Influence of Support for Ru and Water Role on Product Selectivity in

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Influence of Support for Ru and Water Role on Product Selectivity in the Vapour Phase Hydrogenation of Levulinic Acid to #Valerolactone: Investigation by Probe Adsorbed FT-IR Spectroscopy Vijay Kumar Velisoju, Ganga Bhavani Peddakasu, Naresh Gutta, Venu Boosa, Manasa Kandula, Komandur V. R. Chary, and Venugopal Akula J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06003 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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The Journal of Physical Chemistry

Influence of Support for Ru and Water Role on Product Selectivity in the Vapour Phase Hydrogenation of Levulinic Acid to γ-Valerolactone: Investigation by Probe Adsorbed FT-IR Spectroscopy Vijay Kumar Velisoju*, Ganga Bhavani Peddakasu, Naresh Gutta, Venu Boosa, Manasa Kandula, Komandur VR Chary, Venugopal Akula*

Catalysis and Fine Chemicals Division, CSIR - Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, Telangana - 500 007 India.

ACS Paragon Plus Environment

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

ABSTRACT: Ru supported on activated carbon, Al2O3 and MgO were assessed for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). Role of H2O on the hydrogenation activity of Ru was studied by probe adsorbed diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Ru supported on activated carbon showed a maximum productivity of 1.18 kgGVL kgcatalyst-1 h-1 with an insignificant loss in the activity after 72 h of continuous operation in presence of H2O. Using pure LA, GVL rate was decreased by an order of magnitude (0.12 kgGVL kgcatalyst-1 h-1) with in 6 h of reaction time. The physico chemical characteristics of the catalysts were examined by temperature programmed desorption (TPD) of NH3, CO pulse chemisorption, H2-temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) techniques. H2O adsorbed DRIFT spectroscopic data revealed the reversible generation of surface –OH groups when aqueous LA was used as the substrate; consequently, Ru/C catalyst stability was also improved. Finally, based on the kinetic and in-situ spectroscopic data, a plausible surface reaction mechanism is proposed for the vapour phase LA hydrogenation to GVL in presence of H2O over the carbon supported Ru catalyst.


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1. INTRODUCTION Hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL; fuel additive and green solvent) received much attention in the process technology of biomass conversion to fine chemicals and liquid transportation fuels.1 LA is an attractive platform chemical that can be produced from inedible biomass feedstock such as lignocellulose.2 Different strategies were proposed for LA production and its conversion to GVL, which offers many applications such as fuel additive and green solvent.3-5 Recently studies were focussed on the hydrogenation of LA to GVL at high H2 pressures in the batch processes using different solvents.6-10 Therefore, LA hydrocyclisation to GVL in the vapour phase catalytic method also seems to be useful from the environmental and safety perspectives.11 In liquid phase, mostly Ru based catalysts were proven to be active and stable using hydrogen and formic acid as hydrogen source and achieved 100% GVL yields at different reaction temperatures and pressures.12-14 Based on the kinetic data, two reaction pathways were proposed such as angelica lactone (AL) and 4-hydrooxypentanoic acid as intermediates.15, 16 Studies in liquid phase hydrogenation of LA to GVL over the supported Ru catalysts were reported recently.17,18 However, vapour phase catalytic hydrogenation is a simpler method than conventional batch processes in liquid phase at elevated H2 pressures.19,20 The important aspect of aqueous LA conversion to GVL should be achieved at moderate temperatures with minimum by-products over a stable catalyst. Most of the base metal catalysts suffer deactivation in presence of large excess of H2O present in the LA feed stream that could be anticipated from biomass-derived 5-HMF to LA.21-23 Hence, the catalyst system that sustains longer life in presence of H2O under vapour phase conditions is necessary from atom economy to environmental standpoint. In addition, many reports are appeared on LA to GVL over the Ru/C catalyst, but few reports emphasized the reasons for stable activity in the vapour phase conversion of LA to GVL. Role of H2O on the conversion


