Introduction

Hydrogenation reactions are widely studied due to their im- portant application in the chemical industry, not only in the hydrogenation of petroleum derivate compounds but also in synthesis of a great variety of fine chemicals used as agro- chemicals, dyes, fragrances, foods, drugs, etc1-3.

In the last years, studies are leading to the development of new supported systems that can retain its selectivity in homo- geneous systems and stability with easy separation like heter- ogeneous catalysts4-7. Metal transitions complexes that have shown good results in homogeneous catalysis and meso- porous materials are being used in heterogeneous catalyst systems, to synthesize new 'hybrid catalysts' by different methods, such as sol-gel or in situ co-condensation and the post-synthesis method8. The intrinsic properties of meso- porous solids (zeolites, MCM, SBA) with high surface area, high thermal stability, and high mechanical strength, make these materials attractive for supporting metals complexes. Specifically, Mobile Crystallinity Materials (MCM) like MCM-41 and MCM-48, have been studied as support of ruthenium complexes for olefins hydrogenation reactions, showing high activity and selectivity, similar or better than their homologous homogeneous9-12. This work contributed in the development of two new hybrid catalysts to hydrogena- tion of 1-hexene, in which the complexes RuCl2(Py)4 and RuCl2(DMSO)2(NC5H4CO2Na)2 were supported by post- synthesis method on MCM-48: using [3-(2-aminoethyl- amino)propyl]trimethoxysilane as anchor ligand, and tri- ethoxymethylsilane as passivation ligand, both catalysts were characterized and studied in the hydrogenation of 1-hexene.

Experimental

Reagents and equipment

All reagents and solvents were obtained from Aldrich, Merck, Riedel de Haën, Fisher Chemicals and AGA of Venezuela; the reagents were used without previous purification and solvents were dried by methods described in the literature13. FTIR spectra were recorded with a Perkin Elmer 1725-X FTIR spectrometer; the samples were prepared by 5 wt. % KBr discs. Mass spectra were carried out on a Hewlett Pack- ard System MSGC-5988 spectrometer, electronic impact ionization technique was used. Thermogravimetric analysis (TGA, DSC) were performed in an SDT Q600 equipment; measurements were realized from room temperature to 800ºC with a heating rate of 20ºC/min and Air flow of 100 mL/min. N2 adsorption-desorption measures were carried out on a Micrometrics ASAP 2010 sorptometer. SEM images were done on a FEI Quanta 200 FEG microscope with an EDX SDD EDAX Apollo X detector.

Analysis of catalytic tests were done by gas chromatography in a Perkin-Elmer Autosystem 900 GC with a methyl silicone Quadrex capillary column, 50 m. long, 2 mm diameter, 0.52 μm thin film. Patterns of hexane, 1-hexene, cis- and trans-2- hexene were used as reference to detect its, and the percent- age of products were calculated by turbochrom software.

Preparation of hybrid catalysts

ybrid catalysts were prepared by post-synthesis method described by Soundiressane et al.14 with the successive sup- port of the different species like anchor and passivation lig- ands and coordination complexes.

RuCl2(Py)4 and RuCl2(DMSO)2(NC5H4CO2Na)2 were syn- thesized following methods described by Wilkinson et al15, 16 and Suárez et al.9 respectively. MCM-48 was prepared using the method developed by Galarneau et al17.

Support of [3-(2-aminoethylamino)propyl] trimethoxysilane, triethoxymethylsilane and the ruthenium complexes

1.0000 g of MCM-48 was added to 50 mL of dry toluene, the solution was vigorously stirred during 15 min at room tem- perature. Subsequently, the anchor ligand [3-(2-amino- ethylamino)propyl]trimethoxy-silane, the passivation ligand or ruthenium complexes was slowly added (0.1000 g of each of them), the mixture was refluxed during 24 h under argon atmosphere. The solids obtained were filtered and then it was purified by Soxhlet extraction for 6 hours using dichloro- methane as a solvent; finally, the solid was dried in vacuum and labeled as NH2-MCM-48 and NH2-P-MCM-48, RuCl2(Py)4-NH2-P-MCM-48 (I) and RuCl2(DMSO)2 (NC5H4CO2Na)2-NH2-P-MCM-48 (II).

