Evaluation of perhydrodibenzyltoluene dehydrogenation parameters and durability using noble metal catalysts for hydrogen production

Abstract

Hydrogen storage using liquid organic hydrogen carrier (LOHC) technology offers numerous advantages over conventional hydrogen storage systems. LOHCs are organic compounds that can store and release hydrogen through catalytic hydrogenation and dehydrogenation reactions. Globally, there are a few key-industry players involved in this technology. In South Africa, there are identified opportunities for the future application of LOHC technology. Probably the most widely used LOHC material for commercial applications is dibenzyltoluene. However, dibenzyltoluene (technical grade) (H0-DBT) consists of various structural isomers that are difficult to analyse by conventional gas chromatography. Products of reaction mixtures of H0-DBT contain numerous isomeric compounds (>20). This results in overlapping of peaks in conventional gas chromatography. Further challenging topics, associated with the use of H0-DBT as a LOHC, include the following: development and optimization of suitable catalytic materials for the dehydrogenation reaction, durability of the H0-DBT molecule, and the unknown number of hydrogenation and dehydrogenation cycles that a H0-DBT molecule can withstand. Investigations into these and other areas in this field are of a great importance. Specific topics to be addressed here included the following: determination of a suitable chromatographic method and optimum conditions for the separation, identification and quantitative analysis of isomeric reaction mixtures of H0-DBT-based LOHC; evaluation and characterization of monometallic catalysts (Pt/Al₂O₃and Pd/Al₂O₃) and their bimetallic counterparts (Pt–Pd/Al₂O₃) for the dehydrogenation of perhydrodibenzyltoluene (H18-DBT); optimization of the catalyst metal loading and reaction temperature for the reaction, towards achieving a reasonably high degree of dehydrogenation (dod), coupled with high-purity hydrogen; and determination of the durability of H0-DBT-based LOHCs. Suitable analytical methods were successfully developed for the analysis of isomeric mixtures of H0-DBT using the state-of-the-art techniques: two-dimensional gas chromatography coupled with time of flight mass spectrometry and single quadrupole-mass spectrometry gas chromatography. Results of chromatography data were confirmed by spectrometry techniques (direct analysis in real time mass spectrometry and proton nuclear magnetic resonance spectrometry). Furthermore, good use was made of a refractometer to obtain measurements regarding the dod. This analytical technique enables us to obtain rapid measurements of the degree of hydrogenation (i.e., amount of hydrogen available in the molecule) or the dod (i.e., amount of hydrogen released from the molecule.) Results of chromatographic analyses revealed that at higher dehydrogenation temperatures fewer partially dehydrogenated products were formed (hexahydro-dibenzyltoluene and dodecahydro-dibenzyltoluene isomers) but higher amounts of H0-DBT were formed; hence, a higher dod value is obtained. On the other hand, at lower dehydrogenation temperatures a higher amount of partially dehydrogenated products formed but a lower amount of H0-DBT; hence, a lower dod value is obtained.