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of LA to GVL was first explained by Tan et al. describing the influence of surface –OH group involved in the reaction medium in liquid phase.24 In the present study, an attempt has been made to examine the hydrogenation activity of Ru supported on carbon, Al2O3 and MgO supports in the vapour phase and detailed studies are carried out to explain the reversible generation of –OH groups and their involvement in the hydrogenation of LA to GVL by using probe adsorbed DRIFT spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Levulinic acid (LA; 98%), RuCl3 .xH2O (99.9%), %), αangelica lactone (AL; 98%), γ-valerolactone (GVL; 99%), acetone (>99%), methanol (>99%), formic acid (98%), anisole (99.7%) and pyridine (99.8%) obtained from Sigma-Aldrich (AR grade) used without further purification. De-ionised water was used for the dilution of substrates and for other experiments. The Activated Carbon (Sigma-Aldrich; BET-surface area (SBET) = 344 m2 g-1), γ-Al2O3 (Harshaw Al-3945; SBET = 209 m2 g-1) and MgO (Sud Chemie. Pvt. Ltd.; SBET = 87 m2 g-1) obtained were used as supports for the preparation of supported Ru catalysts.

2.2. Catalyst Preparation. The Ru supported on carbon, Al2O3 and MgO catalysts were prepared by an incipient wetness impregnation method.25 In a typical procedure, required amount of aqueous RuCl3.xH2O precursor corresponding to give 2wt%Ru was instantaneously mixed with a known amount of solid support under continuous stirring at 100 °C until the excess water was evaporated. The obtained powder samples were dried at 120 °C for overnight and calcined/annealed at 450 °C for 4 h.


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2.3. Catalyst Characterization and Activity Measurements. The calcined samples were reduced at 450 °C in 5%H2/Ar for 3 h and were characterized by powder XRD, BET-surface area, H2-TPR, NH3-TPD, CO-pulse chemisorption, CHNS, AAS, XPS and H2O/pyridine adsorbed DRIFT spectroscopy. Hydrogenation activities were carried out in a continuous flow fixed-bed quartz reactor under atmospheric H2 pressure. The experimental conditions and the methods adopted in the present study for the preparation, characterization and catalytic screening of the catalysts are identical to the experimental conditions used in our earlier studies and the complete details are also given in supporting information.26-28

3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffraction (XRD). The powder XRD patterns of supported Ru catalysts are presented in Figure S1. Ru/C showed a broad diffraction signal in the 2θ range from 2030° is attributed to the amorphous nature of activated carbon.29 No diffraction peaks due to Ru were observed in all the samples which is explained either due to fine dispersion of Ru or due to a lower loading of Ru on the support. The remaining diffraction lines present in the XRD patterns of Ru/Al2O3 and Ru/MgO samples are assigned to the diffraction pattern of Al2O3 (ICDD # 85-1337) and MgO (ICDD # 87-0652) phases respectively.

3.2. H2-Temperature Programmed Reduction (H2-TPR) Analysis. The reduction behaviour of supported Ru catalysts is analyzed by H2-TPR (Figure 1) and the corresponding H2 uptakes are reported in Table 1. The TPR pattern of Ru/C sample showed a major reduction signal centred at 317 °C which is due to the reduction of ionic Ru and the shoulder peak around 366 °C is ascribed to a large cluster of ruthenium species.21 In case of Ru/MgO and Ru/Al2O3 catalysts, broad reduction signal (deconvoluted to two reduction signals) is observed relatively at high temperatures compared to Ru/C sample. This broad reduction


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signals in Ru/MgO and Ru/Al2O3 samples is an indication of large clusters of ionic Ru strongly interacted with Al2O3 and MgO. This argument is however evidenced from the COpulse chemisorption analysis of these catalysts which showed relatively large particle size of Ru over Ru/MgO and Ru/Al2O3 samples compared to Ru/C (Table S1).

Figure 1. H2-TPR profiles of calcined a) Ru/Al2O3, b) Ru/MgO and c) Ru/C catalysts.

