Heterogeneous Catalysts in N-Heterocycles and Aromatics as Liquid Organic Hydrogen Carriers (LOHCs): History, Present Status and Future

The continuous decline of traditional fossil energy has cast the shadow of an energy crisis on human society. Hydrogen generated from renewable energy sources is considered as a promising energy carrier, which can effectively promote the energy transformation of traditional high-carbon fossil energy to low-carbon clean energy. Hydrogen storage technology plays a key role in realizing the application of hydrogen energy and liquid organic hydrogen carrier technology, with many advantages such as storing hydrogen efficiently and reversibly. High-performance and low-cost catalysts are the key to the large-scale application of liquid organic hydrogen carrier technology. In the past few decades, the catalyst field of organic liquid hydrogen carriers has continued to develop and has achieved some breakthroughs. In this review, we summarized recent significant progress in this field and discussed the optimization strategies of catalyst performance, including the properties of support and active metals, metal-support interaction and the combination and proportion of multi-metals. Moreover, the catalytic mechanism and future development direction were also discussed.

Low-Pt-Based Sn Alloy for the Dehydrogenation of Methylcyclohexane to Toluene: A Density Functional Theory Study

Spin-polarized van der Waals corrected density functional theory calculations were applied to Sn–Pt alloys with Pt content ≤ 50% (referred to as low Pt alloys) to evaluate their catalytic activity towards the dehydrogenation of methylcyclohexane (MCH), with the formation of toluene as product. The calculated adsorption energies of MCH, its intermediates and toluene showed that these molecules bind on the considered Sn–Pt alloys. Sn–Pt alloys had the lowest dehydrogenation energetics, indicating that the activity of this catalytic material is superior to that of a pristine Pt catalyst. Desorption of the intermediate species was feasible for all Sn–Pt alloy configurations considered. The catalytic dehydrogenation reaction energetics for the various Sn–Pt alloy configurations were more favourable than that achieved with pristine Pt surfaces. The current study should motivate experimental realization of Sn–Pt alloys for the catalytic dehydrogenation reaction of MCH.

Preparation of a Novel NiAlO Composite Oxide Catalyst for the Dehydrogenation of Methylcyclohexane

A series of NiAlO composite oxide catalysts with high surface areas and high Ni dispersion were prepared through an improved co-precipitation method. The new preparation method effectively improved the specific surface area and pore volume of the catalyst, promoted the dispersion of nickel species, alleviated the agglomeration of the catalyst, and improved the stability of the catalyst by strengthening the interaction between Ni and Al. The typical catalyst Ni20Al had a specific surface area of 359 m2/g and a NiAl2O4 phase. In the dehydrogenation of methylcyclohexane over the Ni20Al catalyst, the conversion of methylcyclohexane could reach 77.4%, with toluene selectivity of 85.6%, and a hydrogen release rate of 63.94 mmol g−1 h−1, and did not show any significant inactivation during the stability test over 29 h under the reaction conditions of reaction temperature 450 °C and LHSV = 4 mL g−1 h−1. However, the conversion of methylcyclohexane with the IM-NiAl catalyst prepared through the traditional impregnation method was only 50.75%, with toluene selectivity of 70.5%, and with a hydrogen release rate of 35.84 mmol g−1 h−1, and the lifetime of the catalyst was only 15 h.

Hydrogen Generation from Organic Hydrides under Continuous-Flow Conditions Using Polymethylphenylsilane-Aluminum Immobilized Platinum Catalyst

Hydrogen is an important resource for realizing the goal of a hydrogen-based society as well as for synthetic organic chemistry. Catalytic dehydrogenation of organic hydrides such as methyl cyclohexane is attractive for hydrogen storage and transportation in terms of reversibility and selectivity of catalytic reactions and hydrogen storage density. We developed a highly active polymethylphenylsilane-aluminum immobilized platinum catalyst (Pt/MPPSi-Al2O3) for dehydrogenation of organic hydrides. Organic hydrides were fully converted into the corresponding aromatic compounds under reactive distillation conditions at 200 °C or under circulation-flow conditions using the Pt/MPPSi-Al2O3 catalyst packed in a column at 260 °C. The dehydrogenation reaction reached a maximum conversion at equilibrium (ca. 60%) under continuous-flow conditions at 260 °C. This catalytic continuous-flow dehydrogenation was applied to a formal hydrogen transfer from organic hydrides to unsaturated organic substrates under sequential and continuous-flow conditions for practical flow hydrogenation reactions by connecting two different heterogeneous catalysts packed in columns.

