Dehydrogenation of perhydrodibenzyltoluene for hydrogen production: the effect of Mg and Zn dopants on the catalytic activity of Pt/Al₂O₃

Abstract

Liquid organic hydrogen carriers (LOHC) are hydrocarbon molecules that have the capacity to store and release hydrogen through catalytic hydrogenation and dehydrogenation reactions. The dibenzyltoluene/perhydrodibenzyltoluene (H0-DBT/H18-DBT) system, has been identified as the most promising LOHC molecules for hydrogen storage. However, conventional Pt/Al2O3 catalyst used for dehydrogenation of H18-DBT still need to be further improved to obtain high throughput hydrogen production with low deactivation. Modification of Pt/Al2O3 is another way of improving the catalytic performance (productivity, degree of dehydrogenation, selectivity and conversion). Therefore, in this contribution the effect of Mg and Zn dopants on the catalytic performance of Pt/Al2O3 is investigated for the dehydrogenation of H18-DBT. Firstly, the ү-Al2O3 supports were modified with Mg(NO3).6H2O and Zn(NO3).6H2O precursors to produce Mg-Al2O3 and Zn-Al2O3 aiming at 3.8 wt % metal loading. Thereafter, the unmodified and modified ү-Al2O3 supports were impregnated using H2PtCl6.xH2O to produce Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 aiming at 0.5 wt % Pt loading. The catalysts were characterised using inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), hydrogen temperature programmed reduction (H2-TPR), ammonia temperature programmed desorption (NH3-TPD), CO pulse chemisorption, Branuer-Emmett-Teller (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, the catalysts' performance for dehydrogenation of H18-DBT was evaluated using a batch and fixed bed reactor. The samples resulting from dehydrogenation reaction were analysed using gas chromatography and refractive index techniques. The initial four dehydrogenation runs (cycles) using a batch reactor at temperatures from 260-300 °C indicated that an increase in temperature increases the performance of all catalysts. Notably, the performance of the catalysts is in the following order: Pt/Mg-Al2O3 > Pt/Zn-Al2O3 > Pt/Al2O3 which suggests that Mg is a suitable and promising dopant. However, all catalysts showed deactivation during the initial four dehydrogenation runs. Furthermore, initial catalysts stability was investigated using a fixed bed reactor at weight hourly space velocity (WHSV) of 0.61 h-1, 22 h time-on-stream and 300 °C. Pt/Mg-Al2O3 catalyst showed improved performance when compared to Pt/Al2O3 and Pt/Zn-Al2O3, and it also showed lower deactivation. The calculated turnover frequencies (TOF) for dehydrogenation of H18-DBT at 300 °C using Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 are: 202, 586 and 269 min-1, respectively. This indicate that Pt/Mg-Al2O3 catalyst has the highest amount of active metal sites available for dehydrogenation reaction. This was also confirmed by a high frequency factor (2.3 x 1011) min-1 which suggests high rate of molecular collisions. The dehydrogenation of H18-DBT followed first order reaction kinetics and the obtained activation energy of Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 are 101, 151 and 131 kJ/mol, respectively. In this case, high amount of active sites corresponds to high value of

frequency factor and this is connected to high activation energy. Therefore, lower activation energy for dehydrogenation of H18-DBT will not always produce high reaction rate. Mg lowered the support acidity (as proven by NH3-TPD) and the less acidic catalyst is found to be more selective towards the product H0-DBT. Therefore, Mg weakens the adsorption of H0-DBT on Pt surface and promotes H0-DBT desorption. However, the most active catalyst (Pt/Mg-Al2O3) produced high amounts of by-products. This is because more aromatic molecules produced are susceptible to the acidic sites of the support which provides the C-C cracking function.