3.3. NH3-Temperature Programmed Desorption (NH3-TPD) Studies. The acid site distribution on catalyst surface is determined by temperature programmed desorption of NH3 (Figure 2). The NH3 desorption profiles show mainly 2 to 3 signals one at less than 250 °C ascribed to weak; peaks in the range 250-400 °C is attributed to moderate and >450 °C signals are assigned as the interaction of NH3 with strong acid sites.30 The NH3 uptakes obtained from this study (Table 1) revealed a high ratio of acid sites on Ru/C followed by Ru/Al2O3 and Ru/MgO samples. Relatively high amount of acidity present in all the catalysts may also partly attributed to the presence of trace amounts of chloride ions on the catalyst surface. The results obtained in this analysis are also in good correlation with the reported studies.8,21 The lower NH3 uptake on Ru/Al2O3 and Ru/MgO samples is obviously due to the


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low surface area of Al2O3 and MgO. These patterns clearly revealed that Ru/C presented a higher number of weak to moderate acid sites compared the Ru/Al2O3 and Ru/MgO samples. However, the normalized NH3 uptakes showed relatively a higher specific acidity on Ru/Al2O3 (0.529 µmol/m2) followed by Ru/C (0.314 µmol/m2) and Ru/MgO (0.451 µmol/m2).

Figure 2. NH3-TPD profiles of reduced a) Ru/MgO, b) Ru/Al2O3 and c) Ru/C catalysts.

3.4. Pyridine Adsorbed DRIFT Spectroscopy. Pyridine adsorbed DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy is used to identify the type and strength of acid sites (Figure 3). The quantified (using Kubelka-Munk function) DRIFT spectra (Figure 3A) showed vibrational bands in the range of 1400-1700 cm-1. These bands are assigned to the interaction of pyridine with both Brønsted and Lewis acid sites present on the catalyst surface.31 The spectra show a high ratio of Lewis acid sites (co-ordinated pyridine: ~1450 and 1610 cm-1) compared to Brønsted acid sites (protonated pyridine: ~1540 and 1640 cm-1).32,33 A low ratio of Brønsted acid sites on these samples is explained due to lower number of surface -OH groups on the catalyst surface. The ratios of BAS to LAS obtained


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from these spectra revealed slightly high BAS/LAS ratio on Ru/Al2O3 and Ru/MgO samples compared to Ru/C (Table 1). The stability of acid sites on Ru/C surface is also analyzed from the DRIFT spectra of pyridine adsorbed on Ru/C sample by subjecting it to degassing at different temperatures (150 to 300 °C; Figure 3B). The results revealed insignificant changes either in peak position or the peak intensity even at high degassing temperatures up to 275 °C. The normalized peak ratios attributed to the vibrational bands at 1445 and 1589 cm-1 (calculated using the FWHM of these IR signals) revealed that there is no significant change in the peak intensities collected at various degassing temperatures. These results thus confirm that the acid sites present on the catalyst surface are stable enough to be involved in the reaction under the experimental conditions used in this study (275 °C).

Figure 3. Pyridine adsorbed DRIFT spectra of reduced (A) a) Ru/C, b) Ru/Al2O3 and c) Ru/MgO catalysts and (B) Ru/C sample at different degassing temperatures.

3.5. X-Ray Photoelectron Spectroscopy (XPS). The survey scan of the catalysts (Figure S3A) revealed the presence of all elements (Ru, O, C, Mg and Al) in their respective spectra and the binding energy (BE) values are in good agreement with the reported values.34-35


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Presence of two XPS signals in O 1s spectra (Figure S3B) of Ru/MgO and Ru/Al2O3 indicates that the ruthenium particles present on the catalyst surface is only partially converted to metallic state and some of the species are in ionic state on both Ru/MgO and Ru/Al2O3 samples. In contrast, Ru/C showed one major peak along with a small hump indicating that the extent of Ru reduction is far greater than that is observed for other two catalysts. The Ru 3d spectra (Figure S3C) showed two main signals corresponding Ru 3d5/2 and 3d3/2 at a binding energy of 280.1 and 284.0 eV respectively.34 These results are in good correlation with the results obtained from CO-pulse chemisorption which indicated a high Ru metal surface area on Ru/C catalyst. In addition, Ru/C sample showed a sharp signal for Ru 3d whilst the other two samples exhibited broad signals with significantly low in intensity. This observation further supplements from the TPR analysis (Figure 2) which revealed a high (Tmax) temperature reduction profiles for Ru/MgO and Ru/Al2O3 catalysts compared Ru/C.

