Assessment of Reaction Kinetics for the Dehydrogenation of Perhydro-Dibenzyltoluene Using Mg-and Zn-Modified Pt/Al 2 O 3 Catalysts

: The catalysts utilized for the dehydrogenation of dibenzyltoluene-based liquid organic hydrogen carriers (LOHCs) remain crucial. The state-of-the-art catalyst for dehydrogenation of dibenzyltoluene-based LOHC still suffers from deactivation and by-product formation. This is crucial in terms of the efficiency of the industrial dehydrogenation plant for hydrogen production, cyclability as well as the cost of replacing the catalyst. The development of catalysts with optimum performance, minimum deactivation and low by-product formation is required to attain the full benefits of the LOHC technology. Therefore, in this study, the effect of Mg and Zn modification on Pt/Al 2 O 3 catalyst is investigated for the catalytic dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). In addition, an assessment of reaction kinetics is also conducted. High dehydrogenation performance was obtained for Mg-doped Pt/Al 2 O 3 using a batch reactor at 300 ◦ C and 6 h reaction time. In this case, the degree of dehydrogenation (dod), productivity and conversion obtained are 100%, 1.84 gH 2 /g Pt /min and 99.9%, respectively. Moreover, the Mg-doped catalyst has resulted in a high turnover frequency (TOF) of 586 min − 1 compared to the Zn-doped catalyst (269 min − 1 ) and the undoped catalyst (202 min − 1 ) at the reaction temperature of 300 ◦ C. The amount of by-products increased with an increase in the catalytic activity, with the Pt/Mg-Al 2 O 3 catalyst possessing the highest amount of by-products. The dehydrogenation of H18-DBT followed first-order reaction kinetics. In addition, the activation energy obtained using the Arrhenius model is 102, 130 and 151 kJ/mol for Pt/Al 2 O 3 , Pt/Zn-Al 2 O 3 and Pt/Mg-Al 2 O 3 , respectively. Although the Mg-doped Pt/Al 2 O 3 shows high activation energy, the higher performance of the catalyst suggests that mass transfer limitations have no major effect on the dehydrogenation reaction under the conditions used.


Introduction
Hydrogen is one of the most pursued alternative energy carriers due to its high gravimetric energy density (33 kWh/kg), which is higher than most conventional fuels [1].It is also a clean energy carrier and a feedstock that has the potential to decarbonize various fossil-based sectors, such as petrochemicals, chemicals, glass, cement, steel, etc.Specifically, Hydrogen has a low density (0.0813 g/L at STP), resulting in low energy per volume; hence, efficient storage methods are needed [2].Several hydrogen storage technologies, such as high-pressure gas cylinders, liquefied hydrogen, ammonia, metal hydrides and liquid organic hydrogen carriers (LOHCs), have been explored to mitigate the storage challenges [3].Amongst the different storage mediums, LOHC technology offers numerous advantages over conventional hydrogen storage technologies.This is because hydrogen can be stored on a large scale for long periods of time without losses when using LOHC technology, whilst hydrogen storage in cylinders or as liquified hydrogen has potential losses.Dibenzyltoluene (H0-DBT) has been identified as one of the most promising LOHC ultimately, the improvement of the reactor design.It is reported that the dehydrogenation of H18-DBT depends on the mass transport of the molecules, which involves the diffusion of reactants and products to and from the catalyst pores [10].The kinetic models that have been developed for the dehydrogenation of H18-DBT are based on typical Pt/Al 2 O 3 catalysts, but the effect of dopants on the kinetic parameters has not been fully explored.Peters et al. [22] developed a kinetic model for H18-DBT dehydrogenation using 5 wt % Pt/Al 2 O 3 in a fixed bed reactor at temperatures from 290 to 350 • C. First-order reaction kinetics was obtained, with activation energy (Ea) and turnover frequency (TOF) of 117 kJ/mol and 120 s −1 , respectively.The Pt/Al 2 O 3 catalyst was also used by Park et al. [23] for the dehydrogenation of H18-DBT at temperatures ranging from 250 to 320 • C using a continuous flow reactor.The reaction rate orders obtained were between 2.3 and 2.4, with an activation energy of 171 kJ/mol.Modisha et al. [18] obtained an activation energy of 201 kJ/mol using a batch reactor.A microchannel reactor indicated improved reaction kinetics compared with conventional reactors [8].According to the literature, metal doping could lead to changes in the electronic, structural and chemical properties of the catalyst, which affects the catalyst's performance [14,15,24,25].In this study, the effects of Mg and Zn dopants on the Pt/Al 2 O 3 catalyst performance and reaction kinetics were investigated for the dehydrogenation of H18-DBT using a batch reactor.

