Molecular sieves for gas separation Lin, Jerry Y. S.
Science (American Association for the Advancement of Science),
07/2016, Letnik:
353, Številka:
6295
Journal Article
Recenzirano
Metal-organic framework materials enable efficient separation of similar gases
Separation and purification are critical industrial processes for separating components of chemical mixtures, and these ...processes account for about half of industrial energy usage (
1
). Gas mixtures of compounds with very similar physical properties are particularly difficult to separate. On pages
137
and
141
of this issue, Cadiau
et al.
(
2
) and Cui
et al.
(
3
), respectively, show that microporous materials can be designed to have high adsorption capacity and selectivity for particular hydrocarbons, enabling energy-efficient separation.
The ability to tailor the pore structure of metal–organic framework (MOF) membranes enables synthesis of new or modified MOF membranes with enhanced separation characteristics. This work employs a ...modified version of solvent-assisted ligand exchange, termed membrane surface ligand exchange (MSLE), to modify the pore structure of zeolitic imidazolate framework-8 (ZIF-8) membranes. This paper is the first to perform a time-based, ex situ characterization and gas permeation study of ZIF-8 MSLE with 5,6-DBIM (DBIM, dimethylbenzimidazole) to effectively narrow the ZIF-8 pores, enhance light hydrocarbon gas-phase separations, and give insight into the exchange mechanism with respect to time and temperature. The results show that relatively fast exchange kinetics occur mainly at the outer surface of the ZIF-8 membrane during the initial 30 min of exchange and enables significant (40–70%) increases in propylene/propane selectivity with minimal (10–20%) propylene permeance losses for the modified ZIF-8 membranes. We postulate as the reaction time proceeds, the ligand-exchange rate slows as the DBIM linker diffuses into the ZIF-8 membrane beyond the external surface, exchanges with the original linker, disrupts the original framework’s crystallinity, and then increases long-range order/crystallinity as the reaction proceeds. The H2/C2 separation factor increases with increased 5,6-DBIM content in the ZIF-8 framework which is facilitated by increased MSLE time and reaction temperature.
ZIF‐8 membranes have emerged as the most promising candidate for propylene/propane (C3H6/C3H8) separation through its precise molecular sieving characteristics. The poor reproducibility and ...durability, and high cost, thus far hinder the scalable synthesis and industrial application of ZIF‐8 membranes. Herein, we report a semi‐solid process featuring ultrafast and high‐yield synthesis, and outstanding scalability for reproducible fabrication of ZIF‐8 membranes. The membranes show excellent C3H6/C3H8 separation performance in a wide temperature and pressure range, and remarkable stability over 6 months. The ZIF‐8 membrane features dimethylacetamide entrapped ZIF‐8 crystals retaining the same diffusion characteristics but offering enhanced adsorptive selectivity for C3H6/C3H8. The ZIF‐8 membrane was prepared on a commercial flat‐sheet ceramic substrate. A prototypical plate‐and‐frame membrane module with an effective membrane area of about 300 cm2 was used for efficient C3H6/C3H8 separation.
A semi‐solid process, with ultrafast, high‐yield synthesis and outstanding scalability, is developed to fabricate ZIF‐8 membranes on commercial flat‐sheet ceramic substrates. The membrane is highly selective and durable for C3H6/C3H8 separation. A prototypical plate‐and‐frame membrane module with effective membrane area of about 300 cm2 has been implemented for efficient C3H6/C3H8 separation.
•Humidity independent H2S detection is achieved with adsorbent-free CuO sensor.•Resistive Cu2S forms on CuO in wet condition, offsetting the drop of adsorbed oxygen.•Monolayer film structure is a key ...factor for achieving humidity independent response.•The sensor is sensitive and selective to H2S, and stable in dry and wet conditions.
