Selective Hydrogenation of Dibenzo-18-crown-6 ether over Highly Active Monodisperse Ru /-Al 2 O 3 Nanocatalyst

Ru/-Al2O3 nanocatalyst with different metal loading was synthesized by microwave irradiated solvothermal technique. Synthesized nanocatalyst (4-14 nm of metal particle size) was then successfully implemented for the hydrogenation of dibenzocrown-18-crown-6 ether (DB18C6) at 9 MPa, 393 K temperature and 3.5 h. It was observed that the metallic small nanoclusters produced at 4 wt% metal concentration exhibited higher catalytic activity and resulted 96.7% conversion with 100% selectivity towards cis-syn-cis-dicyclohexano-18-crown-6 ether (CSC DCH18C6). © 2015 BCREC UNDIP. All rights reserved


Introduction
Strontium-90 ( 90 Sr) is produced from nuclear reactor due to nuclear fission.It undergoes β − decay into yttrium-90 and has half-life of 28.8 years.It is present in significant amount in spent nuclear fuel and in radioactive waste from nuclear reactors [1].It is a heat generat-ing nuclide and its removal from liquid waste stream is important for improved waste management strategies.Recovered Strontium 90 has applications in nuclear medicine and as compact power and heat source [2].
Crown ethers, a class of macrocyclic ligands, are found to be effective solvent as they possessed high affinity and selectivity for alkali and alkaline earth metal ions [3].The selectivity in complexation of crown ether with a particular cation depends on the size of the ring and its stereochemistry.Reduction of Diben-zocrown-18-crown-6 ether (DB18C6) produces five stereoisomers [cis-syn-cis (CSC), cis-anticis (CAC), trans-syn-trans (TST), trans-antitrans (TAT), cis-trans (CT)] of dicyclohexano-18-crown-6 ether (DCH18C6) [4].Literature shows that CSC DCH18C6 can extract radioactive ions more effectively from solutions of nuclear fuel reprocessing than other isomers [5,6].As separation of different diasteoisomers is a complex and expensive process, so selective hydrogenation of DB18C6 to produce CSC DCH18C6 is worth considering.For stereoselective hydrogenation, various supported metal catalyst or colloidal metal particles were used.Pederson [7] carried out reduction of DB18C6 using Ru/Al2O3 catalyst under 70 atm of H2 pressure but obtained a mixture of isomers.Laskorin et al. [8] obtained five isomers using Raney Ni catalyst.Gurskii et al. [9] used Ru, Rh, Pd and Pt catalysts supported on carbon and but failed to achieve selectivity towards CSC DCH18C6.Landre et al. used colloidal Rh catalyst and obtained 100% conversion, 86% chemoselectivity and syn:anti ratio 34:66 in presence of Aliquat-336 as phase transfer catalyst [10].Recently, Gao et al. [11] carried out hydrogenation of DB18C6 using ruthenium nanocatalyst at 408 K and 10.0 MPa and obtained 99.4% conversion with 6:1 ratio of the syn/anti DCH18C6 as product.
From the above literature it has been found that ruthenium nanocatalyst shows high activity and stereoselectivity simultaneously due to its zero valence state and nano-scale size.Ru nanoparticles were synthesized by different methods such as microemulsion, solvothermal, sol-gel, co-precipitation, sonochemical reduction, microwave irradiation etc. Amongst this microwave irradiated solvothermal technique was found to be most effective as it is cleaner, greener, time saving and simple method of synthesis for metal nanoparticles.Zawadzki and Okal [12] and J. Okal [13] synthesized ruthenium nanoparticles by reduction of ruthenium chloride in ethylene glycol using a microwave assisted solvothermal method, dispersed on γ-Al2O3 and characterized by different technique.Recently, Antonetti et al. [14] reported synthesis of cabon nanotubes supported ruthenium nanoparticles using ethanol/water as solvent as well reducing agent under microwave irradiation.Rini et al. [15] synthesized Ru/γ-Al2O3 nanocatalyst by microwave irradiation technique in the presence and absence of stabilizer and characterized by different technique.Following the same line of action, in our previous study [16], we reported the microwave assisted synthesis of ruthenium nanoparticle using eth-ylene glycol as solvent and also as reducing agent.Effect of reaction temperature, time, PVP/RuCl3 molar ratio and irradiation power on particle size and size distribution were also studied.
In this research work, we have synthesized Ru/γ-Al2O3 catalyst, dispersing ruthenium metal nanoparticles on γ-alumina support, characterized and also reported herein a straightforward and efficient catalyst system for challenging hydrogenation of DB18C6 to CSC DCH18C6 without adding any quaternary ammonium salt as phase transfer catalyst.