Results and Discussion
This section addresses the results of the effect of reaction temperature and dopants on the catalytic dehydrogenation performance of H18-DBT and the dehydrogenation kinetics.

Evaluation of Catalytic Performance for Dehydrogenation of H18-DBT Using a Batch Reactor
The effect of the dehydrogenation reaction temperature on catalytic performance was evaluated using a batch reactor at temperatures in the range of 260-300 • C. The catalysts were subjected to four dehydrogenation cycles (runs) to determine the catalytic performance.The four dehydrogenation runs (run 1, run 2, run 3 and run 4) were also used to determine the stability of the catalysts.To determine the reaction kinetics for the dehydrogenation of H18-DBT, run 4 was used as it showed stability.Stability in this instance means the activity of catalysts was constant for at least two consecutive runs.
Figure 1 displays the effect of reaction temperature on hydrogen flow, productivity and the dod between 260 and 300 • C for the Pt/Al 2 O 3 , Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 catalysts.When the dehydrogenation reaction temperature was increased, there was an increase in hydrogen flow, productivity and dod.This is expected because an increase in temperature increases the rate of reaction, resulting in higher activity.A linear increase was observed for hydrogen flow and productivity in all runs.The activity of the catalysts decreased after the first run, yet some stability was observed between runs 3 and 4 (see also hydrogen flow and dod vs. time in Figure S1).This decrease is due to catalyst deactivation.In run 1, there was no apparent difference in hydrogen flow and productivity between 260 and 280 • C; however, at 290 and 300 • C, slight differences were observed.In runs 2-4 and at temperature ranges of 260-290 • C, the activity of all catalysts was similar, but the Mg-doped catalyst exhibited the highest activity at 300 The unmodified Pt/Al 2 O 3 used in this work showed a productivity of 1.62 gH 2 /g Pt /min, whereas literature work [14,15] reported a range of 0.6-1.2gH 2 /g Pt /min.The Mg-and Zndoped catalysts showed productivities of 1.84 and 1.68 gH 2 /g Pt /min, respectively.This is comparable with the values obtained by Auer et al. [14] and Chen et al. [15].These authors found that doping the 0.3 wt % Pt/Al 2 O 3 with sulphur resulted in productivities of 2 and 1.5 gH 2 /g Pt /min, respectively.An improved productivity of 2 gH 2 /g Pt /min was ob-tained with low Pt loading (0.3 wt %) [14] compared to the productivity of 1.84 gH 2 /g Pt /min obtained in this study using 0.5 wt % Pt/Al 2 O 3 (see Figure 2).This is because high Pt dispersion is achieved with low Pt loading, which results in improved catalytic activity [14,15].Nonetheless, it is considered that the performances recorded in this study are comparable with literature work.The unmodified Pt/Al2O3 used in this work showed a productivity of 1.62 gH2/gPt/min, whereas literature work [14,15] reported a range of 0.6-1.2gH2/gPt/min.The Mg-and Zn-doped catalysts showed productivities of 1.84 and 1.68 gH2/gPt/min, respectively.This is comparable with the values obtained by Auer et al. [14] and Chen et al. [15].These authors found that doping the 0.3 wt % Pt/Al2O3 with sulphur resulted in productivities of 2 and 1.5 gH2/gPt/min, respectively.An improved productivity of 2 gH2/gPt/min was obtained with low Pt loading (0.3 wt %) [14] compared to the productivity of 1.84 gH2/gPt/min obtained in this study using 0.5 wt % Pt/Al2O3 (see Figure 2).This is because high Pt dispersion is achieved with low Pt loading, which results in improved catalytic activity [14,15].Nonetheless, it is considered that the performances recorded in this study are comparable with literature work.The unmodified Pt/Al2O3 used in this work showed a productivity of 1.62 gH2/gPt/min, whereas literature work [14,15] reported a range of 0.6-1.2gH2/gPt/min.The Mg-and Zn-doped catalysts showed productivities of 1.84 and 1.68 gH2/gPt/min, respectively.This is comparable with the values obtained by Auer et al. [14] and Chen et al. [15].These authors found that doping the 0.3 wt % Pt/Al2O3 with sulphur resulted in productivities of 2 and 1.5 gH2/gPt/min, respectively.An improved productivity of 2 gH2/gPt/min was obtained with low Pt loading (0.3 wt %) [14] compared to the productivity of 1.84 gH2/gPt/min obtained in this study using 0.5 wt % Pt/Al2O3 (see Figure 2).This is because high Pt dispersion is achieved with low Pt loading, which results in improved catalytic activity [14,15].Nonetheless, it is considered that the performances recorded in this study are comparable with literature work.[14,15,26,27].