Promoting effect of Zn in high-loading Zn/Ni-SiO2 catalysts for selective hydrogen evolution from methylcyclohexane

The dehydrogenation of methylcyclohexane to toluene was investigated over high-loading monometallic Ni-SiO₂ and bimetallic Zn/Ni-SiO₂ catalysts. The catalysts were prepared by the impregnation coupled with the advantageous heterophase sol-gel technique. Their performance was tested in a fixed-bed flow reactor at 250-350 °C, 0.1 MPa pressure, equimolar ratio H₂/Ar (24 nL h^-1 in total), and a methylcyclohexane feed rate of 12 mL h^-1. Information regarding the structure of Ni-Zn catalysts was obtained by N₂ and CO adsorption, temperature-programmed reduction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, in situ X-ray diffraction, and in situ X-ray absorption spectroscopy. The results have shown that the addition of zinc leads to the hindrance of Ni reducibility along with weakening the Ni interaction with the silica matrix. This behavior particularly indicated the formation of solid oxide nickel-zinc solutions. The catalytic properties of Zn-modified catalysts in the dehydrogenation of methylcyclohexane appeared significantly superior to their Ni-Cu counterparts. For example, the selectivity of Zn/Ni-SiO₂ catalysts toward toluene formation increased markedly with a decrease in the Ni : Zn mass ratio, achieving 97% at 350 °C over the sample with Ni : Zn = 80 : 20. This is attributed to the promoting geometric and electronic effects arising from the formation of bimetallic Ni-Zn solid solutions. Moreover, a deeper reduction of zinc and a more efficient formation of solid bimetallic solutions are observed after the catalytic tests.

Fabrication of Pt-Loaded Catalysts Supported on the Functionalized Pyrolytic Activated Carbon Derived from Waste Tires for the High Performance Dehydrogenation of Methylcyclohexane and Hydrogen Production

The pyrolytic activated carbon derived from waste tires (PTC) was functionalized to fabricate the high performance of Pt-based catalysts in the dehydrogenation of methylcyclohexane and hydrogen production. Structural characterizations evidenced that the modification partially influenced the surface area, the pore structure, and the oxygen-containing functional groups of the supports. The techniques of CO pulse, transmission electron microscopy, and hydrogen temperature-programmed reduction were utilized to investigate the dispersion degrees and particle sizes of the active component Pt, and its interaction with the various functionalized supports, respectively. The results manifested that Pt particles loaded on the functionalized PTC-S had the largest dispersion degree and the smallest size among those loaded on PTC and other functionalized PTC (i.e., PTC-K and PTC-NH). Finally, the Pt-based catalysts were successfully applied in the dehydrogenation reaction of methylcyclohexane to yield hydrogen. The results revealed that the Pt catalyst over the functional PTC-S support exhibited a more excellent conversion of methylcyclohexane (84.3%) and a higher hydrogen evolution rate (991.5 mmol/gPt/min) than the other resulting Pt-based catalysts.

Dehydrogenation of methylcyclohexane over Pt-based catalysts supported on functional granular activated carbon

Herein, we developed the dehydrogenation of methylcyclohexane over Pt-based catalysts supported on functional granular activated carbon. Sulphuric acid, hydrogen peroxide, nitric acid and aminopropyl triethoxy silane were adopted to modify the granular activated carbon. The structural characterizations suggested that the carbon materials had a large surface area, abundant pore structure, and a high number of oxygen-containing functional groups, which influenced the Pt-based catalysts on the particle size, dispersion and dehydrogenation activity. The hydrogen temperature-programmed reduction technique was utilized to investigate the interaction between the active component Pt and the various functionalized granular activated carbon materials. The CO pulse technique revealed the particle sizes and dispersion of the as-prepared Pt-based catalysts. Finally, the Pt-based catalysts were successfully applied to study their catalytic activity in the dehydrogenation reaction of methylcyclohexane. The results showed that the Pt-based catalyst over granular activated carbon functionalized with sulphuric acid groups had a higher conversion of methylcyclohexane (63%) and a larger hydrogen evolution rate (741.1 mmol g_Pt ^-1 min^-1) than the other resulting Pt-based catalysts at 300 °C.

Recent Trends on the Dehydrogenation Catalysis of Liquid Organic Hydrogen Carrier (LOHC): A Review

Considering the expansion of the use of renewable energy in the future, the technology to store and transport hydrogen will be important. Hydrogen is gaseous at an ambient condition, diffuses easily, and its energy density is low. So liquid organic hydrogen carriers (LOHCs) have been proposed as a way to store hydrogen in high density. LOHC can store, transport, and use hydrogen at high density by hydrogenation and dehydrogenation cycles. In this review, we will focus on typical LOHCs, methylcyclohexane (MCH), 18H-dibenzyltoluene (DBT), and 12H-N-ethylcarbazole (NECZ), and summarize recent developments in dehydrogenation catalytic processes, which are key in this cycle.