3.6. Physicochemical Characteristics of the Catalysts. N2-sorption analysis of the samples showed high surface area for the Ru/C followed by Ru/Al2O3 and Ru/MgO catalysts (Table 1) which are consistent with the surface area of the supports (Section 2.1). The H2-uptakes estimated from H2-TPR analysis showed more or less similar which can be explained due to a fixed loading of Ru emphasizing the extent of reduction of ionic Ru seems to be same over all the three supports. CO-pulse chemisorption data (Table 1 and Table S1) showed a high metal surface area for Ru/C compared to Ru/Al2O3 and Ru/MgO samples. The relatively high dispersion of Ru on carbon support is however substantiated by the N2-sorption analysis which showed a higher surface area for carbon support. The larger Ru particles on Ru/MgO and Ru/Al2O3 samples are explained due to agglomeration of Ru on the low surface area supports such as MgO and Al2O3. NH3 uptakes obtained from TPD analysis revealed a relatively higher uptake over the Ru/C followed by Ru/Al2O3 and Ru/MgO samples. The


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higher NH3 uptake over the Ru/C sample is however attributed to higher number of surface Ru species (Table 1) and is most likely contributed as Lewis acid sites on the surface. This is further confirmed using pyridine adsorbed DRIFTS data which revealed a lower BAS/LAS ratio (high proportion of LAS) for Ru/C followed by Ru/Al2O3 and Ru/MgO catalysts.

Table 1. Physico chemical properties of supported Ru catalysts. 2wt%Ru SBET H2 uptake CO uptake SRu supported on (m2/g)a (µmol/gcat)b (µmol/gcat)d (m2/gRu)e Carbon 270.5 182.4 76.8 189.9 Al2O3 135.2 179.6 63.1 156.0 MgO 62.7 178.3 60.9 150.6

NH3 uptake (mmol/g)c 0.085 0.071 0.028

BAS/LAS ratiof 0.014 0.024 0.032

a

Determined from BET surface area analysis; Obtained from H2-TPR analysis; c Determined from NH3-TPD analysis; d Obtained from CO-pulse chemisorption experiments; e Calculated using CO-uptakes; f Calculated using pyridine adsorbed DRIFT spectra.

b

3.7. Catalytic Activity Measurements. The LA hydrogenation was carried out in continuous fixed-bed reactor over the carbon, Al2O3 and MgO supported Ru catalysts at 275 °C under atmospheric H2 pressure (Table 2). The results of this investigation revealed a high GVL selectivity with high conversion of LA over the Ru/C catalyst compared to Ru/Al2O3 and Ru/MgO catalysts. High LA conversion over Ru/C catalyst is probably due to a high Ru metal surface area of the catalyst revealed by CO-pulse chemisorption data (Table 1). While comparing the GVL productivity with reported studies (Table S2), a high GVL productivity of 1.18 kgGVL kgcatalyst-1 h-1 is found over Ru/C compared to earlier reports.36-38 In case of Al2O3 and MgO supports, α-angelica lactone (AL) is found as a by-product which is relatively higher on Ru/MgO catalyst. Presence of AL in the product mixture indicates that the reaction proceeds via dehydration of LA followed by hydrogenation of AL to form GVL which is more feasible at moderate reaction temperatures.20 Using carbon as a support for Pt, Pd and Ni catalysts, GVL rate is very high on Ru/C (Table 2). The LA conversion is almost similar over the Pt/C and Pd/C catalysts compared to Ru/C, whereas the selectivity towards