In this study, Mg appears to be the best dopant for the dehydrogenation of H18-DBT.The improved performance of the Pt/Mg-Al 2 O 3 catalyst could be due to the reduction in deactivation by the Mg dopant, as will be expanded upon in the discussion below.The comparison of the productivity of the catalysts prepared in this work and the literature work is presented in Figure 2.
The effects of temperature and dopants on the conversion, selectivity and by-product formation were investigated by analyzing the LOHC samples using gas chromatography single quadrupole mass spectroscopy (GC-SQ-MS).During the dehydrogenation of H18-DBT, the intermediates H12-DBT and H6-DBT are formed and then also become dehydrogenated to produce H0-DBT.Therefore, the dehydrogenation of H18-DBT follows the following order: H18-DBT-3H 2 → H12-DBT-3H 2 →H6-DBT-3H 2 → H0-DBT.Figure 3 shows the total ion chromatogram of the LOHC reaction mixture.
In this study, Mg appears to be the best dopant for the dehydrogenation of H18-DBT The improved performance of the Pt/Mg-Al2O3 catalyst could be due to the reduction in deactivation by the Mg dopant, as will be expanded upon in the discussion below.The comparison of the productivity of the catalysts prepared in this work and the literature work is presented in Figure 2.
The effects of temperature and dopants on the conversion, selectivity and by-product formation were investigated by analyzing the LOHC samples using gas chromatography single quadrupole mass spectroscopy (GC-SQ-MS).During the dehydrogenation of H18-DBT, the intermediates H12-DBT and H6-DBT are formed and then also become dehydrogenated to produce H0-DBT.Therefore, the dehydrogenation of H18-DBT follows the following order: H18-DBT-3H2 → H12-DBT-3H2 →H6-DBT-3H2 → H0-DBT.Figure 3 shows the total ion chromatogram of the LOHC reaction mixture.In the dehydrogenation of H18-DBT at 300 °C, fresh and used catalysts (from run 1 to run 4) showed conversions of >90% for Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 used in this study (Figure 4) (Figure 5).However, the H18-DBT conversion of the used catalyst was significantly low (~40% decrease) at temperatures from 260 °C to 270 °C.This phenomenon can be explained by the fact that the dehydrogenation of H18-DBT is endothermic and sensitive to temperature changes.Although the difference in conversion for all catalysts at the same temperature is less pronounced, the H0-DBT selectivity for the Pt/Mg-Al2O3 catalysts is remarkable.At the reaction temperature of 290 °C, the Pt/Mg-Al2O3, Pt/Al2O3 and Pt/Zn-Al2O3 catalysts showed H0-DBT selectivities of 87%, 72% and In the dehydrogenation of H18-DBT at 300 • C, fresh and used catalysts (from run 1 to run 4) showed conversions of >90% for Pt/Al 2 O 3 , Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 used in this study (Figure 4) (Figure 5).However, the H18-DBT conversion of the used catalyst was significantly low (~40% decrease) at temperatures from 260 • C to 270 • C.This phenomenon can be explained by the fact that the dehydrogenation of H18-DBT is endothermic and sensitive to temperature changes.Although the difference in conversion for all catalysts at the same temperature is less pronounced, the H0-DBT selectivity for the Pt/Mg-Al 2 O 3 catalysts is remarkable.At the reaction temperature of 290 • C, the Pt/Mg-Al 2 O 3, Pt/Al 2 O 3 and Pt/Zn-Al 2 O 3 catalysts showed H0-DBT selectivities of 87%, 72% and 77%, respectively.A comparable selectivity was obtained for all catalysts with an apparent difference at 300 • C. In run 4, Pt/Al 2 O 3 showed the lowest selectivity (35%), while Pt/Zn-Al 2 O 3 and Pt/Mg-Al 2 O 3 showed selectivities of 49% and 62%, respectively, at 300 • C. 77%, respectively.A comparable selectivity was obtained for all catalysts with an apparent difference at 300 °C.In run 4, Pt/Al2O3 showed the lowest selectivity (35%), while