Plasma-Induced Catalytic Conversion of Nitrogen and Hydrogen to Ammonia over Zeolitic Imidazolate Frameworks ZIF-8 and ZIF-67.

Microporous crystals have emerged as highly appealing catalytic materials for the plasma catalytic synthesis of ammonia. Herein, we demonstrate that zeolitic imidazolate frameworks (ZIFs) can be employed as efficient catalysts for the cold plasma ammonia synthesis using an atmospheric dielectric barrier discharge reactor. We studied two prototypical ZIFs denoted as ZIF-8 and ZIF-67, with a uniform window pore aperture of 3.4 Å. The resultant ZIFs displayed ammonia synthesis rates as high as 42.16 µmol NH3/min gcat. ZIF-8 displayed remarkable stability upon recycling. The dipole-dipole interactions between the polar ammonia molecules and the polar walls of the studied ZIFs led to relatively low ammonia uptakes, low storage capacity, and high observed ammonia synthesis rates. Both ZIFs outperform other microporous crystals including zeolites and conventional oxides in terms of ammonia production. Furthermore, we demonstrate that the addition of argon to the reactor chamber can be an effective strategy to improve the plasma environment. Specifically, the presence of argon helped to improve the plasma uniformity, making the reaction system more energy efficient by operating at a low specific energy input range allowing abundant formation of nitrogen vibrational species.

Metal-Organic Framework HKUST-1 Promotes Methane Hydrate Formation for Improved Gas Storage Capacity.

The large demand of natural gas consumption requires an effective technology to purify and store methane, the main component of natural gas. Metal-organic frameworks and gas hydrates are highly appealing materials for the efficient storage of industrially relevant gases, including methane. In this study, the methane storage capacity of the combination of methane hydrates and HKUST-1, a copper-based metal-organic framework, was studied using high pressure differential scanning calorimetry. The results show a synergistic effect, as the addition of HKUST-1 promoted hydrate growth, thus increasing the amount of water converted to hydrate from 5.9 to 87.2% and the amount of methane stored, relative to the amount of water present, from 0.55 to 8.1 mmol/g. The success of HKUST-1 as a promoter stems mainly from its large surface area, high thermal conductivity, and hydrophilicity. These distinctive properties led to a kinetically favorable decrease in hydrate growth induction period by 4.4 h upon the addition of HKUST-1. Powder X-ray diffraction and nitrogen isotherm suggests that the hydrate formation occurs primarily on the surface of HKUST-1 rather than within the pores. Remarkably, the HKUST-1 crystals show no significant changes in terms of structural integrity after many cycles of hydrate formation and dissociation, which results in the material having a long life cycle. These results confirm the beneficial role of HKUST-1 as a promoter for gas hydrate formation to increase methane gas storage capacity.

Separation of Light Gases from Xenon over Porous Organic Cage Membranes.

Herein, we demonstrate the successful synthesis and separation ability of CC3 porous organic cage membranes grown on tubular supports for light gases He, CO2, CH4, and Kr over xenon. CC3 membranes were synthesized using secondary seeded growth and displayed different separation performances depending on the crystal size, size distribution of the seeds, and membrane thickness. CC3 membranes as thin as ∼2.5 µm resulted in high single gas permeances of 2114, 1962, 1705, 773, and 162 GPU, for He, CH4, CO2, Kr, and Xe, respectively. The highest ideal selectivities for He/Xe, CH4/Xe, CO2/Xe, and Kr/Xe gas pairs were 13, 12, 10.5, and 4.8, respectively. Mechanistically, the membranes separated He, CO2, Kr, and CH4 from Xe mainly via gas diffusivity differences. Therefore, the separation was kinetically driven.

Recovery of xenon from air over ZIF-8 membranes.

Continuous ZIF-8 membranes effectively separated air/Xe gas mixtures. These membranes showed air permeances as high as 3.94 × 10-8 mol m-2 s-1 Pa-1 and separation selectivities as high as 12.4 for an air/Xe molar feed composition of 9 : 1. These membranes separated air from Xe via molecular sieving, preferential adsorption, and diffusivity differences.

SAPO-34/5A Zeolite Bead Catalysts for Furan Production from Xylose and Glucose.

SAPO-34 zeolite crystals were grown on zeolite 5A beads, characterized, and then used to produce furfural from xylose and 5-hydroxymethylfurfural (HMF) from glucose. The SAPO-34/5A bead catalysts resulted in moderate furfural and HMF yields of 45% from xylose and 20% from glucose (463 K; 3 h) and were easier to recover than the SAPO-34 powder catalyst. At 463 K, the SAPO-34/5A beads were more selective than 0.02 M sulfuric acid for producing HMF and, unlike the sulfuric acid system, no levulinic acid was formed. The SAPO-34/5A bead catalysts had no significant loss in activity after three rounds of recycle when water washed or heated overnight between reactions; however, the heat-treated beads did show signs of thermal stress after the second reuse. The SAPO-34/5A bead catalysts show promise for dehydration reactions to produce furfural and HMF from xylose and glucose, respectively, and tailoring the catalyst and the support bead could lead to even higher selectivities and yields.