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AL and other compounds are higher over Pt/C, Pd/C and Ni/C. Particularly, the Pd/C and Pt/C catalysts showed a significant amount valeric acid (VA) and 2-methyltetrahydrofuran (MTHF) which occurred due to the ring opening and/or deep hydrogenation of GVL respectively; consequently a decrease in GVL selectivity is observed. Formation of VA over Pd supported on carbon can be explained by hydrogenolysis of GVL at 275 °C and MTHF formation over Pt/C is occurred due to deep hydrogenation of GVL, although fairly high metal surface area is observed for these catalysts (Table S1). In case of Ni/C, the AL selectivity is slightly high suggesting its poor hydrogenation activity compared to the noble metal catalysts maybe due to low metal surface area as observed in the CO pulse chemisorption experiments (Table S1). Although Ni and Cu are proven to be active for this conversion when they are supported on TiO2, H-ZSM-5 and SiO2, but the Ni/C showed relatively poor activity towards GVL.36-38 These results clearly suggested that the Ru/C is active and selective catalyst for the LA hydrogenation to GVL compared to Pt, Pd and Ni catalysts in presence of H2O.37 Table 2. LA conversion to GVL over supported metal catalysts. Reaction conditions: H2 flow: 20 cc/min; GHSV = 16.2 mL s-1 gcat-1; Reaction temperature: 275 °C; Feed: 10wt% LA in H2O; Data collected after 6 h of continuous operation.

(2wt% metal)

LA conversion Selectivity (mol%) (mol%) GVL AL VA MTHF Pd/C 9.8 72.2 3.9 22.0 Pt/C 16.2 62.9 4.5 7.1 24.3 Ni/C 2.5 69.3 23.9 5.4 Ru/Al2O3 10.5 83.2 8.9 7.5 Ru/MgO 9.5 77.9 16.6 5.2 Ru/C 16.8 99.0 1.0 Ru/Ca 9.2 98.7 1.3 Ru/Cb 18.0 90.3 0.8 6.9 c Ru/C 19.9 82.1 1.2 13.5 a,b,c Reaction carried out at 250, 300 and 325 °C respectively. VA and MTHF: Valeric acid and 2-methyltetrahydrofuran.

Productivity (kgGVL kgcatalyst-1 h-1) 0.51 0.72 0.13 0.62 0.53 1.18 0.64 1.15 1.16

Upon increasing the reaction temperature from 250 to 275 °C, LA conversion was increased with no change in GVL selectivity (Table 2). Further increase from 275 to 325 °C,


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a drop in GVL selectivity is observed with a small increase in the LA conversion (Table 2). The decrease in GVL selectivity was at the cost of valeric acid production occurred via ring opening of GVL as this step is more feasible at high reaction temperatures (Table 2).9, 20 The product distribution over the Ru/C catalyst is also investigated at different LA conversions and the results are illustrated in Figure S4. In this analysis, the activity tests were conducted by adjusting the GHSV at 275 °C. At and above 10% conversion (low GHSV), the Ru/C catalyst showed no significant changes in the GVL selectivity (>99%). Whereas at 5% LA conversion (higher GHSV), a small decrease in GVL selectivity is however observed which is probably due to less contact time. In such case, angelica lactone (AL) was observed as the major product (8%) indicating the conversion of LA to AL is a fast step in this reaction and AL to GVL is the rate determining step. Therefore, the GVL selectivity is decreased at lower LA conversion (higher GHSV) which is also supported by the reported literature.20

3.8. Catalysts Stability Studies. The time on stream analysis is also tested over these catalysts at higher LA conversions (Table 3). In the comparative analysis, Ru/C exhibited long-term stability even after 72 h of continuous operation compared to other catalysts emphasizing its robustness in presence of huge amounts of H2O in the feed. On the other hand, a decrease in both LA conversion and GVL selectivity was observed after 30 h over the Ru/MgO and Ru/Al2O3 catalysts (Table 3) which is attributed to the coke deposition occurred during the course of the reaction. Table 3. Catalysts activity as a function of time; Reaction conditions: H2 flow: 20cc min-1; GHSV = 4.87 mL s-1 gcat-1; Reaction temperature: 275 °C; Feed: 10wt% LA in H2O. Catalyst Time LA conversion GVL selectivity Carbon Ru content (wt%)b a (%) (%) (wt%) Reduced Used