Pt/Zn-Al2O3 and Pt/Mg-Al2O3 showed selectivities of 49% and 62%, respectively, at 300 °C.77%, respectively.A comparable selectivity was obtained for all catalysts with an apparent difference at 300 °C.In run 4, Pt/Al2O3 showed the lowest selectivity (35%), while Pt/Zn-Al2O3 and Pt/Mg-Al2O3 showed selectivities of 49% and 62%, respectively, at 300 °C.The amount of by-products increased with an increase in both the dehydrogenation temperature and the catalyst activity.By-products, in this case, are a combination of high and low boiling point compounds, as reported in our previous work [18].Table 1 shows that the highest amount of by-products (12 mol %) was produced by the Pt/Mg-Al 2 O 3 catalyst, while Pt/Zn-Al 2 O 3 and Pt/Al 2 O 3 produced only 5 and 1.2 mol %, respectively, at 300 • C. In run 4, the amounts decreased to 0.68%, 2.1% and 1.9 mol % for Pt/Al 2 O 3 , Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 , respectively.Therefore, a decrease in the amount of byproducts can be correlated with low catalytic activity observed in run 4.These by-products are due to the cracking and cyclization of dibenzyltoluene molecules and it was found that the by-products are chemically similar throughout the runs (with the only difference being the quantity).The higher the catalytic reaction, the higher the dod and the formation of aromatic molecules.These aromatic molecules are susceptible to the acidic sites of the catalyst, which provides the cracking function.A combination of side reactions leading to the formation of by-products as well as the intermediates certainly affect the selectivity to H0-DBT.The formation of by-products increases with an increase in the degree of dehydrogenation over time, as shown in Figure 6.This indicates that as the dehydrogenation reaction takes place, the amount of partially and fully dehydrogenated compounds are prone to side reactions; hence, by-products are formed.In this study, the aliphatic by-products have not been obtained.The deactivation parameter, X = (X i − X f )/X i × 100, was calculated based on the H18-DBT conversion to determine the percentage deactivation based on the reaction time of 6 h (four runs of 90 min per run).Figure 8 shows the percentage deactivation of the catalysts at 300 • C. The undoped Pt/Al 2 O 3 had the highest deactivation of 7.4%, while Zn-doped and Mg-doped catalysts had deactivations of 4.9% and 1.9%, respectively.This suggests that Mg doping suppresses deactivation in a batch reactor, hence leading to an improved performance.A summary of the performance of catalysts in a batch reactor is presented in Table 2.The highest activity for all catalysts was observed at 300 • C. At this temperature, the addition of Mg and Zn dopants significantly increased activity, conversion, selectivity and by-products.For example, the dod improved from 82 to 88% for Zn-doped Pt/Al 2 O 3 and from 82 to 100% for Mg-doped Pt/A 2 O 3 .Moreover, full dehydrogenation was achieved within 75 min when using the Mg-doped catalysts, whereas undoped and Zn-doped catalysts did not result in full dehydrogenation within 90 min.For all catalysts, a decrease in activity was observed after the first run.The loss of activity can be attributed to catalyst deactivation.The better performance of the Pt/Mg-Al 2 O 3 catalysts can be explained by the low deactivation parameter of 1.9%, which suggests that Mg reduced deactivation.Characterization results obtained from our previous work [13] indicate that Mg reduced the acidity of alumina by 37% and this could have led to better performance than in the cases of Zn-doped and undoped catalysts with similar acidity values (see Table 2).The strong acidic sites inhibit the desorption of products; hence, the ability of Mg to decrease these sites led to improved selectivity to H0-DBT and the ultimate enhancement of performance.Due to the high catalytic activity of Pt/Mg-Al 2 O 3 , a high amount of by-products were observed.Therefore, in this study, Pt/Mg-Al 2 O 3 showed improved catalytic performance for the dehydrogenation of H18-DBT compared to Pt/Al2O3 and Pt/Zn-Al 2 O 3 .This study is an extension of our previously published work and the physicochemical properties of the catalysts studied in this work can be obtained from our previous work [13].

