A new energy efficient process for hydrogen purification using ZIF-8/glycol–water slurry: Experimental study and process modeling

Effective recovery of hydrogen from refinery off-gas and coke oven gas, in which hydrogen and methane are key components, is increasingly important for the development of hydrogen energy. In this paper, we introduced a new energy efficient process for hydrogen purification from CH₄/H₂ mixture. Firstly, we conducted a phase equilibrium experimental study for CH₄/H₂ in zeolitic imidazolate framework-8 (ZIF-8)/glycol–water slurry. The simple absorption and desorption configuration is adopted for the continuous separation of CH₄/H₂ using ZIF-8/glycol–water slurry. The multi-stage pseudo-absorption modeling approach was introduced for the modeling and simulation of the absorption–adsorption and desorption columns via using multiple flash modules in Aspen Plus. The binary interaction parameters in the thermodynamic model were fitted by experimental data within an acceptable error (4.93%). The operating conditions (i.e., the number of theoretical stages, feed stage, flash pressure, and desorption pressure) were determined to increase H₂ concentration in product and H₂ recovery ratio. The energy performance of the process was also evaluated. Given the feed gas contains 50 mol% H₂, the gas–slurry volume ratio of 43.27 is required to produce 95 mol% H₂ with a high recovery of 97.94%. The total energy consumption per unit volume of product is 0.06254 kW·h/Nm³. Results indicate that the hybrid absorption-adsorption process is a promising energy efficient technique to separate CH₄/H₂ in the future.     

Facile synthesis of hybrid porous composites and its porous carbon for enhanced H2 and CH4 storage

The anticipated energy crisis due to the extensive use of limited stock fossil fuels forces the scientific society for find prompt solution for commercialization of hydrogen as a clean source of energy. Hence, convenient and efficient solid-state hydrogen storage adsorbents are required. Additionally, the safe commercialization of huge reservoir natural gas (CH₄) as an on-board vehicle fuel and alternative to gasoline due to its environmentally friendly combustion is also a vital issue. To this end, in this study we report facile synthesis of polymer-based composites for H₂ and CH₄ uptake. The cross-linked polymer and its porous composites with activated carbon were developed through in-situ synthesis method. The mass loadings of activated carbon were varied from 7 to 20 wt%. The developed hybrid porous composites achieved high specific surface area (SSA) of 1420 m²/g and total pore volume (TPV) of 0.932 cm³/g as compared to 695 m²/g and 0.857 cm³/g for pristine porous polymer. Additionally, the porous composite was activated converted to a highly porous carbon material achieving SSA and TPV of 2679 m²/g and 1.335 cm³/g, respectively. The H₂ adsorption for all developed porous materials was studied at 77 and 298 K and 20 bar achieving excess uptake of 4.4 wt% and 0.17 wt% respectively, which is comparable to the highest reported value for porous carbon. Furthermore, the developed porous materials achieved CH₄ uptake of 8.15 mmol/g at 298 K and 20 bar which is also among the top reported values for porous carbon.     

Catenated metal-organic frameworks: Promising hydrogen purification materials and high hydrogen storage medium with further lithium doping

Based on the first-principles derived force fields and grand canonical Monte Carlo simulations, we find that the catenated metal-organic frameworks outperform the noncatenated structures, in terms of H₂ separation from other gases (CH₄, CO and CO₂) and H₂ adsorption by Li doping. A system utilizing IRMOF-11 (or IRMOF-13) for hydrogen separation and Li-doped IRMOF-9 for hydrogen storage is therefore proposed, with hydrogen uptake of 4.91 wt% and 36.6 g/L at 243 K and 100 bar for Li-doped IRMOF-9, which is close to the 2017 DOE target. It is promising to find appropriate microporous materials for hydrogen purification and storage at ambient conditions with structure catenated.     