Catalyst Preparation
Ruthenium (III) chloride as precursor salt and Poly (N-vinyl-2-pyrrolidone) (PVP as a capping agent) were dissolved in ethylene glycol, which played a dual role for synthesis, as solvent and also as reducing agent was irradiated in microwave reactor (300 W, 200 • C) and cooled down by ice chilled water.Ru/γ-Al2O3 nanocatalyst was synthesized by transferring the Ru nanoparticles onto the support, redispersed in methanol, mechanically stirred (6500 rpm using Ultraturax) for 24 h, washed with acetone and water to remove the organic and inorganic impurities and dried at 100 °C for 6 h.Dark brown coloured powdered catalyst was calcinated at 300 °C for 8 h in an oven [17].

Catalytic Hydrogenation of DB18C6
The hydrogenation of DB18C6 to DCH18C6 was carried out in a 100 ml stainless steel autoclave reactor (Amar Technology, Mumbai.India) built with a thermocouple, hydrogen inlet valve, pressure gauge and magnetic stirrer.30 ml butanol, 3 g of DB18C6 and known quantity (0.3 g) of prepared activated (1hr in continuous flow of H2) catalyst (3-5 wt% (as 1 and 2 wt% not found effective) were charged into the reactor at optimum condition (9 MPa H2, 393 K temperature and 3.5 h).

TEM Analysis
The surface morphology of the Ru/γ-Al2O3 (3-5 wt.% Ru) catalyst were studied by TEM. Figure 1 showed that ruthenium nanoparticles imaged as black dark dots and were uniformly dispersed on the γ-Al2O3 support, with size of the ruthenium nanoparticles ranging from 1-11 nm. Figure 1(b) and 1(c) indicated the formation of small clusters due to aggregation of nanoparticles at higher (4.5 wt%) metal content.

XRD Analysis
Figure 2 showed the characteristic peaks for ruthenium metal particles around 2θ = 44.08°(101) (highlighted by Δ) of the crystal structure of metal ruthenium (JCPDS file no.06-0663).The presence of support material γ-alumina was confirmed by the peak at 2θ = 66.5° (JCPDS file No. 10-0425).The average particle size of the ruthenium nanoparticles was calculated from the Scherrer equation, was in the range of 4-14 nm which was closed to the value found from TEM analysis.

SEM and EDAX Analysis
SEM analysis (Figure 3) of 4 wt% Ru/γ-Al2O3 revealed that ruthenium nanoparticles of various sizes were uniformly dispersed on the surface, covering the whole surface of γ-Al2O3 and the EDAX analysis (Figure 3) of the catalyst indicated the presence of the ruthenium, alumina, oxygen on the surface of the catalyst.

BET Analysis and Metal Surface Area
Table 1 showed that the specific surface area (SBET) of catalyst decreased with increasing ruthenium loading from 3-5 wt%.BET data showed that the metal atoms were mainly deposited on the surface of the γ-alumina, not in the pore of the support.So small change in pore volume (0.24 -0.156 cm 3 /g) was observed and almost all the atom would be accessible to the reaction.To determine ruthenium dispersion and metal surface area (SM) of the Ru/γ-Al2O3
3.1.5.Small-angle X-ray Scattering (SAXS) Size of the ruthenium nanoparticles dispersed on γ-Al2O3 support and commercial catalyst was compared using SAXS (Figure 4).With an increase in concentration of ruthenium from 3-5 wt% metal nanoparticles size was found to increase slightly from 2 nm to 3.5 nm as at the time dispersion tiny particles agglomerated to form larger particles due to lower stabilizer concentration compared to precursor concentration.