Reaction Kinetics for Dehydrogenation of H18-DBT
The effects of temperature on the kinetic parameters (e.g., reaction rate, rate constant, TOF) were then investigated.Experiments were conducted at dehydrogenation temperatures in the range of 260-290 • C and the dod was <70%.This was to avoid back-reactions that occur at high conversions [20].
As depicted in Figure 9, the concentration of H18-DBT decreased with time for all catalysts and the decrease in H18-DBT concentration is a function of temperature.Hence, the concentration gradient becomes steeper with an increase in temperature.At 300 • C, a H18-DBT concentration of 0.84 M was observed for Pt/Mg-Al 2 O 3 , while Pt/Zn-Al 2 O 3 and Pt/Al 2 O 3 generated final concentrations of 1.30 and 1.42 M, respectively.
TOF is defined as the number of reactant molecules converted by each Pt molecule per unit time.
Here, r is the initial rate of reaction (M min −1 g cat −1 ), k is the rate constant (min −1 ), C 0 is the initial concentration of H18-DBT (M), Mwt Pt is the molecular mass of Pt (g/mol), M Pt is the Pt loading (wt %) and D is the Pt dispersion (%).
The dehydrogenation kinetics of H18-DBT followed first-order reaction kinetics, as seen by the linear fit of ln (C/C 0 ) vs. time in Figure 10.This is in accordance with the literature [12,18].Rate constants were obtained from the slopes of the respective curves and then used to calculate the rates of reaction and TOFs, as shown in Table 3.The activity increases with an increase in temperature.This is due to the collision of molecules induced by high temperatures, which then increases the reaction rate.Undoped Pt/Al 2 O 3 produced higher rate constants at low temperatures (260 and 270 • C) than Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 .In specific, at 260 • C, Pt/Al 2 O 3 generated a rate constant of 0.00180 min −1 , while Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 generated rate constants of 0.00140 and 0.00170 min −1 , respectively.However, at temperatures > 280 • C, this changed drastically; the activity of the doped catalysts, particularly with Mg, increased from 0.0049 to 0.0137 min −1 .The TOF followed a similar trend.A high TOF value indicates a high activity due to an increase in the number of available active sites for the reaction.A Mg-doped catalyst exhibited the highest activity, i.e., a TOF of 586 min −1 , which is more than double compared to the Zn-doped catalyst (269 min −1 ) and almost three times than that obtained with an undoped catalyst (202 min −1 ) (calculated for dehydrogenation reactions at 300 • C).
by high temperatures, which then increases the reaction rate.Undoped Pt/Al2O3 produced higher rate constants at low temperatures (260 and 270 °C) than Pt/Mg-Al2O3 and Pt/Zn-Al2O3.In specific, at 260 °C, Pt/Al2O3 generated a rate constant of 0.00180 min -1 , while Pt/Mg-Al2O3 and Pt/Zn-Al2O3 generated rate constants of 0.00140 and 0.00170 min -1 , respectively.However, at temperatures > 280 °C, this changed drastically; the activity of the doped catalysts, particularly with Mg, increased from 0.0049 to 0.0137 min -1 .The TOF followed a similar trend.A high TOF value indicates a high activity due to an increase in the number of available active sites for the reaction.A Mg-doped catalyst exhibited the highest activity, i.e., a TOF of 586 min -1 , which is more than double compared to the Zn-doped catalyst (269 min -1 ) and almost three times than that obtained with an undoped catalyst (202 min -1 ) (calculated for dehydrogenation reactions at 300 °C).The activation energy of the catalysts was calculated using the Arrhenius model, where the graph of ln k vs. 1/T is plotted (see Figure 11).The slope of the graph provided activation energies of 102, 130 and 151 kJ/mol for Pt/Al 2 O 3 , Pt/Zn-Al 2 O 3 and Pt/Mg-Al 2 O 3 , respectively (see Table 4).A comparison indicates that our results are within the range reported in the literature.The regression was found to be >0.99 for all catalysts.Pt/Al 2 O 3 had the lowest activation energy but low catalytic performance when compared to its counterparts (Pt/Zn-Al 2 O 3 and Pt/Mg-Al 2 O 3 ).This suggests that factors such as mass transfer limitations could have had an impact because a lower activation energy indicates that a reaction barrier is reduced and the reaction can proceed more rapidly.Interestingly, the pre-exponential factor, which indicates the frequency of the H18-DBT molecule encountered on the active sites, is directly proportional to the activation energy.The high pre-exponential factor associated with Pt/Mg-Al 2 O 3 suggests an increase in the number of active sites.If the active sites are less effective (lower energy), then this decrease in energy of the active sites would lead to a growth in the activation energy [29].The high activation energy observed for Pt/Mg-Al 2 O 3 means more energy was used to overcome the reaction barrier when using this catalyst.Jorschick et al. argue that this high activation energy indicates that mass transfer effects have no dominant effect in the dehydrogenation reaction under the used conditions [30].active sites.If the active sites are less effective (lower energy), then this decrease in energy of the active sites would lead to a growth in the activation energy [29].The high activation energy observed for Pt/Mg-Al2O3 means more energy was used to overcome the reaction barrier when using this catalyst.Jorschick et al. argue that this high activation energy indicates that mass transfer effects have no dominant effect in the dehydrogenation reaction under the used conditions [30].A similar trend was observed by Seidel [16]: the dehydrogenation of H18-DBT at 270-320 °C with 0.5 wt % Pt/Al2O3, 0.3 wt % Pt/Al2O3 and 0.3 wt % Pt/Al2O3 (0.14 wt % S) generated activation energies of 97, 131 and 143 kJ/mol, respectively.High activation energies (131 and 143 kJ/mol) for catalysts with large pore diameter (24.6 nm) were obtained (compared with 97 kJ/mol for a pore diameter 3.2 nm).Therefore, the addition of Mg and Zn dopants reduced the diffusion barrier, as evidenced by the low activation energy (102 kJ/mol) of the undoped catalyst.Mg also increases basic surface OH groups on catalysts, A similar trend was observed by Seidel [16]: the dehydrogenation of H18-DBT at 270-320 • C with 0.5 wt % Pt/Al 2 O 3 , 0.3 wt % Pt/Al 2 O 3 and 0.3 wt % Pt/Al 2 O 3 (0.14 wt % S) generated activation energies of 97, 131 and 143 kJ/mol, respectively.High activation energies (131 and 143 kJ/mol) for catalysts with large pore diameter (24.6 nm) were obtained (compared with 97 kJ/mol for a pore diameter 3.2 nm).Therefore, the addition of Mg and Zn dopants reduced the diffusion barrier, as evidenced by the low activation energy (102 kJ/mol) of the undoped catalyst.Mg also increases basic surface OH groups on catalysts, which enhances fast kinetics, as evidenced by the work of Kim et al. [31].The Ru/MgO catalysts had better activity and kinetics compared with Ru/Al 2 O 3 during the hydrogenation of benzyltoluene because of the low surface acidity of the MgO support.We are thus able to confirm the higher catalytic activity of the Mg-doped catalyst compared with the other catalysts considered here.Since the catalysts studied here had the same textural properties (see Table 2), the ability of the Mg catalyst to slightly suppress pore size reduction may have contributed to improved performance.Optimal Mg loading could further contribute to the retention of the pore size and subsequently result in better performance.n is the order of reaction, k 0 is the frequency or pre-exponential factor in min −1 and Ea is the activation energy in kJ/mol.