Ru/C 72 h 99 98 0.4 1.87 Ru/Al2O3 30 h 32.7 82.9 7.5 1.92 Ru/MgO 30 h 24.4 70.6 6.4 1.90 a,b Obtained from CHNS and AAS analysis of the catalysts respectively.


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1.91 1.90 1.95

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The significant coke deposition observed in CHNS analysis of used Ru/Al2O3 and Ru/MgO catalysts is most likely due to the irreversible adsorption of substrate and/or intermediates i.e. LA and AL.23 Therefore, the Ru/C is found suitable for the hydrogenation of aqueous LA to GVL under the experimental conditions adopted. AAS analysis of Ru/C and Ru/Al2O3 samples (after 72 and 30 h respectively) showed no significant leaching of Ru species during the hydrogenation of LA unless otherwise stated. From the above results, it is clear that coking may be a possible reason for the decline in activity of Ru/MgO and Ru/Al2O3 catalysts.

3.9. Role of H2O on the LA Conversion to GVL. The previous reports suggested that the role of water in the reaction media play an important role in the LA hydrogenation to GVL in liquid phase.24, 39 Tan et al. reported high GVL yields while LA in water as a reaction media and proved that water would be involved in the reaction by H spillover on the catalyst surface.24 Michael et al. reported that water is responsible for enhanced reactivity of Ru in the conversion of LA to GVL using DFT calculations and experimental data.39 Hence, we made an attempt to investigate the role of H2O vapour in the continuous phase LA hydrogenation to GVL (Figure 4). For this purpose, different LA concentrations (5, 10, 20, 30, 40, 50, 70wt% in H2O and pure LA) were tested over the Ru/C catalyst and the results are presented in Figure 4. With increase in LA concentration, a gradual decrease in both LA conversion and GVL selectivity is observed. In parenthesis, an increase in AL selectivity is observed. Although, few by-products such as valeric acid and pentane diols are formed, no significant amounts of these compounds are seen and their selectivity is also found not to be a significant extent (< 5%). To conclude, the maximum LA conversion and GVL selectivity (high productivity) are obtained at 5 and 10wt% LA concentrations, above which a significant drop in activity is observed. This is explained due to irreversible chemisorption of


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intermediate compound i.e. AL on the catalyst surface which might be blocking the surface active species in the LA conversion to GVL. This statement is however supported by the elemental analysis (CHNS) of the used Ru/C catalysts which showed a significant amount of coke deposition ca. 2.1 and 6.9wt% carbon in case of 50wt% LA in H2O and the activity data obtained using pure LA as substrate respectively. Experiments are also carried out at isoconversion levels under similar protocol, the results revealed no significant differences in GVL selectivity.

Figure 4. Influence of LA concentration on the activity of Ru/C catalyst. Reaction conditions: H2 flow: 20 cc min-1; Reaction temperature: 275 °C; Feed: 5-100wt% LA in H2O.

This activity data substantiate the role of H2O in the selective formation of GVL from LA. Therefore, the role of H2O on the LA conversion is examined by H2O adsorbed IR spectroscopic studies by mimicking the reaction conditions adopted for the hydrogenation of aqueous LA (Figure 5). The normalized DRIFT spectra of the catalysts after H2O adsorption showed the intense vibrational bands around 3200-3400 and 1620-1640 cm-1 region the