Metal organic frameworks for hydrogen purification

High purity hydrogen is one of the key factors in determining the lifetime of proton exchange membrane (PEM) fuel cells. However, the current industrial processes for producing high purity hydrogen are not only expensive, but also come with low energy efficiencies and productivity. Finding more cost-effective methods of purifying hydrogen is essential for ensuring wider scale deployment of PEM fuel cells. Among various hydrogen purification methods, adsorption in porous materials and membrane technologies are seen as two of the most promising candidates for the current industrial hydrogen purification methods, with metal organic frameworks (MOF) being particularly popular in research over the last decade. Despite many available reviews on MOFs, most focus on synthesis and production, with few reports focused on performance for hydrogen purification. This review describes the working principle and performance parameters of adsorptive separations and membrane materials and identifies MOFs that have been reported for hydrogen purification. The MOFs are summarised and their performance in separating hydrogen from common impurities (CO₂, N₂, CH₄, CO) is compared systematically. The challenges of commercial application of MOFs for hydrogen purification are discussed.     

Ruthenium supported on MIL-96: An efficient catalyst for hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

For the first time, ultrafine Ru nanoparticles with mean diameter of 2 nm are successfully deposited on MIL-96 by using a simple liquid impregnation strategy, and tested for catalytic hydrolysis of ammonia borane. The powder X-ray diffraction, N₂ physical adsorption, transmission electron microscopy, energy-dispersive X-ray spectroscopy and inductively coupled plasma-atomic emission spectroscopy measurements are employed to characterized the Ru/MIL-96 catalysts. Thanks to the unique 3D structure of MIL-96, Ru NPs supported on MIL-96 exhibit much enhanced catalytic activity compared with other commercial supported materials and graphene, with the TOF value of 231 mol H₂ min⁻¹ (mol Ru)⁻¹, which is among the highest value ever reported. Moreover, this simple method can be extended to facile synthesis of other MOFs supported monometallic and polymetallic NPs for more application.     

Dual application of Ti-catalyzed Li-RHC composite for H2 purification and CO methanation

2LiH + MgB₂ composite doped with TiO₂ (Li-RHC-Ti) is employed with a two-fold purpose: hydrogen purification under a H₂–CO (0.1 mol%) mixture and CO methanation. Upon dynamic cycling under CO–H₂ mixture, hydrogen release curves display a quite stable amount of pure hydrogen of about 10 wt%, short release times of around 60 min, and minor degradation. Gas analysis by Fourier transform infrared spectroscopy (FTIR) after a thermal dehydrogenation process of MgH₂ and LiBH₄ under CO evidence the conversion of CO to CH₄. Li-RHC-Ti dehydrogenated under CO shows the simultaneous formation of CH₄, CH₃OH, and B(CH₃)₃ in the gas phase. X-ray powder diffraction (XRPD) and FTIR characterizations of the solid phases of Li-RHC-Ti after both H₂–CO mixture and CO interactions demonstrate the formation of MgO, LiBO_2, and HCOO⁻ species. Li-RHC-Ti acts as a hydrogen source and promoter for the CO conversion. Reaction pathways are proposed based on experimental results and equilibrium composition calculations.     

Development of hydrogen-selective CAU-1 MOF membranes for hydrogen purification by ‘dual-metal-source’ approach