Characterization
This study is a continuation of the previous work already published, where methods and detailed characterization of the catalysts used in this study can be found in the reported literature [13].Characterization techniques, such as CO pulse chemisorption, Hydrogen temperature programmed reduction (H 2 -TPR), Ammonia temperature programmed desorption (NH 3 -TPD), transmission electron microscopy (TEM) and inductively coupled plasma-optical emission spectrometry (ICP-OES) are described in our previous study.The Pt dispersion percentage, a critical factor for catalytic efficiency, is highest in Pt/Al 2 O 3 (38%), indicating a better spread of platinum particles across the support.However, the Mg-doped catalyst (34% dispersion) outperformed a highly dispersed Pt/Al 2 O 3 .Therefore, a 4% difference in dispersion has not led to a significant change in catalyst performance.Despite differences in dispersion, both Pt/Al 2 O 3 and Pt/Mg-Al 2 O 3 show identical metallic surface areas (0.42 m 2 /g), suggesting that the metallic surface area is not a determining factor for the improved performance shown by Pt/Mg-Al 2 O 3 .The hydrogen consumption values could also reflect the catalytic activity, which is notably higher for Pt/Mg-Al 2 O 3 (64 µmol/g) and Pt/Zn-Al 2 O 3 (55 µmol/g).High hydrogen consumption in TPR indicates the presence of a significant amount of reducible metal oxides, which in a metallic state are responsible for the catalytic activity.There is a slight variation in total acidity observed among the three catalysts, with marginally higher acidity in Pt/Zn-Al 2 O 3 .Particle size has an impact on the surface area and reaction kinetics, and Pt/Zn-Al 2 O 3 exhibits slightly smaller particles with a 0.06-0.08nm difference.The consistency in Pt loading across the catalysts is notable, but Mg and Zn dopants present in the latter two suggest a modification in electronic or structural properties, which can considerably affect catalytic performance already discussed.Thus, a comparison depicted in Table 5 emphasizes the importance of choosing a catalyst based on the specific requirements of the H18-DBT dehydrogenation process.

