chepigo, Bull. USSR

Acad. Sci., Div. Chem. Sci. 6 (1957), 392.

5. A. Fukuoka, P. L. Dhepe, Angew. Chem. Int. Ed. 45 (2006) 5161-5163.

6. Y. B. Huang, Y. Fu, Green Chem. 15, 2013, 1095-1111.

7. J. Geboers , S. Van de Vyver , K. Carpentier, P. Jacobs, B. Sels, Green Chem. 13

(2011) 2167-2174.

8. A. Negoi, K. Triantafyllidis, V. I. Parvulescu, S.M. Coman, Catal. Today 223

(2014) 122-128.

9. J. Geboers, S. Van de Vyver, K. Carpentier, P. Jacobs, B. Sels, Chem. Comm. 47

(2011) 5590-5592.

10. M. Käldström, N. Kumar, D. Yu. Murzin, Catal. Today 167 (2011) 91-95.

11. M. Käldström, N. Kumar, M. Tenho, M. V. Mokeev, Y. E. Moskalenko, D. Yu.

Murzin, ACS Catal. 2 (2012) 1381-1393.

12. .P. Mäki-Arvela, T. Salmi, B. Holmbom, S. Willför, D. Yu. Murzin, Chem. Rev.

111 (2011) 5638-5666.

13. B. T. Kusema, G. Hilmann, P. Mäki-Arvela, S. Willför, B. Holmbom, T. Salmi,

D. Yu. Murzin, Catal. Lett. 141 (2011) 408-412. 38

14. P. Demma Carà, M. Pagliaro, A. Elmekawy, D. R. Brown, P. Verschuren, N. R.

Shiju, G. Rothenberg, Catal. Sci. Technol. 3 (2013) 2057-2061.

15. V. A. Sifontes Herrera, O. Oladele, K. Kordas, K. Eränen, J. -P. Mikkola, D.

Yu. Murzin, T.Salmi, J. Chem. Techn. Biotech. 86 (2011) 658-668.

16. V. A. Sifontes Herrera, F. Salem, B. Kusema, K. Eränen, T.Salmi, Top. Catal.,

55 (2012) 550-555.

17. A. Aho, S. Roggan, O. Simakova, T. Salmi, D. Yu. Murzin, Catal. Today 241

(2015), 195-199.

18. B. T. Kusema, L. Faba, N. Kumar, P. Mäki-Arvela, E. Díaz, S. Ordoñez, T.

Salmi, D. Yu. Murzin, Catal. Today 196 (2012) 26-33.

19. B. T. Kusema, L. Faba, N. Kumar, P. Mäki-Arvela, E. Díaz, S. Ordoñez, T.

Salmi, D. Yu. Murzin, Microporous Mesoporous Mater. 189 (2014) 189-199.

20. I.V. Sumerski, S. M. Krutov, M. Ya. Zarubin, Russ. J. Appl. Chem. 83 (2010)

321-328.

21. S. K. R. Patil, C. R. F. Lund, Energy Fuels 25 (2011) 4745-4755.

22. S. K. R. Patil, J. Helzel, C. R. F. Lund, Energy Fuels 26 (2012) 5281-5293.

23. I. van Zandvoort, Y. Wang, C. B. Rasendra, E. R. H. van Eck, P. C. A.

Bruijnincx, H. J. Heeres, B. M. Weckhuysen, ChemSusChem 6 (2013) 1745-

1758.

24. Y. Nakagawa, M. Tamura, K. Tomishige, ACS Catal. 3 (2013) 2655-2668.

25. T. D. Swift, C. Bagia, V. Choudhary, G. Peklaris, V. Nikolakis, D. G. Vlachos,

ACS Catal. 4 (2013) 259-267.

26. A. H. Øygarden, J. Pérez-Ramírez, D. Waller, K. Schöffel, D. Brackenbury,

WO2004/110622, 2004.

27. M. Santiago, A. Restuccia, F. Gramm, J. Pérez-Ramírez, Microporous

Mesoporous Mater. 146 (2011) 76-81. 39

28. F. Frechard, P. Sautet, Surf. Sci. 389 (1997) 131-146.

29. A.S. Boymirzaev, Sh. Shomuratov, A.S. Turaev, Khimija Rastitel’nogo Syr’ja, 2

(2013) 51-55.

30. B. T. Kusema, C. Xu, P. Mäki-Arvela, S. Willför, B. Holmbom, T. Salmi, D.

Yu. Murzin, Int. J. Chem. Reactor Eng. 8 (2010) 1-18.

31. S. Liu, M. Tamura, Y. Nakagawa, K. Tomishige, ACS Sustain. Chem. Eng. 2

(2014) 1819-1827.

32. J. Dijkmans, D. Gabriëls, M. Dusselier, F. de Clippel, P. Vanelderen, K. Houthoofd, A. Malfliet, Y. Pontikes, B. F. Sels, Green Chem. 15 (2013) 2777 - 2785. 33. D. Kubicka, N. Kumar, P. Mäki-Arvela, M. Tiitta, V. Niemi, H. Karhu, T. Salmi, D. Yu. Murzin, J. Catal, 227 (2004) 313-327. 34. D. Kubicka, N. Kumar, T. Venäläinen, H. Karhu, I. Kubickova, H. Österholm, D.Yu. Murzin, J. Phys. Chem. B, 110 (2006) 4937-4946. 35. J.I. Villegas, D. Kubicka, H. Karhu, H. Österholm, N. Kumar, T. Salmi, D.Yu.

Murzin, J. Molec. Catal. A. Chem., 264 (2007) 192-201.

36. L.A. Pedersen, J.H. Lunsford, J Catal. 61 (1980) 39-47.

37. S. Van de Vyver, J. Geboers, W. Schutyser, M. Dusselier, P. Eloy, E.Dornez, J.

Won Seo, C. M. Courtin, E. M. Gaigneaux, P. A. Jacobs, B. F.

Sels, ChemSusChem, 5 (2012) 1549-1558.

38. S. Van de Vyver, J. Geboers, P. A. Jacobs, B. F. Sels, ChemCatChem, 3 (2011)

82–94.

39. R. Palkovits, K. Tajvidi, A. M. Ruppert, J. Procelewska, Chem. Commun., 47,

(2011) 576–578

40. R. M. Ravenelle, F. Schüβler, A. D’Amico , N. Danilina, J. A. van Bokhoven, J.

A. Lercher, C. W. Jones, C. Sievers, J. Phys. Chem. C, 114 (2010) 19582–

19595.

41. T. Ennaert, J.Geboers, E.Gobechiya, C.M.Courtin, M.Murttepeli, K.Houthoofd,

C. E.A. Kirschhock, P. C.M.M. Magusin, S.Bals, P. A. Jacobs, B. F. Sels, ACS

Catal.,5 (2015) 754–768.

42. K. Tajvidi, P. Hausoul, R. Palkovits, ChemSusChem, 7 (2014) 1311-1317.

43. R. Sun, T. Wang, M. Zheng, W. Deng, J. Pang, A. Wang, X. Wang, T. Zhang,

ACS Catal. 5 (2015) 874-883.

44. B. Danon, G. Marcotullio, W. de Jong, Green Chem. 16 (2014) 39-54.

45. R. –J. V. Putten, J. N. M. Soetedjo, E. A. Pidko, J. C. van der Waal, E. J. M.

Hensen, E. de Jong, H. J. Heeres, ChemSusChem 6, (2013) 1681-1687.

46. D. J. Liu, E. Y. -X. Chen, ACS Catal. 4 (2014) 1302-1310.

47. H. Haario, ModEst 6.0, User Guide, Helsinki, 2010.