Hydrogen provides reliable, sustainable, environmental and climatic friendly energy to meet world's energy requirement and it also has high energy density. Hydrogen is relevant to all of the energy sectors-transportation, buildings, utilities and industry. In all of these sectors, hydrogen-rich gas streams are needed. Thus, hydrogen-selective membrane technology with superior performances is highly demanded for separation and purification of hydrogen gas mixtures. In this study, novel [Al₄(OH)₂(OCH₃)₄(H₂N-BDC)₃]·xH₂O (CAU-1) MOF membranes with accessible pore size of 0.38 nm are evaluated for this goal of hydrogen purification. High-quality CAU-1 membranes have been successfully synthesized on α-Al₂O₃ hollow ceramic fibers (HCFs) by secondary growth assisted with the homogenously deposited CAU-1 nanocrystals with a size of 500 nm as seeds. The energy-dispersive X-ray spectroscopy study shows that the HCFs substrates play dual roles in the membrane preparation, namely aluminum source and as a support. The crystals in the membrane are intergrown together to form a continuous and crack-free layer with a thickness of 4 μm. The gas sorption ability of CAU-1 MOF materials is examined by gas adsorption measurement. The isosteric heats of adsorption with average values of 4.52 kJ/mol, 12.90 kJ/mol, 12.82 kJ/mol and 27.99 kJ/mol are observed for H₂, N₂, CH₄, and CO₂ respectively, indicating different interactions between CAU-1 framework and these gases. As-prepared HCF supported CAU-1 membranes are tested by single and binary gas permeation of H₂/CO₂, H₂/N₂ and H₂/CH₄ at different temperatures, feed pressures and testing time. The permeation results show preferential permeance of H₂ over CO₂, N₂, and CH₄ with high separation factors of 12.34, 10.33, and 10.42 for H₂/CO₂, H₂/N₂, H₂/CH₄, respectively. The temperature, pressure and test time dependent studies reveal that HCFs supported CAU-1 membranes possess high stability, resistance to cracking, temperature cycling, high reproducibility, these of which combined with high separation efficiency make this type of MOF membranes are promising for hydrogen recycling from industrial exhausts.     

Porous g-C3N4/TiO2 S-scheme heterojunction photocatalyst for visible-light driven H2-production and simultaneous wastewater purification

In this work, photocatalysts with a novel S-scheme heterojunction were fabricated by coupling MOF-derived TiO₂ with porous g-C₃N₄ (MTO/PCN). The S-scheme heterojunction with matching band gap possesses different advantageous properties, which can not only inhibit photo-generated charge recombination, but also reserve outstanding redox ability. As expected, superior hydrogen evolution efficiency of 40-MTO/PCN was obtained in a TEOA-containing aqueous solution and pure water, which were 5252.9 and 974.6 μmol h⁻¹ g⁻¹, respectively. Meanwhile, the hybrid can also serve as a bifunctional catalyst for H₂ generation (2137.3 μmol h⁻¹ g⁻¹) and organic contaminant removal (the RhB purification efficiency: 35.24%). This work furnishes a feasible method for devising bifunctional photocatalysts that can simultaneously produce hydrogen and purify wastewater to provide both energy-saving and environmental restoration functions.     

Hydrogen bioproduction with anaerobic bacteria consortium from brewery wastewater

Biohydrogen production is a cheap and clean way to obtain hydrogen gas. In subtropical countries such as Brazil the average temperatures of 27 °C can favor the hydrogen producing bacteria growth. A mixed culture was obtained from a subtropical sludge treating brewery wastewater and anaerobic batch reactors were fed with glucose, sucrose, fructose and xylose in low concentrations (2.0, 5.0 and 10.0 g L⁻¹) at 37 °C, initial pH 5.5 and headspace with N₂ (99%) to maintain the anaerobic conditions. The inoculum was a subtropical granulated sludge from UASB (Upflow Anaerobic Sludge Blanket) reactor treating brewery wastewater. The higher H₂ yields were obtained in reactors operated with 2 and 5 g L⁻¹ of fructose and they were 1.5 mol H₂ mol⁻¹ of fructose and 1.3 mol H₂ mol⁻¹ of sucrose, respectively. The volatile fatty acids (VFA) generated at the end of operation were, predominantly, butyric and acetic acid, indicating the favoring of the metabolic route of hydrogen generation by the consortium of anaerobic bacteria from the brewery wastewater. Biomolecular analyses revealed the predominance of hydrogen producing bacteria from Firmicutes phylum distributed in the families Streptococcaceae, Veillonellaceae and uncultured bacteria. These results confirm future applications of subtropical sludges with agroindustrial wastewaters containing low concentrations of sugars on hydrogen generation.     