Experimental
The modified Pt-based catalysts (Pt/Al 2 O 3 , Pt/Mg-Al 2 O 3 , Pt/Zn-Al 2 O 3 ) were prepared following the wet impregnation method as described, characterized and analyzed in our previous work [8].A batch reactor setup, as shown in Figure 12, was used to study the reaction kinetics for the dehydrogenation of H18-DBT.A more detailed description of the experimental setup was reported earlier by our research team [16].
sistency in Pt loading across the catalysts is notable, but Mg and Zn dopants pr the latter two suggest a modification in electronic or structural properties, which siderably affect catalytic performance already discussed.Thus, a comparison dep Table 5 emphasizes the importance of choosing a catalyst based on the specific ments of the H18-DBT dehydrogenation process.

Experimental
The modified Pt-based catalysts (Pt/Al2O3, Pt/Mg-Al2O3, Pt/Zn-Al2O3) were p following the wet impregnation method as described, characterized and analyze previous work [8].A batch reactor setup, as shown in Figure 12, was used to st reaction kinetics for the dehydrogenation of H18-DBT.A more detailed descriptio experimental setup was reported earlier by our research team [16].The degree of dehydrogenation (dod) obtained was calculated based on the total hydrogen produced vs. theoretical hydrogen stored in H18-DBT.This value was confirmed by determining the dod of the remaining reaction mixture using a calibrated refractometer.The dod, productivity (P) and concentration (C) were calculated using Equations ( 3), ( 4) and (5), respectively.

Dod (%) =
Volume of H 2 released Theoretical H 2 volume × 100 C H18−DBT = n H18−DBT 1 − Yield (%) 100 V Total (5) where m, n and V Total are the mass (g), number of moles and total volume (L) of the liquid, respectively.

