Preparation and characterisation of Rh/Al2O3 catalysts and their application in the adiponitrile partial hydrogenation and styrene hydroformylation
Introduction
Rh is a rare element from the Platinum Group Metals, its cost is the highest in this group (about 1300 US $ per troy Oz) and in 2003 about the 84% of the total Rh supply (about 655000 Oz) was used for the production of automotive catalysts [1]. Careful attention should therefore be paid during catalyst preparation to produce highly efficient catalysts with low metal loading, high metal dispersion and a long lifetime. Finally, recovery of the metal from the spent catalyst is also a fundamental step in the life cycle of a catalyst (about 17% of the Rh used for automotive catalysts come from a gross recovery).
Different impregnation methods are available for catalyst preparation such as wet, incipient-wetness and dry impregnation or impregnation with interactions (e.g. ion exchange and adsorption methods). Generally for the preparation of metal supported catalysts the final metal loading and the metal dispersion depends on the preparation technique used and on the final treatments to bring the catalyst in the active form. Rh catalysts were studied in the 1980s mainly for the Fisher–Tropsh reaction.
When the activity and selectivity of a reaction depends on the size of the metal particles, the reaction is called “structure sensitive” and in this case the choice of proper preparation methods becomes of primary importance to obtain the best catalyst performance [2].
In this work, Rh/Al2O3 catalysts were prepared using the “ion exchange method” or the “homogeneous precipitation through urea decomposition method”.
The ion exchange method is an impregnation where ions of a metal precursor electrostatically interact with the charged support after pH correction above or below the isoelectric point [3], [4], [5] depending on the charge of the metal precursor. We have investigated the kinetics of adsorption of the precursor on alumina and the effect of multiple deposition procedure on the final catalyst characteristics. The homogeneous precipitation through the urea decomposition method is based on the slow precipitation in the support porosity of the precursor by thermally decomposing urea to change the pH from neutral to basic:CO(NH2)2 + 3H2O → 2NH4+ + ↑CO2 + 2OH−In alkaline conditions, the metal hydroxide has a low solubility and the slowness of the urea decomposition process avoids the bulk precipitation [3], [4], [6]. This technique was used in order to have the deposition of the metal on the support without interaction between the precursor and the support.
After the precursor deposition using the two techniques cited above, the influence of the reduction conditions (via liquid phase with sodium borohydride or via gas phase with molecular hydrogen) on the metal dispersion was investigated.
The prepared catalysts were characterised in terms of catalyst loading and metal dispersion. Their activity was studied in two industrially important reactions:
- •
Hydrogenation of adiponitrile. We previously [7] showed that a Rh/Al2O3 catalyst prepared by Metal Vapour Synthesis was active in this reaction to produce hexamethylenediamine. The importance of studying a supported catalyst for the adiponitrile (ADN) hydrogenation process is linked to the possibility of substituting Ni Raney as the industrial catalyst and then to technologically improve the process. In fact Ni Raney is very active, but it still suffers from some drawbacks, including pyrophoricity (careful handling is therefore required). Moreover, by tuning the operating conditions, the Rh catalyst was able to give hexamethylenediamine (HMD) or 6-aminocapronitrile (ACN). 6-Aminocapronitrile is an interesting intermediate since ɛ-caprolactam could be obtained from it through an environmentally friendly process, avoiding the formation of the large amount of ammonium sulphate presently generated by the traditional process via cyclohexanone. In this paper we show the activity of the differently prepared catalysts to obtain ACN as main products.
- •
Hydroformylation of styrene. The hydroformylation of olefins is one of the most important reactions catalysed by homogeneous cobalt and rhodium complexes for the industrial production of aldehydes (ca. 7 millions ton/year) [8]. In particular, rhodium compounds, generally modified with phosphorous ligands, give rise to high reaction rates and good selectivity to the desired products [9], [10]. This process, however, suffers some drawbacks such as the difficult separation of products from the solvent and the soluble precious catalyst, and therefore many heterogeneous systems such as metals supported on inorganic carriers have been employed in the hydroformylation of simple olefins [11]. Despite the usefulness of functionalised olefins for the preparation of valuable biologically active compounds [12], the hydroformylation of these substrates has been very poorly studied in heterogeneous phase [13], [14]; therefore, we decided to evaluate the activity of this new Rh/Al2O3 catalyst in the hydroformylation of styrene, a well-known model substrate for functionalised olefins [9].
Section snippets
Catalyst preparation
Al2O3 (Alfa Aesar, cod. 011502) was used as a support for the deposition of rhodium. Two different methods (ion exchange and homogeneous precipitation through urea decomposition methods) were used to deposit Rh onto the alumina support. In both methods the precursor solution was obtained by dissolving RhCl3·3H2O (Aldrich, cod. 260261-1G) in deionised water:
- (A)
Ion exchange method. The concentration of the Rh precursor in solution was low (50–100 mg/l) and its pH was corrected to the desired value
Catalyst characterization
SEM observations showed that the average size of the alumina granules was about 100 μm. The isoelectric point of alumina was found to be 8 within the range reported in literature (IP = 7–9) for γ-Al2O3 [5].
Fig. 1 shows the adsorption–desorption isotherm of N2 at 77 K and the pore size distribution was calculated using the BJH method. The alumina used was mesoporous with an average pore diameter of about 3.6 nm. The specific surface area of the alumina support was found to be 166 m2/g using the BET
Conclusions
Two methods of preparation for Rh/Al2O3 catalysts were used and the ion exchange method was investigated in greater depth. Using the ion exchange method and reducing the precursor in a H2 atmosphere it is possible to obtain a catalyst with a very high metal dispersion and small Rh crystallites. In the adiponitrile hydrogenation the Rh catalyst prepared showed a performance to produce ACN higher than similar catalyst reported in literature but used in different operating conditions. Moreover no
Acknowledgements
The authors would like to acknowledge the financial support of both MIUR (PRIN 2002) and Radici Chimica S.p.A.
References (20)
- et al.
Appl. Catal. A
(2001) - et al.
Catal. Today
(2003) - et al.
Catal. Today
(1997) - et al.
J. Mol. Catal. A: Chem.
(2003) - et al.
J. Mol. Catal. A: Chem.
(1996) - et al.
J. Mol. Catal. A: Chem.
(1996) - et al.
J. Mol. Catal. A: Chem.
(2002) - et al.
Talanta
(2002) - et al.
Hydrometallurgy
(1996) - et al.
Appl. Catal.
(1989)
Cited by (49)
Regioselectivity regulation of styrene hydroformylation over Rh-based Phosphides: Combination of DFT calculations and kinetic studies
2022, Chemical Engineering JournalRecent development towards alkene hydroformylation catalysts integrating traditional homo- and heterogeneous catalysis
2022, Catalysis Science and TechnologyInfluence of the framework on the catalytic performance of Rh-supported Zr-MOFs in the hydroformylation of n-alkenes
2022, Molecular CatalysisCitation Excerpt :These materials are catalytic active e.g. in oxidation, reduction, hydrogenation, and photocatalysis [38,41,42,43,44,45,46]. Rh-loaded MOFs are tested mostly in the hydrogenation [47,48,49,50]. So far only very few reports deal with the hydroformylation of olefins with Rh-loaded metal-organic frameworks such as ZIF-8, MIL-101, and IRMOF-3 [51,52,53,54,55].