SYNTHESIS Of NANOSIZED PALLADIUM MATERIALS AND THEIR CATALYTIC PROPERTIES IN SELECTIVE HYDROGENATION OF ALKYNES Al-Wadhaf H.A. Hajjah University, Republic of Yemen, E-mail:
[email protected] Supported palladium catalysts are widely used in large-scale processes of petrochemical and refining industries [1]. Of particular interest in recent years are palladium nanocatalysts, which hold much promise due to their performance [2]. Deposition of precursors on supports followed by their reduction with hydrogen is one of the most effective methods for preparing catalysts of this type, including palladium catalysts [3]. Here we were prepared nanocatalysts Pd/C and Pd/SiO2 by loading carbon and Silica Gel with palladium(II) acetate [4]. The nanomaterials obtained with a high Pd content (about 9%) on both carbon and silica gel, in which the metal particle size was 1-3 nm (Fig. 1).
Fig. 1. Transmission electron micrographs of metal particles for the 9.1%Pd catalyst reduced on (a) carbon at 120°C and (b) silica gel at 50°C.
It is interesting that the reduction of Pd(II) diacetate on carbon at a higher temperature (120°C) leads to larger particles with a narrow size distribution (2.8 ± 0.2 nm). On silica gel at a lower temperature (50°C), smaller particles are generated, but the size distribution becomes wider (1.7 ± 0.7 nm). The materials obtained were examined under the equal conditions as catalysts for the selective liquidphase hydrogenation of PA to styrene.
Ph–C≡CH + H2 → Ph-CH=CH2 + H2 → Ph–CH2–CH3 The reaction was carried out in a cylindrical shaken reactor under constant conditions (25°C, 1atm H2), and at reactor shaking speed of 180 cycles/min). The reactor was loaded with 0.002 g of the catalyst having a Pd loading of 9.1% and 10 mL of dimethylformamide (solvent). In special experiments, it was shown that the diffusion limitations can be neglected at reactor shaking speed of 180 cycles/min and a catalyst particle size of less than 0.09 mm. It turned out that they differ in efficiency. The initial rate of PA hydrogenation to styrene in the presence of the catalyst was 0.0071 mol/(L min) on carbon or 0.0026 mol/(L min) on silica gel. At the same time, the selectivity of the former is below that of the latter (Table 1). Note that the high selectivity found for both catalysts drops not considerably when the PA conversion increases to about 100%.
PA conversion, %
Selectivity, % 9,1% Pd/C
9,1% Pd/SiO2
~50
93,3
96,8
~100
84,6
90,3
Table 1. Selectivity of phenylacetylene hydrogenation in the presence of palladium catalysts on carbon and silica gel.
References 1. O. V. Krylov, Heterogeneous Catalysis (Akademkniga, Moscow, 2004) [in Russian]. 2. A. Yu. Stakheev, I. S. Mashkovskii, G. N. Baeva, and N. S. Telegina, Ross. Khim. Zh. (Zh. Ross. Khim. Obva Im. D.I. Mendeleeva) 2009, 53 (2), 68. 3. H. A. Al-Wadhaf, Vestn. MITHT 2012, 7 (1), 3 [in Russian]. 4. A. S. Berenblyum and H. A. Al-Wadhaf, Vestn. MITHT 2010, 5, 43 [in Russian].