Hydrogenation of Toluene to Methyl Cyclohexane over PtRh Bimetallic Nanoparticle-Encaged Hollow Mesoporous Silica Catalytic Nanoreactors

PtRh bimetallic nanoparticle (NP)-encaged hollow mesoporous silica nanoreactors (PtRh@HMSNs) are prepared by employing metal-ion-containing charge-driven polymer micelles as templates. These nanoreactors feature ∼1-2 nm PtRh NPs in ∼11 nm hollow cavities of HMSNs. Among various Pt x Rh y @HMSNs, Pt0.77Rh1@HMSNs show the best catalytic performance for toluene hydrogenation. Under 30 °C, atmospheric H2 pressure, and a toluene/(Pt+Rh) molar ratio of 200/1, Pt0.77Rh1@HMSNs reach 100.0% of methyl cyclohexane yield and demonstrate a much better catalytic performance than monometallic Pt@HMSNs and Rh@HMSNs and their physical mixtures. Moreover, Pt0.77Rh1@HMSNs exhibit a good catalytic stability during recycling experiments. The enhanced performance of Pt0.77Rh1@HMSNs is ascribed to the interaction between Pt and Rh, the beneficial effect of the relatively large mesoporous channels for mass transfer, as well as the confinement effect of functional NPs inside hollow cavities.

Recent developments of nanocatalyzed liquid-phase hydrogen generation

Hydrogen is the most effective and sustainable carrier of clean energy, and liquid-phase hydrogen storage materials with high hydrogen content, reversibility and good dehydrogenation kinetics are promising in view of "hydrogen economy". Efficient, low-cost, safe and selective hydrogen generation from chemical storage materials remains challenging, however. In this Review article, an overview of the recent achievements is provided, addressing the topic of nanocatalysis of hydrogen production from liquid-phase hydrogen storage materials including metal-boron hydrides, borane-nitrogen compounds, and liquid organic hydrides. The state-of-the-art catalysts range from high-performance nanocatalysts based on noble and non-noble metal nanoparticles (NPs) to emerging single-atom catalysts. Key aspects that are discussed include insights into the dehydrogenation mechanisms, regenerations from the spent liquid chemical hydrides, and tandem reactions using the in situ generated hydrogen. Finally, challenges, perspectives, and research directions for this area are envisaged.

Catalytic dehydrogenation of liquid organic hydrogen carrier dodecahydro-N-ethylcarbazole over palladium catalysts supported on different supports

The system based on liquid organic hydrogen carrier (LOHC) is one of the technologies to solve the problem of hydrogen storage and transportation capacity in large-scale applications. In this paper, the catalytic dehydrogenation of LOHC dodecahydro-N-ethylcarbazole (H12-NEC) over supported Pd nanoparticles (NPs) catalyst on four kinds of different supports, such as Pd/C, Pd/Al₂O₃, Pd/TiO₂, and Pd/SiO₂, was studied. It was found from catalyst characterization and dehydrogenation reaction that the volcano-type dependence of the activity on the Pd particle size, catalytic activity improvement with large specific surface area, and high Pd reduction degree indicated that the structure, particle size, and reduction degree of Pd NPs and textural properties of supports had a synergistic effect on the catalytic performance. Among all the catalysts, Pd/C displayed outstanding catalytic performance with the H12-NEC conversion of 99.9% and hydrogen storage capacity of 5.69 wt% at 180 °C after 12 h. The particle size of Pd/C distributes in the range of 1.5-6.0 nm with an average size of 3.0 nm. The results of dehydrogenation reaction kinetics showed that the rate limiting step and rate constant for different catalysts were mainly related to the physicochemical properties and adsorption and activation abilities towards the reactants and intermediates. In terms of the stationarity of dehydrogenation process, Pd/Al₂O₃ was excellent, indicating that it was best for dehydrogenation of H12-NEC.

Low-temperature selective catalytic dehydrogenation of methylcyclohexane by surface protonics

The methylcyclohexane (MCH)–toluene cycle is a promising liquid organic hydride system as a hydrogen carrier. Generally, MCH dehydrogenation has been conducted over Pt-supported catalysts, for which it requires temperatures higher than 623 K because of its endothermic nature. For this study, an electric field was applied to Pt/TiO₂ catalyst to promote MCH dehydrogenation at low temperatures. Selective dehydrogenation was achieved with the electric field application exceeding thermodynamic equilibrium, even at 423 K. With the electric field, “inverse” kinetic isotope effect (KIE) was observed by accelerated proton collision with MCH on the Pt/TiO₂ catalyst. Moreover, Pt/TiO₂ catalyst showed no methane by-production and less coke formation during MCH dehydrogenation. DRIFTS and XPS measurements revealed that electron donation from TiO₂ to Pt weakened the interaction between catalyst surface and π-coordination of toluene. Results show that the electric field facilitated MCH dehydrogenation without methane and coke by-production over Pt/TiO₂ catalyst.

Electric field facilitated MCH dehydrogenation at 423 K without methane and coke by-production over Pt/TiO₂ catalyst by surface protonics.

Irreversible catalytic methylcyclohexane dehydrogenation by surface protonics at low temperature

Surface protonics by applying electric field promotes low temperature methylcyclohexane dehydrogenation for effective hydrogen production.