Detection of ppm and sub-ppm level airborne hydrogen sulfide (H2S) is critical to both environmental monitoring and medical diagnosis. However, conductometric gas sensors based on semiconducting metal oxides are usually susceptible to humidity interference, which limits their performance in those applications. Typically, the hydroxyl groups formed by the chemisorption of water vapor on the surfaces of metal oxides prevent any further redox reaction, and in turn, passivate the sensing material. Departing from the traditional strategy of alleviating humidity interference with moisture adsorbents, we propose to explore the possible surface reactions between H2S and hydroxyl groups on the surface of copper (II) oxide (CuO) in this work. With a monolayer film of CuO nanosheets, we observe an unprecedented humidity independent H2S sensing performance. In addition, the sensor also shows excellent sensitivity and selectivity to H2S and high stability in dry and wet conditions. The mechanism underlying the stable sensing response regardless of humidity variations is investigated with X-ray photoelectron spectroscopy.
•HTL was conducted on Galdieria sulphuraria microalgae for 30 min at 300 °C and 9 MPa.•HTL biochar pyrolysis was investigated using TGA experiments and fixed bed reactor configuration.•Pyrolysis ...experiments were carried out in a batch membrane reactor equipped with Pd77Ag23 hydrogen-selective membrane.•Two thirds of the produced hydrogen is recovered in the permeate stream of Pd77Ag23 membrane.•The retentate stream shows reduced CO and CO2 as well as increased CH4 content.
Algal biomass has recently emerged as a sustainable feedstock that can be converted to liquid fuels and other energy products. Hydrothermal liquefaction (HTL) is considered one of the most efficient thermochemical conversion techniques to produce high-quality biocrude oil that can be upgraded into a variety of liquid fuels. However, failure to identify practical uses for the HTL residual solids (biochar) could make the process economically less attractive especially with low lipid algae. This work investigates the conversion of HTL biochar of microalgae Galdieria sulphuraria to hydrogen under pyrolysis conditions in a membrane reactor capable of selectively separating hydrogen from the reaction zone. HTL biochar pyrolysis was first investigated using thermogravimetric analysis experiments and fixed bed reactor configuration. Batch membrane reactor pyrolysis experiments were then carried out using Pd77Ag23 hydrogen-selective membrane. The involvement of a Pd77Ag23 membrane in the reactor during the pyrolysis of biochar results in the recovery of hydrogen in the permeate stream (~2 times the hydrogen remaining in the retentate) and further facilitates the conversion of biochar to gaseous fuels. The retentate stream shows reduced CO and CO2 as well as increased CH4 content compared to pyrolysis conditions with no membrane.
Short porous ceramic tubes in small quantities are needed for membrane-related research and development efforts and can be synthesized by centrifugal casting (CC) or cold isostatic pressing (CIP) ...methods. This work evaluates the characteristics of porous ceramic tubes prepared by both methods and provides a strategy for selecting the proper method for ceramic tube fabrication, depending on its application. Samarium-doped ceria (SDC) with an average crystallite size of 10 nm was synthesized by a citrate method and used to make porous SDC tubes by the CC and CIP methods under different conditions. Porous SDC tubes were prepared from an aqueous suspension of the SDC powder with polyvinyl alcohol (PVA) using a cylindrical metallic mold, centrifugated at 4000–6000 rpm, followed by sintering in 1200–1500 °C. SDC tubes were also made by the CIP method from the SDC powder placed inside a cylindrical rubber bag followed by isostatic compression at 1379 bar. The pore structure of the SDC tubes was characterized by helium permeation and liquid nitrogen Archimedes method. Due to high compression pressure giving a low porosity green tube, the CIP method with lower sintering temperature produces SDC tubes with much lower porosity and average pore size and similar tortuosity compared to the CC methods. The CIP method should be used to prepare low porosity or dense ceramic tubes, and the CC method should be used for preparing high-porosity ceramic tubes or tubes with pore gradient tube wall.