Catalytic trials

The catalytic reactions were carried out in Parr batch reactors of 10 mL capacity. The temperature was provided by vertical heating ovens and monitored by PID temperature controllers and type K thermocouples. The reactions were performed as follows: 5 mL of dry THF were added in the reactor vessel, then 0.0050 g of hybrid catalyst and 1-hexene substrate were added; the reactor was closed and charged with an appropri- ate H2 pressure. The reactor was placed in the heating oven and the starting time was taken when the reaction reached the established temperature8.

The reuse of catalysts was carried out as follow: a catalytic test was carried out under optimal conditions, the solution that result was filtered and the solid obtained was dried and used again under the same conditions up to five times.

Results and discussion

Characterization of hybrids catalysts

FTIR: FTIR spectrums showed characteristic bands corre- sponding to symmetric and asymmetric v(SiO-H) stretching at about 3450 cm-1 and 810 cm-1, respectively, and a wide and intense band at 1106 cm-1 which is attributed to vs(O-Si-O) stretching. A new band about 2940 cm-1 is assigned to the symmetric v(-C-H) stretching of the alkylic chain, consistent with the structure of the anchor ligand. Hybrid catalyst (I) showed the occurrence of a new series of bands characteristics of the pyridine ligand: symmetric v(C=C) stretching at 1478 cm-1; hybrid catalyst (II) showed two signals (1432 cm-1, 696 cm-1) corresponding to v(C=O) and γ(=C‒H) of sodium nicotinate ligand; these bands suggest the effective anchorage of the ruthenium(II) complexes on the functionalized sup- port18.

SEM/EDX: Table 1 shows chemical analysis of all prepared materials. MCM-48 contains only silicon and oxygen, NH2- MCM-48 and NH2-P-MCM-48 solids contain carbon and nitrogen additionally, indicating that the passivation and an- chor ligands were supported. (I) and (II) ruthenium complex- es, contain silicon, carbon, nitrogen and oxygen besides ruthe- nium and chloride. These results indicate that anchoring of the complexes on functionalized MCM-48 was probably obtained.

Figures 1 and 2 shows SEM photographs of hybrid catalysts (I) and (II), respectively. The morphology of the materials and agglomerates of particles with irregular appearance is observed. This morphology is typical, and it has been report- ed for different hybrids catalysts of coordination complexes on mesoporous MCM-48 and SBA-15 solids16. Therefore, incorporation of different species on the starting material, shows no changes in the morphology of the same; which is consistent with that reported by Sakthivel et al20.

N2 adsorption-desorption

All materials synthesized showed a type IV isotherm ac- cording to the IUPAC nomenclature corresponding to mesoporous materials. The surface area was obtained by applying the BET equation to the nitrogen adsorption iso- therm and pore volume and pore diameter were calculated from the BJH model21; these values indicate that the pore diameters are in the medium porosity range (see table 2). Comparison of these values showed that new species were incorporated on the MCM-48 solid, with a gradual de- crease in all the parameters studied; it indicated that effec- tively anchor ligand, passivating ligand and ruthenium complexes were anchored on the surface and inside the pores22,23.

Thermogravimetric Analysis (TGA-DSC)

Thermogravimetric analysis was performed on all materi-als. MCM-48 showed only one loss between 60 °C and 150 °C, it corresponds to lattice water24. Functionalized materials showed changes in thermal stability, which has been associat- ed with the presence of organic and inorganic compounds (anchor, passivation, and complex ligands), this behavior has been observed in similar hybrid catalysts25. In all thermo- grams (see figure 3 and 4) a mass loss between 60 °C and 120 °C was observed which corresponds to network water; be- tween 120 °C and 800 °C, a series of losses (30 wt.%) was observed, which can be assigned to decomposition of organic material and metal complexes supported, indicating that the MCM-48 support has new species.