Statistical optimization of medium components affecting simultaneous fermentative hydrogen and ethanol production from crude glycerol by thermotolerant Klebsiella sp. TR17

A thermotolerant Klebsiella sp. TR17 for production of hydrogen from crude glycerol was investigated. Results from Plackett–Burman design indicated that the significant variables, which influenced hydrogen production, were KH₂PO₄ and NH₄Cl (for buffer capacity and nitrogen source). Subsequently, the two selected variables and crude glycerol were optimized by the Central Composite design for achieving maximum hydrogen and ethanol yield. The concentration of crude glycerol, KH₂PO₄, and NH₄Cl had an individual effect on both hydrogen and ethanol yield (p < 0.05), while KH₂PO₄ and NH₄Cl had an interactive effect on ethanol yield (p < 0.05). The optimum medium components were 11.14 g/L crude glycerol, 2.47 g/L KH₂PO₄, and 6.03 g/L NH₄Cl. The predicted maximum simultaneous hydrogen and ethanol yield were 0.27 mol H₂/mol glycerol and 0.63 mol EtOH/mol glycerol, respectively. Validation of the predicted optimal conditions exhibited similar hydrogen and ethanol yield of 0.26 mol H₂/mol glycerol and 0.58 mol EtOH/mol glycerol, respectively.     

Electrochemical impedance studies of electrooxidation of methanol and formic acid on Pt/C catalyst in acid medium

Cyclic voltammetry (CV), amperometric i − t experiments, and electrochemical impedance spectroscopy (EIS) measurements were carried out by using glassy carbon disk electrode covered with the Pt/C catalyst powder in solutions of 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ HCOOH at 25 °C, respectively. Electrochemical measurements show that the activity of Pt/C for formic acid electrooxidation is prominently higher than for methanol electrooxidation. EIS information also discloses that the electrooxidation of methanol and formic acid on the Pt/C catalyst at various polarization potentials show different impedance behaviors. The mechanisms and the rate-determining steps of formic acid electrooxidation are also changed with the increase of the potential. Simultaneously, the effects of the electrode potentials on the impedance patterns were revealed.     

Room temperature hydrogen generation from hydrolysis of ammonia–borane over an efficient NiAgPd/C catalyst

NiAgPd nanoparticles are successfully synthesized by in-situ reduction of Ni, Ag and Pd salts on the surface of carbon. Their catalytic activity was examined in ammonia borane (NH₃BH₃) hydrolysis to generate hydrogen gas. This nanomaterial exhibits a higher catalytic activity than those of monometallic and bimetallic counterparts and a stoichiometric amount of hydrogen was produced at a high generation rate. Hydrogen production rates were investigated in different concentrations of NH₃BH₃ solutions, including in the borates saturated solution, showing little influence of the concentrations on the reaction rates. The hydrogen production rate can reach 3.6–3.8 mol H₂ mol_cat⁻¹ min⁻¹ at room temperature (21 °C). The activation energy and TOF value are 38.36 kJ/mol and 93.8 mol H₂ mol_cat⁻¹ min⁻¹, respectively, comparable to those of Pt based catalysts. This nanomaterial catalyst also exhibits excellent chemical stability, and no significant morphology change was observed from TEM after the reaction. Using this catalyst for continuously hydrogen generation, the hydrogen production rate can be kept after generating 6.2 L hydrogen with over 10,000 turnovers and a TOF value of 90.3 mol H₂ mol_cat⁻¹ min⁻¹.     

Enhancement of hydrogen adsorption by alkali-metal cation doping of metal-organic framework-5

Over the past decade, metal-organic frameworks (MOFs) have been extensively studied as a novel approach to store hydrogen. The large surface area and volume of micropores that are intrinsic to MOFs make them ideal for gas adsorption. In addition, we chemically reduced MOF-5 by doping it with alkali metals (Li, Na, and K). We found that the H₂ uptake capacity of MOF-5 materials doped with Li, Na, and K exceeded that of a neutral framework by 24%, 68%, and 70%, respectively. Notably, at the same levels of doping, the Li⁺-doped framework exhibited the strongest H₂ binding, and the binding strength decreased sequentially in the order Li⁺ > Na⁺ > K⁺.     

