Entwicklung heterogener Katalysatorsysteme zur Dehydrierung von perhydro-Dibenzyltoluol

• Development of heterogeneous catalysts for the dehydrogenation of perhydro dibenzyltoluene

Chen, Ximeng; Palkovits, Regina (Thesis advisor); Wasserscheid, Peter (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

In order to move from a carbon-based energy system to a more sustainable one, focus is placed on Liquid organic hydrogen carrier (LOHC) systems for CO2-free hydrogen storage and release. In this work supported catalysts were developed and investigated for the dehydrogenation of the LOHC system perhydro dibenzyltoluene/dibenzyltoluene (H18DBT/H0-DBT) and compared to the currently applied benchmarks Pt/γ-Al2O3 and S-Pt/γAl2O3. The aim was to develop other well performing catalyst systems with regard to activity, stability and selectivity. Therefore, an insight into correlations between catalyst properties and their performance in the dehydrogenation of H18-DBT was gained. First, the active component (metal species) and the support were varied for catalyst synthesis in order to determine the most active catalyst for dehydrogenation experiments carried out in a batch setup. Platinum was found to be more active compared to non-noble metals (Ni, Co, Cu, Mn). Out of all tested supports including metal oxide, structured silica and activated carbon, TiO2 anatase has been proven to be the support resulting in the most active and selective catalyst. Stability tests of TiO2 anatase in H18-DBT und different mixtures of H18DBT/H0-DBT under dehydrogenation conditions revealed formation of coke and coke precursors in similar amounts. The variation of the surface area of TiO2 anatase affects the dispersion of the supported platinum. In addition, the reduction temperature affects the particle size and thus dispersion as well. An optimum Pt dispersion of 0.40-0.45 was found to result in the highest TOF in the dehydrogenation of H18-DBT.Sulfur as a dopant for the Pt/TiO2 catalyst was identified to be a selective poison resulting in high performing catalysts in the dehydrogenation of H18-DBT. S-Pt/TiO2 was found to achieve a high degree of dehydrogenation of 98% which is comparable to the benchmark S-Pt/γ-Al2O3. In addition, analysis of the side products revealed a higher selectivity towards dibenzyltoluene in the presence of S-Pt/TiO than over γ-Al2O3 . Characterization by infrared spectroscopy (IR), transmission electron microscopy (TEM), diffuse reflectance infrared fourier transform spectroscopy with CO as adsorbing molecule (CO-DRIFTS) suggest the presence of strongly chemisorbed sulfur species on Pt and on the support as well as SMSI of the Pt/TiO2 catalysts. As found for the sulfur modified Pt/γ-Al2O3 before, sulfur also has both a geometric as well as an electronic effect on platinum particles on TiO2 and prefers adsorbing at the low-coordinated sites of Pt. While sulfur blocks the sites favoring side reactions, it also withdraws electron density from the Pt particles. The second effect is however opposed by SMSI where the reduced support provides electron density to Pt particles. The modification with the optimal amount of sulfur is crucial to achieve the selectivity and activity promoting effect without using an excess of sulfur so that it turns into a poison. Since the Pt dispersion was one factor found that is linked directly to the dehydrogenation performance of the catalyst, sulfur can be used to receive the desired Pt dispersion on supports with different specific surface areas. The sulfur to accessible Pt molar ratio was found to be around 0.48. In addition, sulfur was found to adsorb on the support. Based on kinetic studies a batch and plug flow reactor for the dehydrogenation reaction were simulated for the Pt/TiO2 and S-Pt/TiO2. According to the calculations, the sulfur doped catalyst displays higher conversions in both batch and plug flow operation as compared to the unmodified system Pt/TiO2.In the recycling studies of the S-Pt/TiO2 catalysts, high degrees of dehydrogenation were gained over all 3 catalytic cycles with prolongation of the reaction time until no more H2 formation was detected. Nevertheless, progressive pore blocking was found to occur in repeating catalytic cycles. The total pore volume as well as the average pore size decrease as well as the Pt dispersion despite washing the used catalysts and reductive treatment. These results suggest that organic remains and coke (precursors) in the catalyst pores dissolve in the LOHC during the dehydrogenation reaction and active surface area is regained.