Graphical abstract
Zeolitic imidazolate framework-8 (ZIF-8) membranes have shown promising potential for the separation of industrially important propylene/propane mixtures. For industrial applications, it is necessary ...to synthesize a thin and defect-free ZIF-8 membrane with good integrity on scalable supports. In this work, we report the synthesis of ZIF-8 membranes on the sol–gel derived mesoporous γ-alumina layer coated on top of the α-alumina support. Thin (2.5 μm), high quality, and stable ZIF-8 membranes can be grown on the γ-alumina support grafted with 3-glycidoxypropyltrimethoxysilane (GLYMO) by the seeded secondary growth method. Propylene/propane equimolar mixture separation experiments were conducted to test the separation performance of membranes. Compared to the ZIF-8 membranes on the bare macroporous α-alumina support, ZIF-8 membranes prepared on the γ-alumina coated α-alumina support show much better separation characteristics, with a maximum propylene/propane separation factor of 225 and propylene permeance of 9.65 × 10–9 mol m–2 s–1 Pa–1. The ZIF-8 membrane synthesized on the scalable γ-alumina coated α-alumina support exhibits separation characteristics and stability similar to the best-performing ZIF-8 membranes on alumina supports with complex synthesis procedures difficult to scale up.
Ceramic-supported Teflon AF2400 hollow fiber membrane contactors for dissolved gas-in-oil extraction has gained importance in online dissolved gas analysis (DGA) monitoring. An improved understanding ...of the mass transfer characteristics is certainly justified to enhance process efficiency and long-term stability. This work presents an investigation of the effects of operating parameters on gas transport in dissolved gas-in-oil extraction for seven key penetrants (i.e., H2, CO, CO2, CH4, C2H6, C2H4, and C2H2) encountered in DGA. During extraction, the permeate fluxes decreased exponentially with on-stream time due to membrane wetting. After stabilization, the overall mass transfer was controlled by gas transport in both the liquid phase and the membrane. Through Wilson plot analysis, it is found that gas transport in the liquid phase is the rate-limiting step. This could be attributed to extremely low gas-in-oil diffusivity and solubility. Furthermore, owing to membrane wetting, gas transport in the membrane is dominated by gas-in-oil solubility. Increasing temperature can enhance the overall mass transfer. However, gas transport performance through the membrane deteriorates with increasing temperature, attributed to the fact that the degree of membrane wetting increases with an increase in temperature. As a result, the rate-limiting step is sensitive to temperature, and the risk of oil leak increases with increasing temperature. Therefore, temperature plays a key role in determining both process efficiency and long-term stability of dissolved gas-in-oil extraction.
Highly b‐oriented, closely packed, MFI zeolite films are prepared on seeded stainless‐steel plates using organic template‐free, secondary growth solutions, containing aluminum sulfate as a ...crystallization agent. The number of a‐oriented twin crystals is significantly reduced, and even eliminated, simply by restricting the pH value of the secondary growth solution to the narrow range of 11.1–11.3. Values of pH can be adjusted through the controlled addition of (NH4)2SO4 or H2SO4 to secondary growth solutions of the composition (1 SiO2:0.57 NaOH:137.5 H2O:0.0050 (Al2(SO4)3⋅18 H2O)) or by simply decreasing the molar composition of NaOH with no extra additives.
Thin films: The effect of compositional constraints on the synthesis of highly b‐oriented MFI zeolite films on stainless‐steel plates using secondary growth solutions, free of organic template, was investigated. The number of twin crystals can be significantly reduced, and even eliminated, simply by restricting the pH value of the secondary growth solutions to the narrow range of 11.1–11.3.
Four different types of amine-attached MCM-48 silicas were prepared and investigated for CO2 separation from N2. Monomeric and polymeric hindered and unhindered amines were attached to the pore ...surface of the MCM-48 silica and characterized with respect to their CO2 sorption properties. The pore structures and amino group content in these modified silicas were investigated by XRD, FT-IR, TGA, N2 adsorption/desorption at 77 K and CHN/Si analysis, which confirmed that in all cases the amino groups were attached to the pore surface of MCM-48 at 1.5−5.2 mmol/g. The N2 adsorption/desorption analysis showed a considerable decrease of the pore volume and surface area for the MCM-48 silica containing a polymeric amine (e.g., polyethyleneimine). The CO2 adsorption rates and capacities of the amine-attached MCM-48 samples were studied employing a sorption microbalance. The results obtained indicated that in addition to the concentration of surface-attached amino groups, specific interactions between CO2 and the surface amino groups, and the resultant pore structure after amine group attachment have a significant impact on CO2 adsorption properties of these promising adsorbent materials.