Direct fermentation of Laminaria japonica for biohydrogen production by anaerobic mixed cultures

A few studies have been made on fermentative hydrogen production from marine algae, despite of their advantages compared with other biomass substrates. In this study, fermentative hydrogen production from Laminaria japonica (one brown algae species) was investigated under mesophilic condition (35 ± 1 °C) without any pretreatment method. A feasibility test was first conducted through a series of batch cultivations, and 0.92 mol H₂/mol hexose_added, or 71.4 ml H₂/g TS of hydrogen yield was achieved at a substrate concentration of 20 g COD/L (based on carbohydrate), initial pH of 7.5, and cultivation pH of 5.5. Continuous operation for a period of 80 days was then carried out using anaerobic sequencing batch reactor (ASBR) with a hydraulic retention time (HRT) of 6 days. After operation for approximately 30 days, a stable hydrogen yield of 0.79 ± 0.03 mol H₂/mol hexose_added was obtained. To optimize bioenergy recovery from L. japonica, an up-flow anaerobic sludge blanket reactor (UASBr) was applied to treat hydrogen fermentation effluent (HFE) for methane production. A maximum methane yield of 309 ± 12 ml CH₄/g COD was achieved during the 90 days operation period, where the organic loading rate (OLR) was 3.5 g COD/L/d.     

Hydrogen and/or syngas production by combined steam and dry reforming of methane on nickel catalysts

Two alumina supported Ni catalysts with pore sizes of 5.4 nm and 9 nm were synthetized, characterized and tested in the Combined Steam and Dry Reforming of Methane (CSDRM) for the production of hydrogen rich gases or syngas. The reaction mixture was designed to simulate the composition of real clean biogas, the addition of water being made in order to have molar ratios of H₂O:CO₂ corresponding to 2.5:1, 7.5:1 and 12.5:1. Structural and functional characterization of catalysts revealed that Ni/Al₂O₃ with larger pore size shows better characteristics: higher surface area, lower Ni crystallite sizes, higher proportion of stronger catalytic sites for hydrogen adsorption, and higher capacity to adsorb CO₂. At all studied temperatures, for a CH₄:CO₂:H₂O molar ratio of 1:0.48:1.2, a (H₂+CO) mixture with H₂:CO ratio around 2.5 is obtained. For the production of hydrogen rich gases, the optimum conditions are: CH₄:CO₂:H₂O = 1:0.48:6.1 and 600 °C. No catalyst deactivation was observed after 24 h time on stream for both studied catalysts, and no carbon deposition was revealed on the used catalysts surface regardless the reaction conditions.     

Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm³) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH₄ and H₂S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH₄ potential than dairy manure on a volatile solids basis. While no synergistic effect on CH₄ production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH₄ production. Initial H₂S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H₂S concentrations (0.10–0.14%) and higher CH₄ content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH₄ yield (372 ± 12 L kg⁻¹ VS). The co-digestion mixture containing 40% radish had a lower specific CH₄ yield (345 ± 2 L kg⁻¹ VS) but also showed significantly less H₂S production at start-up and high quality biogas (58% CH₄). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH₄ production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH₄ production of manure digesters in the fall/winter.     

Morphologies dependence of hydrogen evolution over CeO2 via ultrasonic triggering

Ultrasonic field can lead to cavitation bubbles explosion, which rises a high-frequency oscillation and generates a high-frequency current in semiconductor nanoparticles in suspension. However, the effect of nanoparticle morphology on ultrasonic-triggered H₂ production is still unclear. To this end, herein, nanorods CeO₂ (nrCeO₂), CeO₂ nanocubes (ncCeO₂), and CeO₂ nanospheres (nsCeO₂) were successfully synthesized. Among them, one-dimensional nrCeO₂ had the most abundant O-vacancies. As revealed by the COMSOL simulation, nanoparticle deformation was easier in nanorods compared with nanocubes and nanospheres, resulting in more efficient charge separation and facilitating H₂ production reaction in nrCeO₂. In detail, within a 5 h’ period, nrCeO₂ presented the highest H₂ production activity of 983.1 μmol g^?1 h^?1 with the positive charge (q+) trapping agent of CH₃OH, and that of 278.1 μmol g^?1 h^?1 in pure water. This work presents a new understanding about the relationship between nanoparticle morphology and H₂ production activity, and provides a promising, efficient, and clean H₂ production approach, which can be further extended to multi-field coupling reactor.     