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stretching and bending vibrations of –OH group. All the samples showed intense peaks (corresponding to –OH group vibrations) at 125 °C. When the temperature of IR cell increased to 275 °C (i.e. reaction temperature), a significant decrease in the peak intensity is observed. However, the peak intensity was strong at 275 °C for Ru/C followed by Ru/MgO and Ru/ Al2O3 catalysts. These results thus confirms that the –OH groups are generated on the catalyst surface under the experimental conditions adopted. Although, the stability of these surface hydroxyl groups seems to be less (semi-stable), it is believed that they are still involved in the LA conversion to AL (first step) according to the experimental data obtained from H2O adsorbed DRIFTS. In addition, it was also proposed that the spillover of H on the catalyst surface from H2O further enhances the Ru hydrogenation capacity to convert AL to form GVL as evidenced from a recent study on LA conversion to GVL in H2O.24 The normalized DRIFT spectra emphasized that the relative ratio of 1620 cm-1 signal to 3350 cm-1 is 0.13 on reduced sample and 0.122 on the H2O adsorbed on Ru/MgO sample (Figure 5a). On the other hand, the relative ratio of these bands over Ru/Al2O3 sample indicated 0.078 and 0.097 for the reduced and reduced+H2O adsorbed spectra respectively (Figure 5b). It shows a marginal enhancement in the surface –OH groups over Ru/Al2O3 compared to Ru/MgO sample. Quite contrast to this, a significant increase in surface –OH groups is observed on Ru/C sample with the (I1620/I3350) ratio of 0.12 in case of reduced and 0.23 on the reduced+H2O dosed sample. This improvement in surface –OH groups on Ru/C can also be explained by a high surface area of carbon support and also various functional groups namely –OH stretching frequencies due to hydroxyl, carboxylic and phenolic groups appear in the 3200-3400 cm-1 of IR spectra.40-41


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Figure 5. Steam adsorbed DRIFT spectra of a) Ru/MgO, b) Ru/C and c) Ru/Al2O3 catalysts.

In addition, the vibrational band due to metal co-ordinated H2O molecule at 3550 cm-1 is also compared between reduced and H2O exposed samples (Figure 6). The normalized DRIFT spectra of the reduced and H2O adsorbed spectra showed a H2O co-ordinated to Ru band at ~3550 cm-1.42 An intense Ru-H2O band is appeared on Ru/C and this signal is weak in case of Ru/Al2O3. Although TPR is a bulk technique; a basis of the Tmax (Figure 1) is considered here for comparing the extent of reduction of Ru supported on carbon, Al2O3 and MgO, since the H2O dosed DRIFT spectra was collected after reduction followed by H2O adsorption (Figure 5). These results are in good correlation with the H2-TPR pattern which exhibited a higher Tmax over Ru/Al2O3; quite contrast to this, the Tmax of Ru/C was low. These results thus infer that the reduction of ionic Ru species on carbon support occurred at low temperature as a result of it, an increase in the intensity of Ru-H2O was observed due to high surface coverage of H2O coordinated to surface Ru. On the contrary, Al2O3 supported Ru sample displayed a weak (Ru-H2O) band at 3550 cm-1. Although the Ru-H2O peak was weak over Ru/MgO compared to Ru/C; but was found to be slightly higher when compared to Ru/Al2O3.


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Figure 6. DRIFT spectra of reduced (dotted lines) and H2O adsorbed (thick lines) of Ru/MgO (a, a’), Ru/Al2O3 (b, b’) and Ru/C (c, c’) catalysts.

As matter of interest in this study, the most stable and active Ru/C catalyst is further investigated in detail to find the reasons for its high activity in the vapour phase LA conversion to GVL. The comparison of reduced catalyst with that of H2O adsorbed spectra of Ru/C sample showed more intense peak corresponding to –OH stretching and bending vibrations over H2O adsorbed spectrum. In order to confirm the stability of these surface hydroxyl groups (on Ru/C) formed after H2O exposure, the spectra was also collected in different time intervals at 275 °C in an inert gas flow (Figure 7A). The spectra show a gradual decrease in peak intensity (corresponding to –OH surface hydroxyl groups) with increase in degassing time until 30 min indicating that the regenerated surface hydroxyl groups after H2O adsorption are semi-stable under the experimental conditions. The acidity of Ru/C catalyst was also investigated with pyridine adsorbed DRIFTS of H2O exposed sample


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and the results are compared with the reduced sample (Figure 7B). Interestingly, an additional vibrational band (at 1542 cm-1) corresponding to Brønsted acidity in the H2O adsorbed DRIFT spectra is observed. However, the intensity of this band was small compared to the signals attributed to Lewis acid sites. As the reaction feed contains high amount of H2O (90wt% H2O and 10wt% LA) which has been fed continuously, the surface hydroxyl groups are generated reversibly on the catalyst surface which are most likely involved in the LA conversion to AL step of the reaction (Figure 5).