Conclusions
The evaluation of the catalytic performance of Mg-and Zn-modified Pt/Al 2 O 3 catalysts in a batch reactor indicated a strong dependence of activity and performance on temperature.The highest activities were obtained at 300 • C for all catalysts.The Pt/Mg-Al 2 O 3 was the best-performing catalyst with dod, productivity, conversion and selectivity of 100%, 1.84 gH 2 /g Pt /min, 99.9% and 87%, respectively.After run 4, the catalytic activity for all catalysts decreased due to catalyst deactivation.The extent of deactivation, expressed as a deactivation parameter, indicated that Mg reduced the deactivation of Pt/Al 2 O 3 by a factor of three, which then led to improved stability and better performance.The presence of by-products was a function of catalyst activity.Mg-doped catalysts performed better because of their favourable acidity feature, which suppressed deactivation and facilitated easy diffusion of reactants and products (high activation term), thus resulting in better catalytic activity.The dehydrogenation activity of H18-DBT followed first-order reaction kinetics.The obtained activation energy of Pt/Al 2 O 3 , Pt/Mg-Al 2 O 3 and Pt/Zn-Al 2 O 3 were 101, 151 and 131 kJ/mol, respectively.In this case, a high number of active sites corresponds to a high frequency factor value, which relates to high activation energy.Therefore, a lower activation energy for the dehydrogenation of H18-DBT will not always produce a high reaction rate.The high activation energy of the doped catalysts, particularly Pt/Mg-Al 2 O 3 , suggests that mass transfer effects have no dominant effect in the dehydrogenation reaction under the used conditions.The catalysts in stable form meet the set criteria in terms of H18-DBT conversion (>90%).However, there is a need for further improvement since the catalyst productivity and selectivity do not meet the set criteria provided in the introduction section.

Figure 7 .
Figure 7. Possible side reaction pathways for the formation of by-products.

Figure 7 .
Figure 7. Possible side reaction pathways for the formation of by-products.

Figure 12 . 12 .
Figure 12.Schematic representation of a batch reactor setup used for dehydrogenation experim Figure 12.Schematic representation of a batch reactor setup used for dehydrogenation experiments [6].

Author Contributions:
Conceptualization, P.M. and D.B.; Methodology, R.G.; Formal analysis, R.G.; Data curation, R.G.; Writing-original draft, R.G. and P.M.; Supervision, P.M. and D.B.; Project administration, D.B.; Funding acquisition, D.B.All authors have read and agreed to the published version of the manuscript.Funding: This research received no external funding.
• C. The fresh Pt/Mg-Al 2 O 3 catalyst produced a 100% dod at 290 • C for run 1, while, at the same temperature, the Pt/Al 2 O 3 and Pt/Zn-Al 2 O 3 catalysts produced a dod of 78%.Pt/Zn-Al 2 O 3 and Pt/Al 2 O 3 did not provide full dehydrogenation at the studied temperatures.After the first run, Pt/Mg-Al 2 O 3 still exhibited the highest dod of 73% compared with values of 60% and 50% observed for Pt/Zn-Al 2 O 3 and Pt/Al 2 O 3 , respectively.

Table 1 .
Quantities of by-products obtained for dehydrogenation of H18-DBT using different catalysts at 300 • C.

Table 2 .
Summary of the effects of Mg and Zn dopants on Pt/Al 2 O 3 for the H18-DBT dehydrogenation performance at 300 • C in a batch reactor.i and X run 4 are the initial and final H18-DBT conversions (%), S i and S run 4 are the initial and final H0-DBT selectivities (%), P i and P run 4 are initial and final productivities, Y i and Y are the initial and final dod (%), X is the deactivation parameter (%), acidity is in mmol NH 3 /g cat . X )

Table 4 .
Comparison of kinetic parameters of Pt/Al 2 O 3 catalysts between this study and the literature.

Table 5 .
Catalyst properties determined by various characterization techniques.

Table 5 .
Catalyst properties determined by various characterization techniques.