*1.1. Hydrogenation Catalysts*

Catalyst selection and preparation is one of the most important stages in process design and development. Generally, Pd, Pd-Ag, and Pd-Au supported on α-Al2O<sup>3</sup> have been designed to use in the industrial acetylene hydrogenation process [5–7]. Ravanchi et al. reviewed the theoretical and practical aspects of catalysis for the selective hydrogenation of acetylene to ethylene and the potential ways to improve catalyst formulation [8]. Bos et al. investigated the kinetics of the acetylene hydrogenation on a commercial Pd catalyst in a Berty type reactor [9]. The considered reaction network consists of acetylene hydrogenation and ethylene hydrogenation reactions. They proposed different rate expressions and calculated the parameters of rates, based on the experimental data. The results showed that the classical Langmuir-Hinshelwood rate expressions could not fit the data well, when there is a small amount of carbon monoxide in the feed stream. Borodzi´nski focused on the hydrogenation of acetylene and mixture of acetylene and ethylene on the palladium catalyst [10]. The results showed that two different active sites are detectable based on the palladium size. The results showed that, although acetylene and hydrogen are adsorbed on the small active site, ethylene did not adsorb, due to steric hindrance. In addition, all reactants were adsorbed on the large sites and butadiene as coke precursor was produced on that site. Zhang et al. investigated the performance of Pd-Al2O<sup>3</sup> nano-catalyst in the acetylene hydrogenation [11]. The results showed that dispersing Ag as a promoter on the catalyst surface increases ethylene selectivity from 41% to 60% at 100 ◦C. Typically, adding Au to Pd-Al2O<sup>3</sup> can tolerate carbon monoxide concentration swing, and improve the selectivity, and temperature resistant [12]. Schbib et al. investigated the kinetics of acetylene hydrogenation over Pd-Al2O<sup>3</sup> in the presence of a large excess of ethylene in a laboratory flow reactor [13]. They claimed that C2H<sup>2</sup> and C2H<sup>4</sup> compounds are adsorbed on the same site and they react with the adsorbed hydrogen atoms to form C2H4, and C2H6, respectively. It appeared that the presence of a trace amount of silver on Pd-Al2O<sup>3</sup> catalyst decreases the rate of ethylene hydrogenation as a side reaction [14]. Khan et al. studied adsorption and co-adsorption of ethylene, acetylene, and hydrogen on Pd-Ag, supported on α-Al2O<sup>3</sup> catalyst by temperature programmed desorption [15]. The TPD (temperature programmed desorption) results showed that, although the presence of Ag on the catalyst suppresses overall hydrogenation activity, it increased the selectivity towards ethylene [16]. Pachulski et al. investigated the effect green oil formation and coke build-up has on the deactivation of Pd-Ag, supported on α-Al2O<sup>3</sup> catalyst, applied in the C2-tail end-selective hydrogenation [17]. It was found that the catalyst contains low Ag to Pd ratio presents the highest long-term stability. The characterization results showed that the regenerated samples present the same stability. Currently, the use of non–toxic and inexpensive metals such as Fe, Ti, Cu or Zr, instead of Pd and Ag based commercial catalysts is an attractive topic. In this regard, Serrano et al, focused on the embedding FeIII on an MOF to prepare an efficient catalyst for the hydrogenation of acetylene under front–end conditions [18]. The experimental results showed that the prepared catalyst presents similar activity to Pd catalyst and could control acetylene concentration at the desired level.
