**2.1. Catalysts preparation**

**1. Introduction**

16 New Advances in Hydrogenation Processes - Fundamentals and Applications

reactions.

Lindlar catalyst.

Petroleum cuts contain mixtures of unsaturated and aromatic compounds. Among them, acetylenic hydrocarbons are very unstable; so they must be transformed to olefins. Alkenes have industrial and academic relevance on a large scale; industries such as petrochemical, pharmaceutical and agrochemical use these compounds as raw materials. Partial hydrogenation reactions using catalytic materials allow a reduction in operational costs and also enable high selectivity to alkenes. Specifically, the catalytic selective hydrogenation of alkynes using either homogeneous or heterogeneous catalysts has been widely studied in the past several years [1]. The hydrogenation of any alkyne conduces naturally to the alkene formation as the former trends to bind more strongly than the latter on the supported metal catalyst, thus blocking the possibility of the alkene readsorption or displacing it from the support surface. Many natural products, such as biologically active compounds [2], are synthesized through this kind of

Classical heterogeneous catalysts used to hydrogenate multiple carbon-carbon bonds contain noble metals such as Pd, Pt, Ru and Rh, which are highly active and selective [3, 4]. Many authors have found that supported palladium catalysts present the highest catalytic activity for the partial hydrogenation of alkynes to alkenes [5, 6]. One of the most used catalytic systems for these kinds of reactions is the Lindlar catalyst (Pd/CaCO3 modified with Pb(OAc)2), developed in 1953 [7]. During decades, much research has been carried out modifying this type of catalyst in order to increase the activity and selectivity to alkenes of low molar weight. Several materials have been used as supports, and they are usually classified as organic (macroreticular/macroporous polymers) or inorganic (silica, alumina, zeolites and clays). Besides, modified palladium [8] or nanoparticles of Pd have also been investigated [9]. Another kind of material, not clearly included in any of these groups, is carbonaceous species, whose outstanding properties as a catalyst support are well recognized [10], among them are the possibility of modifying the specific surface area, the porosity and the surface chemistry; moreover, carbon supports present the advantage of being inert in liquid reaction media [11]. Many catalysts, mono- or bimetallic as well as complexes of several transition metals, have also been proposed for these kinds of reactions [2, 12–14]. A major part of research efforts have been devoted to the partial hydrogenation of short-chain alkynes such as ethyne [15], with few works related to longer chain alkynes. As Pd has increased its cost, it is a challenge to synthesize cheaper catalysts. In this context, using nickel catalysts during the selective hydrogenation of

Based on the above considerations, the objectives of this chapter are to evaluate the effects of different factors on the activity and selectivity during the selective hydrogenation of 1-heptyne, a long-chain terminal alkyne. The factors studied are (a) pretreatment reduction temperature, (b) reaction temperature, (c) type of support, (d) metal loading, (e) precursor salt and (f) addition of a second metal (such as nickel) to monometallic palladium catalyst. Last but not least, all the catalytic performances are compared against those obtained with the commercial

alkyne is less studied and has been recently researched [12].

All the monometallic catalysts used in this work were prepared by the incipient wetness technique, and for bimetallic catalyst the successive impregnation technique was used following a procedure previously indicated [12]. γ-Al2O3 (Ketjen CK 300, cylinders of 1.5 mm diameter) and a pelletized activated carbon (AC: GF-45 provided by NORIT) were used as supports.

All of the supports were impregnated with acid aqueous solutions of PdCl2 or Pd(NO3)2 (Fluka, purity of >99.98%), and Ni(NO3)2.6H2O (Fluka, purity of >98.5%) was used as the precursor salt for the bimetallic catalyst.

The monometallic alumina-based catalysts were calcined at 823 K for 3 h. Before its catalytic evaluation, all the catalysts were reduced for 1 h under a hydrogen stream at 573 K. Only the High loaded Pd catalyst prepared with chlorine precursor salt was reduced at 373 and 573 K to study the effect of pretreatment temperature on the catalytic behaviour. Besides, the bimetallic catalyst was reduced for 1 h at 673 K.
