Effect of N₃⁻ species on selective acetylene hydrogenation over Pd/SAC catalysts

Highlights

• NaN₃ was added to Pd/SAC catalysts as Lewis base promoter.

• NaN₃ addition increased acetylene adsorption dramatically.

• Dual function sites with N₃⁻ and Pd sites increased acetylene adsorption.

• Acetylene adsorption type was critical for selective hydrogenation.

Abstract

Selective hydrogenation of acetylene to ethylene is very important for removing a trace amount of acetylene from ethylene. Tuning catalyst acidity is critical for hydrogenation reaction to prevent oligomerization. In this work, NaN₃ was introduced to Pd catalyst serving as Lewis base sites. NaN₃-Pd/SAC catalysts with different N₃⁻ loadings (0–15 wt.%) were prepared by impregnation method and analyzed with BET, temperature-programmed reduction (TPR), Fourier-transferred Infrared (FTIR) spectroscopy, CO pulse chemisorption, temperature-programmed desorption (TPD) of hydrogen and acetylene. The results showed NaN₃ covered some Pd species and increased the reduction temperatures of catalysts. Also NaN₃ addition decreased the catalyst surface areas due to blocking some pores on the support. N₃⁻ on the catalysts lowered hydrogen adsorption capacity but showed superior adsorption capability for acetylene, the selectivity and conversion of the acetylene hydrogenation was negatively affected by N₃⁻ species under the current operation conditions. The amount of the adsorbed hydrogen and the types of acetylene adsorption played important roles for the hydrogenation reaction. This work provides insights on how to prepare an effective catalyst for removing acetylene impurity from ethylene.

Introduction

Selective hydrogenation of acetylene to ethylene is a very important industrial purification process for removing a trace amount of acetylene from ethylene. The selective hydrogenation of acetylene with Pd catalyst is commonly used for this purpose [1], [2], [3]. Pd catalysts supported on carbon materials such as activated carbon (AC) have been extensively employed as the catalysts for hydrogenation in industrial reactions [4], [5], [6], [7]. Most recent work provided clear evidence that the carbon support can play an important role in the selective hydrogenation of acetylene [8].

As for Pd catalysts, significant efforts have been made toward developing different additives that are capable of promoting acetylene selectivity to ethylene. The role of the additives is generally considered to be derived from two factors: geometric and electronic. For example, researchers [9] reported the insertion of Cu into the Pd matrix decreased the number of multi-coordination sites of the Pd atoms that is responsible for the dissociative adsorption of acetylene and suppresses the formation of beta phase Pd hydride; both are damaging to ethylene selectivity. Modification of Pd/Al₂O₃ catalysts with an additive such as silver, cobalt or zinc [10], [11], [12] was also explored to reduce high acidity of the catalyst to minimize carbonaceous deposits on the catalyst surface and increase the catalyst lifetime.

Azide anion N₃⁻, found in 1890, was usually attracted by its high energy density [13], [14]. Also, it was found that controlled thermal decomposition of impregnated sodium azide on γ-alumina could yield a super basic catalyst [15]. And our recent study proved the lone pairs from N₃⁻ are very active and could play the role as electron donor in oxygen-reduction reactions [16]. Thus, N₃⁻ may be employed in the Pd/AC catalyst as an additive to serve as Lewis base for adsorbing acetylene which is a weak Lewis acid [17]. Increasing acetylene coverage on the catalyst is similar to a low H₂/acetylene ratio that is used in the reactant stream, which may improve ethylene selectivity proved by experimental and simulation results [18], [19]. Therefore, it is prompt for us to examine N₃⁻ as an additive for selective hydrogenation of acetylene on Pd/SAC catalysts.