Catalytic Activity Towards DB18C6 Hydrogenation Reaction
Figure 5 showed the concentration profile of DB18C6 for gas-liquid hydrogenation of DB18C6 to DCH18C6 in presence of continuous flow of H2 at different time interval.Initially, the reaction pressure, temperature and time were optimized to achieve maximum conversion and was found to be 9 MPa H2, 393 K temperature and 3.5 h.Table 3 showed the effect of ruthenium loading (3-5 wt%) on DB18C6 conversion.Product mixture was analysed by GC-MS (MS peaks at 372, 201, 171, 143, 99, 72 and

Bulletin of Chemical Reaction Engineering & Catalysis, 10 (1), 2015, 27
Copyright © 2015, BCREC, ISSN 1978-2993 54 a.m.u), NMR [ 1 H: (200 MHz; CDCl3): 1.3-2.0(m, 16H, -CH2-), 3.4-3.7(m,4H, -CH-) and 3.7-3.9(m, 16H, -CH2O-)] and liquid XRD (Figure 6).Product analysis showed maximum 96.7% conversion with 100% selectivity towards CSC DCH18C6 using 4 wt% catalyst which indicated better result in terms of selectivity than other reported data.Actually reduced ruthenium has higher density of electron around the atom, and thus larger repulsion to the cyclohexyl ring, the first reduced benzene ring of DB18C6.As the half-hydrogenated crown ether molecule getting close to the Ru catalyst, cyclohexyl ring will be easily pushed to the opposite side and as far as possible to the catalyst surface and produced mainly CSC DCH18C6 (Figure 7).This reaction could be considered as pseudo first order with respect to DB18C6 as concentration of hydrogen will be more compared to DB18C6 (Equation (1)): where, CA = concentration of the reactant, DB18C6 at time t, CA0 = initial concentration of the reactant and k = rate constant for the firstorder reaction.Rate constant for first-order reaction can be calculated by Equation (2): It was found from Figure 5 that 80-90% reduction occurs within 10 min of reaction time.From the ln (CA/CA0) versus t plot (not shown), the initial rate constant (up to 10 min of reaction time) was calculated at different ruthenium loading on the catalyst.It was found from Table 3 that the rate constant increased linearly with increasing ruthenium loading from 3 to 4 wt% (0.574 to 0.997 min -1 ) and then decreased at 5 wt% metal loading (0.722 min -1 ).
Comparing the catalytic activities with different loading ruthenium, 4 wt% ruthenium loading was considered to be the optimum because it provided the highest % conversion of   DB18C6 (even better than 5 wt% commercial catalyst).The smaller ruthenium nanoparticles (up to 4 wt% loading) would be in bulk of the active surface area and with an increase of metal loading, a decrease in the number of small particles is seen in the structure region, which was responsible for lower catalytic activity.Larger loading also led to more agglomeration and larger average particle size.TEM images (Figure 1b) showed that the 4 wt% ruthenium loading, though forming metallic nanoclusters to some extent, was the optimum because the ruthenium particles were not too dense and dispersed uniformly on the support.It was hypothesized that this provided the maximum active sites for hydrogenation of DB18C6.Chemisorptions results (Table 2) also showed that 4 wt% ruthenium loaded catalyst had the highest (SM = 47.47 m 2 /gcat) active metal site to carry out the hydrogenation reaction at faster rate.It is also found from Figure 5 that the 5 wt% Ru/-Al2O3 showed higher activity than that of 3 wt% Ru/-Al2O3 although the metal surface area of 3 wt% ruthenium loaded catalyst (42.23 m 2 /g) is higher than that of 5 wt% ruthenium loaded catalyst (11.28 m 2 /g).It indicated that ruthenium nanoclusters formed at higher (5 wt%) metal loading (Figure 1c) were highly active sites for hydrogenation reaction, which resulted higher conversion (95.3%) and larger initial rate constant (0.722 min -1 ).

Conclusion
Supported ruthenium metal nanocatalyst was synthesized by microwave irradiated solvothermal technique.Synthesized catalysts activity was then successfully tested for the hydrogenation reaction of DB18C6 to DCH18C6.4wt% Ru/γ-Al2O3 showed 96.7% conversion with 100% selectivity towards CSC DCH18C6 which exhibited even better result compared to 5wt% commercial catalyst.

Figure 6 .
Figure 6.Liquid XRD analysis of CAC & CSC DCH18C6 and synthesized product