Catalytic Consequences of Supported Pd Catalysts on Dehydrogenative H₂ Evolution from 2-[(n-Methylcyclohexyl)methyl]piperidine as the Liquid Organic Hydrogen Carrier

Abstract

Herein, nanoporous Al2O3, CeO2, TiO2, ZrO2, and SnO2 were used as the supports for Pd nanoparticles, and effects of surface characteristics on catalytic performances for dehydrogenation of 2-[(n-methylcyclohexyl)­methyl]­piperidine (H12-MBP) as the H2-rich liquid organic hydrogen carrier were investigated. The H2 yield, dehydrogenation rate, product selectivity, and recyclability of the supported Pd catalysts depended on the metal oxide support and Pd loading. The H2 yield and reaction rate of the Al2O3 supporting 5 wt % Pd with a mean size of 5.76 nm were the highest (75.8%) and the fastest (k 1 = 0.076 min–1), respectively, of all the catalysts. CeO2 exhibited the highest reducibility and the best supporting ability for Pd nanoparticles, which thus dispersed Pd with the smallest mean size of 3.45 nm. Although this catalyst exhibited a lower H2 yield (67.1%) and a slower reaction rate (k 1 = 0.030 min–1) than Al2O3, it showed the best recyclability without a significant loss of activity during four consecutive runs, which could be attributed to the strong metal–support interaction of Pd to the surface of CeO2. The H2 yield and the dehydrogenation rate were systematically correlated with the surface characteristics of the metal oxides, such as acidity, adsorption affinity (adsorption energy), and charge transfer value of H12-MBP, which were determined via combined experimental and theoretical studies.

Enhanced Dehydrogenative H2 Release from N-Containing Amphicyclic LOHC Boosted by Pd-Supported Nanosheet MFI Zeolites Having Strong Acidity and Large Mesoporosity

“…Liquid organic hydrogen carriers (LOHCs) are organic compounds that exist in the liquid state under ambient conditions. They can reversibly release and store ∼6 wt % of H 2 via dehydrogenation and hydrogenation, respectively. − Because they facilitate the safe storage and transportation of H 2 , LOHCs, in conjunction with compression and liquefaction technologies have attracted significant attention as a means to the realization of a H 2 economy. − Various rationally designed and commercially viable LOHCs have been reported to date, which can be classified into three different molecular categories depending on their elementary compositions: − (i) homocyclic LOHCs containing cyclic hydrocarbon moieties, − (ii) heterocyclic LOHCs containing heteroatoms, such as nitrogen, as part of their cyclic hydrocarbon structures, − and (iii) amphicyclic LOHCs containing homocyclic hydrocarbon ring and heterocyclic heteroatom-containing ring linked via C–C covalent bridge. − Methylcyclohexane (H 6 -MCH), perhydro-diphenylmethane (H 12 -DPM), perhydro-benzyltoluene (H 12 -BT), and perhydro-dibenzyltoluene (H 18 -DBT) are representative homocyclic LOHCs, which have relatively higher stabilities and lower reactivities than N -heterocyclic LOHCs. − Perhydro- n -ethylcarbazole (H 12 -NEC) with a fused ring can be classified as N -heterocyclic LOHCs . Perhydro-2-( n -methylbenzyl)­pyridine (H 12 -MBP) and perhydro-2-benzylpyridine (H 12 -BP) can be considered as the amphicyclic LOHCs (Scheme ).…”

“…Perhydro-2-( n -methylbenzyl)­pyridine (H 12 -MBP) and perhydro-2-benzylpyridine (H 12 -BP) can be considered as the amphicyclic LOHCs (Scheme ). − Although the heterocyclic LOHCs and the amphicyclic LOHCs have lower stabilities than homocyclic LOHCs due to the presence of C–N bonds, they have higher reactivities than homocyclic LOHCs owing to the presence of the heteroatom. ,,− …”

“…To save energy and time, the H 2 -rich LOHC should discharge its releasable H 2 rapidly at the H 2 demand sites. ,,− In terms of dehydrogenation enthalpy and kinetics, N -containing heterocyclic or amphicyclic LOHCs deliver superior performance due to their high reactivities. − Among various N -containing LOHCs, H 12 -MBP is the first amphicyclic LOHC that was rationally designed via theoretical calculations of dehydrogenation enthalpies and molecular properties of various LOHC candidates and produced via organic synthesis . H 12 -MBP is the perhydro structure of 2-( n -methylbenzyl)­pyridine containing a homocyclic cyclohexane ring and N -heterocyclic piperidine ring.…”

“…Both dehydrogenation steps follow the first-order kinetics. However, the dehydrogenation rates and rate constants ( k 1 and k 2 in Scheme ) for piperidine and cyclohexane rings are significantly different due to the different reactivities of the homocycle and heterocycle. ,, In general, the homocyclic cyclohexane ring is dehydrogenated at a slower rate than the heterocyclic piperidine ring (typically, k 1 ≫ k 2 ). ,,, Therefore, the overall dehydrogenation rate corresponding to the maximum H 2 yield is strongly dependent on the rate and extent of H 2 discharge from the cyclohexane moiety of H 6 -MBP, which is formed as the intermediate species. − , …”

Controlled Metal–Support Interactions in Au/CeO2–Mg(OH)2 Catalysts Activating the Direct Oxidative Esterification of Methacrolein with Methanol to Methyl Methacrylate

“…The SMSI effect can not only enhance catalytic activity but can also control product selectivity [10][11][12]. For example, with specific metal nanoparticles as the main catalytic site, variation of the metal oxide support can change the catalytic behavior [13,14].…”