Photocatalytic hydrogen evolution assisted by aqueous (waste)biomass under simulated solar light: Oxidized g-C3N4 vs. P25 titanium dioxide

Oxidized graphitic carbon nitride (o-g-C₃N₄) and Evonik AEROXIDE^® P25 TiO₂ were compared for lab-scale photocatalytic H₂ evolution from aqueous sacrificial biomass-derivatives, under simulated solar light. Experiments in aqueous starch using Pt or Cu–Ni as the co-catalysts indicated that H₂ production is affected by co-catalyst type and loading, with the greatest hydrogen evolution rates (HER) up to 453 and 806 μmol g⁻¹ h⁻¹ using TiO₂ coupled with 3 wt% Cu–Ni or 0.5 wt% Pt, respectively. Despite the lower surface area, o-g-C₃N₄ gave HERs up to 168 and 593 μmol g⁻¹ h⁻¹ coupled with 3 wt% Cu–Ni or 3 wt% Pt. From mono- and di-saccharide solutions, H₂ evolution was in the range 504–1170 μmol g⁻¹ h⁻¹ for Pt/TiO₂ and 339–912 μmol g⁻¹ h⁻¹ for Cu–Ni/TiO₂, respectively; o-g-C₃N₄ was efficient as well, providing HERs of 90–610 μmol g⁻¹ h⁻¹. The semiconductors were tested in sugar-rich wastewaters obtaining HERs up to 286 μmol g⁻¹ h⁻¹. Although HERs were lower compared to Pt/TiO₂, a cheap, eco-friendly and non-nanometric catalyst such as o-g-C₃N₄, coupled to non-noble metals, provided a more sustainable H₂ evolution.     

Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells

Hydrogen represents a promising clean fuel for future applications. The biocathode of a two-chambered microbial electrolysis cell (biotic MEC) was studied and compared with an abiotic cathode (abiotic MEC) in order to assess the influence of naturally selected microorganisms for hydrogen production in a wide range of cathode potentials (from −400 to −1800 mV vs SHE). Hydrogen production in both MECs increased when cathode potential was decreased. Microorganisms present in the biotic MEC were identified as Hoeflea sp. and Aquiflexum sp. Supplied energy was utilized more efficiently in the biotic MEC than in the abiotic, obtaining higher hydrogen production respect to energy consumption. At −1000 mV biotic MEC produced 0.89 ± 0.10 m³ H₂ d⁻¹ m⁻³NCC (Net Cathodic Compartment) at a minimum operational cost of 3.2 USD kg⁻¹ H₂. This cost is lower than the estimated market value for hydrogen (6 USD kg⁻¹ H₂).     

Fabrication of graphene/CaIn2O4 composites with enhanced photocatalytic activity from water under visible light irradiation

A series of graphene/CaIn₂O₄ composites were synthesized using a facile solvothermal method to improve the photocatalytic performance of CaIn₂O₄. The reduction of graphene oxide to graphene and the deposition of CaIn₂O₄ nanoparticles on the graphene sheets can be achieved simultaneously during the solvothermal process. The photocatalytic activities of as-prepared graphene/CaIn₂O₄ composites for hydrogen evolution from CH₃OH/H₂O solution were investigated under visible light irradiation. It was found that graphene exhibited an obvious influence on the photocatalytic activity of CaIn₂O₄. The graphene/CaIn₂O₄ composite reached a high H₂ evolution rate of 62.5 μmol h⁻¹ from CH₃OH/H₂O solution when the content of graphene was 1 wt%. Furthermore, the 1 wt% graphene/CaIn₂O₄ composite did not show deactivation for H₂ evolution for longer than 32 h. This work could provide a new insight into the fabrication of visible light driven photocatalysts with efficient and stable performance.