Mass Spectrometry

Under the experimental working conditions, only the or- ganic fragments (anchor and passivation ligand, and the ligands of supported complexes) get to the ionization cam- era26, 27. In the mass spectra of hybrid catalysts (I) and (II), a series of ions with unpaired electrons can be observed28. These were formed by Si-C bond rupture from the alkylic anchor chain. The masses and isotopic patterns of these ions most probably indicated that the ruthenium complexes were linked to the anchor ligand chain through the nitrogen atoms, in a bidentate mode forming a five membered ring with the metal, which is consistent with the "chelating effect"29. The (I) catalyst shows a fragment ion at 419.30 m/z. This ion with odd electron number is observed that corresponds to a fragment coming from a C-C β bond breaking off from the silicon atom of the anchoring ligand; this ion indicates that a double substitution of the pyridine ligands in ruthenium(II) complex is coming from the nitro- gen of the anchoring ligand; this reaction is thermodynam- ically favored because of the chelating effect of the diamine ligand. After the formation of the molecular ion, the consecutive CH2 loss of the remaining alkylic chain, followed by the loss of the two pyridine ligands.

he fragmentation pattern of the catalyst (II) shows an odd electron number fragment due to the breaking off from the Si-C on the anchoring ligand at 719.9 m/z which in this case corresponds to the fragment ion; this ion suggests the substitution of the two dimethylsulfoxide molecules by two nitrogen atoms from the anchoring ligands forming a chelating complex. Following the formation of the molecu- lar ion, a loss of two fragments at 145 m/z is observed, this is consistent with the loss of two sodium nicotinate ligands bonded to the metal, releasing fragments at 574.15 m/z and 429.65 m/z.

Catalytic Tests

In order to optimize the parameters used in the catalytic as- says using (I), the reactions were carried out with the follow- ing variable conditions ranges: reaction time: 0 to 4 h, tem- perature: 60 °C to 140 °C; substrate catalyst ratio 400:1 to 2000:1; hydrogen pressure: 200 to 1000 psi. The optimal parameters for catalyst (I) were: Reaction time: 4 h; Tempera- ture: 100 °C; substrate/catalyst ratio: 1000:1 and hydrogen pressure: 600 psi. For the catalyst (II), the reactions were carried at the following variable conditions ranges: reaction time: 0 to 24 h, temperature: 80 °C to 140 °C; substrate cata- lyst ratio: 400:1 to 2000:1; hydrogen pressure: 400 to 1000 psi. The optimal parameters for catalyst (II) were: reaction time: 16 h; temperature: 100 °C; substrate:catalyst ratio: 2000:1 and hydrogen pressure: 400 psi. The principal product is 1-hexene hydrogenation to n-hexane.

Under the optimal conditions the recycling of the catalysts was done. These results are show in table 3. The conver- sion diminishes somewhat which is common for this type of system due to leaching of the metal complexes30 and decomposition31 of the metallic complexes. However, it is important to remark that after five runs, a high conversion percentage was still observed. The mercury assays were done in the fourth run and we found not to diminish the catalytic activity which showed that reaction occurs with the metallic complexes instead of metallic particles coming from metal complex decomposition.

Catalyst (I) showed a TOF of 300 h-1 and the catalyst (II) 166 h-1; if we compare this value with the reported catalyst in homogeneous phase9 (TOF 150 h-1 for RuCl2(DMSO)2 (NC5H4CO2Na)2), the hybrid catalyst showed better cata- lytic activity, this can be explained by the synergic effect between the ruthenium complex and the MCM-48 support, in this case the pores of the support acts as microreactors32 offering more actively concentrated sites and increasing efficiency in the process.

Conclusions

The functionalization through the post synthesis method of the mesoporosous MCM-48 and its immobilization of the Ru complexes (I) and (II) were done successfully, obtain- ing the hybrid catalysts. Both were characterized and their catalytic activity in the 1-hexene hydrogenation reaction was carried out showing high catalytic activity. Catalyst (I) showed more catalytic activity than complex (II) consider- ing its lower content of supported metal percentage. TOF was higher than 150 h-1 for both catalysts, being better than the results obtained with a homogeneous catalyst.

Acknowledgements

To CDCHTA ULA, Project N° C-1936-15-08-A.

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