Enhanced selectivity and stability of Pt-Ge/Al₂O₃ catalysts by Ca promotion in propane dehydrogenation

Highlights

• Calcium increases Pt-Ge alloy extent by enhancing Ge reducibility.

• Calcium significantly enhances catalyst stability and propylene selectivity.

• Calcium selectively suppresses strong acid sites.

• Calcium-promoted catalyst is less active for coke deactivation, (ψ_d).

• Steady state activity is increased since Ca decreases ψ_d more than ψ_r.

Abstract

The promoting effect of Ca on the catalytic performance of Pt_0.5-Ge_1.5/Al₂O₃ during propane dehydrogenation (PDH) reaction was studied using experimental methods and kinetic modeling. Different characterization techniques including H₂-TPR, NH₃-TPD, C₃H₆-TPD, XPS, BET, CO-chemisorption, CO-DRIFT, and TGA were used to unravel the properties of catalysts. Enhanced propylene selectivity (98.5% Vs 96.4% @ 600 °C and 24 h TOS) and catalyst stability were achieved due to the decrease of acid centers of Al₂O₃ along with an increase of the degree of alloy formation upon addition of Ca. The experimental data of propane conversion vs. time were successfully fitted with the Deactivation Model with Residual Activity (DMRA), allowing for the calculation of the intrinsic deactivation kinetic parameters for both catalysts. For the Ca-promoted catalyst, a deactivation function of 0.05 h⁻¹ along with a residual activity of 0.82 at 600 °C were obtained. Whereas, for the non-promoted sample, these values were 0.22 h⁻¹ and 0.63, respectively. The activation energy of the PDH reaction for the Pt_0.5-Ge_1.5/Al₂O₃-CaO catalyst was slightly higher than that of the unmodified one (74.8 kJ/mol vs. 66.4 kJ/mol) due to the enhanced reduction of the GeO_x species on this sample, which enhanced the formation of a Pt-Ge alloy, decreasing the initial activity of the catalyst. However, the higher resistance against the deactivation by coke of the promoted catalyst was invoked to explain the better performance of this catalyst in terms both of activity and selectivity at steady state conditions.

Introduction

Propane dehydrogenation for the production of propylene has many advantages over traditional routes [1], such as lower cost due to huge availability of cheap light alkane feedstock from shale gas and less harmful to the environment due to less CO₂ emission [2]. Supported Pt on Al₂O₃ is one of the most widely used commercial catalysts for this reaction due to its high activity for C–H bond cleavage [3]. However, catalyst deactivation by coke formation is still a major challenge [4], [5]. Coke formation generally occurs as a result of side reactions during propane dehydrogenation reaction, such as hydrogenolysis and cracking, as well as isomerization [6]. The geometric and electronic properties of Pt can be tailored by adding promotors such as Sn [7], In [8] and Cu [9], resulting in the suppression of detrimental side reactions and better catalytic performance. Besides of the active metal, the catalyst support also plays a vital role in this reaction. Support properties, including surface area, structure, as well as acid-base properties, have substantial effects on the metal support interaction, the dispersion of the active component, and in modulating geometric and electronic properties of Pt, significantly affecting the catalytic performance [10], [11], [12]. In addition, the support properties can also be significantly modified by incorporating metals. For this purpose, rare earth metals, alkali and alkaline earth metals have been commonly used [13], [14], [15], [16], [17]. Bai et al. used a Ca-modified ZSM-5 as support for a bimetallic Pt-Sn in PDH reaction [17]. In this work, it was reported that adding an optimal amount of Ca modifies the properties of both the active metal and the support. Indeed, Ca suppresses strong acidic sites of ZSM-5 and accelerates Sn reduction, resulting in a high percentage of alloy formation. Long et al. reported that small amount of yttrium does not have much effect on γ-Al₂O₃ structure, but neutralizes the strong acidic centers of the support, increases the metal dispersion and reduces the particle size, leading to improved selectivity [18]. Several works have applied Ca to modify Al₂O₃ properties. Choong et al. [16] reported a Ca-modified Al₂O₃ as support for Ni and Pt metal particles for ethanol steam reforming, resulting in a decrease of acidity, an increase of particle size and a different reaction mechanism. Yu et al. [19] reported a better catalytic performance on a Ca-modified support during dehydrogenation of cyclohexane. Neutralizing strong acidic sites and modifying electronic properties of Pt were some of the positive effects ascribed to Ca, since it suppresses deep dehydrogenation and facilitates coke desorption due to the increased H/C ratio of H-deficient coke. Although Ca has been commonly used to modify properties of Al₂O₃ support in different reactions, only one work is available in the literature using Ca to modify the Al₂O₃ support in PDH reaction reported by Long et al. [20]. They used different amount of Ca to modify the Al₂O₃ support in a tri-metallic Pt-Sn-In catalyst. Three positive effects, attributed to Ca addition, have been invoked in that work: i- decrease in acidity, ii- increase of Pt dispersion, and iii- prevention of sintering of metal particles. However, precise understanding of Ca effects on the deactivation behavior of the catalyst in PDH reaction needs to be explored further. Particularly, kinetic of catalyst deactivation in presence of Ca during PDH reaction is still lacking.

In our previous work, we have reported for the first time a bimetallic Pt_0.5-Ge_1.5/Al₂O₃ catalyst showing outstanding catalytic performance during PDH reaction [21]. However, this catalyst has to be further improved for long Time-On-Stream (TOS). Accordingly, in this contribution, we report the effect of Ca on catalyst deactivation of Pt-Ge samples. Along with a thorough characterization of the bimetallic catalysts, a deactivation model (DMRA) was used to evaluate the effect of Ca on the deactivation behavior during PDH.