Figure 7. A) DRIFT spectra of Ru/C catalyst at 275 °C before and after steam exposure followed by degassing at different time intervals; B) Pyridine adsorbed DRIFT spectra of Ru/C (a) reduced and (b) reduced+H2O dosed samples at 275 °C.

From above observations, it can be summarized that the hydroxyl groups are involved in the LA dehydration to produce AL over acid sites and further conversion of AL to GVL is occurring on the Ru metal sites that were activated by H spillover from H2O (Scheme 1).24 When alcohols such as CH3OH and EtOH were used as solvent apart from water, it was reported that the formation of undesired products viz. alkyl levulinates was observed and are ACS Paragon Plus Environment

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found that the alcohols are not suitable solvents for the desired GVL selectivity.20-21 These results are in accordance from the activity data obtained over various loadings of LA in H2O that showed a gradual decrease in both LA conversion and GVL selectivity upon decreasing the H2O concentration (Figure 4). The DRIFT spectra of H2O adsorbed catalysts, the involvement of surface –OH groups is well evidenced from the vibrational band observed at both ~ 3550 and 1638 cm-1. The regeneration of these hydroxyl groups is responsible for first step of the LA conversion to AL, whilst further conversion of AL to GVL is initiated by the surface metal species present in the catalysts. It is therefore, well understood that the high activity, stability towards GVL from LA over the Ru/C catalyst is explained due to its surface which is more facile for regeneration of surface –OH groups (H2O adsorbed DRIFTS peak at 3550 cm-1) and presence of more active metal surface area (CO pulse chemisorption) as compared to other two catalysts.

Scheme 1. Surface reaction mechanism involving H2O in the LA conversion to GVL.

4. CONCLUSIONS The support effect for Ru was studied using various types of supports for continuous phase hydrocyclisation of aqueous LA to GVL. Among the different supports tested; activated carbon was found to be a suitable support for Ru in this reaction. The Ru/C also showed


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sustained activity towards GVL for 72 h of continuous operation. The used catalyst was found to be free from coke or carbon deposition and retained its structure and active sites after the reaction showing its robust nature in aqueous LA conversion. The high activity of Ru/C catalyst was obviously due to higher Ru metal surface area compared to Ru on Al2O3 and MgO. Presence of H2O in the stream caused the generation of active surface hydroxyl groups which was observed from the various LA loaded H2O substrates wherein at lower LA concentration, GVL yields were high. The DRIFT spectra of Ru/C revealed the reversible surface –OH groups that were almost doubled upon H2O dose on the reduced sample. Pyridine adsorbed DRIFT spectra emphasized the emergence of Brønsted acid sites on Ru/C sample. It was also found that the other supports such as MgO and Al2O3 for Ru showed low activity and stability due to the differences in surface properties of these supports in the conversion of LA.

ASSOCIATED CONTENT Supporting Information Experimental conditions: Preparation, characterization and testing of the catalysts, powder XRD patterns, CO-pulse chemisorption data of the catalysts, XPS analysis and activity results of the catalysts.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] ORCID: 0000-0003-1182-8332


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* E-mail: [email protected] ORCID: 0000-0002-2299-1824 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS All the authors thank DST New Delhi for funding under India-Australia (AISRF: DST/INT/AUS/P-64/2015) program. VVK and KVRC thank CSIR New Delhi for research funding through Emeritus Scientist Scheme.

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Influence of Support for Ru and Water Role on Product Selectivity in

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