Recent Advances on Carbon Molecular Sieve Membranes (CMSMs) and Reactors

Pillared Carbon Membranes Derived from Cardo Polymers

Carbon molecular sieve membranes (CMSMs) were prepared by carbonizing the high free volume polyimide BTDA-BAF that is obtained from the reaction of benzophenone-3,3',4,4'-tetracarboxylic dianhydride (BTDA) and 9,9-bis(4-aminophenyl) fluorene (BAF). The bulky cardo groups prevented a tight packing and rotation of the chains that leads to high permeabilities of their CMSMs. The incorporation of metal-organic polyhedra 18 (MOP-18, a copper-based MOP) in the BTDA-BAF polymer before pyrolysis at 550 °C prevented the collapse of the pores and the aging of the CMSMs. It was found that upon decomposition of MOP-18, a distribution of copper nanoparticles minimized the collapse of the graphitic sheets that formed the micropores and mesopores in the CMSM. The pillared CMSMs displayed CO2 and CH4 permeabilities of 12,729 and 659 Barrer, respectively, with a CO2/CH4 selectivity of 19.3 after 3 weeks of aging. The permselectivity properties of these membranes was determined to be at the 2019 Robeson upper bound. In contrast, the CMSMs from pure BTDA-BAF aged three times faster than the CMSMs from MOP-18/BTDA-BAF and exhibited lower CO2 and CH4 permeabilities of 5337 and 573 Barrer, respectively, with a CO2/CH4 selectivity of 9.3. The non-pillared CMSMs performed below the upper bound.

Tailoring the Stabilization and Pyrolysis Processes of Carbon Molecular Sieve Membrane Derived from Polyacrylonitrile for Ethylene/Ethane Separation

For ethylene/ethane separation, a CMS (carbon molecular sieve) membrane was developed with a PAN (polyacrylonitrile) polymer precursor on an alumina support. To provide an excellent thermal property to PAN precursor prior to the pyrolysis, the stabilization as a pre-treatment process was carried out. Tuning the stabilization condition was very important to successfully preparing the CMS membrane derived from the PAN precursor. The stabilization and pyrolysis processes for the PAN precursor were finely tuned, and optimized in terms of stabilization temperature and time, as well as pyrolysis temperature, heating rate, and soaking time. The PAN stabilized at >250 °C showed improved thermal stability and carbon yield. The CMS membrane derived from stabilized PAN showed reasonable separation performance for ethylene permeance (0.71 GPU) and ethylene/ethane selectivity (7.62), respectively. Increasing the pyrolysis temperature and soaking time gave rise to an increase in the gas permeance, and a reduction in the membrane selectivity. This trend was opposite to that for the CMS membranes derived from other polymer precursors. The optimized separation performance (ethylene permeance of 2.97 GPU and ethylene/ethane selectivity of 7.25) could be achieved at the pyrolysis temperature of 650 °C with a soaking time of 1 h. The separation performance of the CMS membrane derived from the PAN precursor was comparable to that of other polymer precursors, and surpassed them regarding the upper bound trade off.

The Phase Structural Evolution and Gas Separation Performances of Cellulose Acetate/Polyimide Composite Membrane from Polymer to Carbon Stage

Blending and heat-treatment play significant roles in adjusting gas separation performances of membranes, especially for incorporating thermally labile polymers into carbon molecular sieve membranes (CMSMs). In this work, cellulose acetate (CA) is introduced into polyimide (PI) as a sacrificial phase to adjust the structure and gas separation performance from polymer to carbon. A novel result is observed that the gas permeability is reduced, even when the immiscible CA phase decomposes and forms pores after heat treatment at 350 °C. After carbonization at 600 °C, the miscible CA has changed without contribution, while the role of the immiscible CA phase has changed from original hindrance to facilitation, the composite-based CMSM at a CA content of 10 wt.% shows highest performances, a H₂ permeability of ~5300 Barrer (56% enhancement) with a similar H₂/N₂ permselectivity of 42. The structural analyses reveal that the chain interactions and phase separation behaviors between CA and PI play critical roles on membrane structures and gas diffusion, and the corresponding phase structural evolutions during heat treatment and carbonization determine gas separation properties.

Cellulose-Based Carbon Molecular Sieve Membranes for Gas Separation: A Review

In the field of gas separation and purification, membrane technologies compete with conventional purification processes on the basis of technical, economic and environmental factors. In this context, there is a growing interest in the development of carbon molecular sieve membranes (CMSM) due to their higher permeability and selectivity and higher stability in corrosive and high temperature environments. However, the industrial use of CMSM has been thus far hindered mostly by their relative instability in the presence of water vapor, present in a large number of process streams, as well as by the high cost of polymeric precursors such as polyimide. In this context, cellulosic precursors appear as very promising alternatives, especially targeting the production of CMSM for the separation of O₂/N₂ and CO₂/CH₄. For these two gas separations, cellulose-based CMSM have demonstrated performances well above the Robeson upper bound and above the performance of CMSM based on other polymeric precursors. Furthermore, cellulose is an inexpensive bio-renewable feed-stock highly abundant on Earth. This article reviews the major fabrication aspects of cellulose-based CMSM. Additionally, this article suggests a new tool to characterize the membrane performance, the Robeson Index. The Robeson Index, θ, is the ratio between the actual selectivity at the Robeson plot and the corresponding selectivity—for the same permeability—of the Robeson upper bound; the Robeson Index measures how far the actual point is from the upper bound.

Metal-Organic Framework Membranes and Membrane Reactors: Versatile Separations and Intensified Processes

Metal-organic frameworks are an emerging and fascinating category of porous solids that can be self-assembled with metal-based cations linked by organic molecules. The unique features of MOFs in porosity (or surface areas), together with their diversity for chemical components and architectures, make MOFs attractive candidates in many applications. MOF membranes represent a long-term endeavor to convert MOF crystals in the lab to potentially industry-available commodities, which, as a promising alternative to distillation, provide a bright future for energy-efficient separation technologies closely related with chemicals, the environment, and energy. The membrane reactor shows a typical intensified process strategy by combining the catalytic reaction with the membrane separation in one unit. This review highlights the recent process of MOF-based membranes and the importance of MOF-based membrane reactors in relative intensified chemical processes.

Advances in membranes and membrane reactors for the Fischer-Tropsch synthesis process for biofuel production

The biomass-to-liquid (BtL) process is a promising technology to obtain clean, liquid, second-generation biofuels and chemicals. The BtL process, which comprises several steps, is based upon the gasification of biomass and the catalytic transformation of the syngas that is obtained via the Fischer-Tropsch synthesis (FTS) reaction, producing a hydrocarbon pool known as syncrude. The FTS process is a well-established technology, and there are currently very large FTS plants operating worldwide that produce liquid fuels and hydrocarbons from natural gas (NG) (gas-to-liquids, GtL process) and coal (coal-to-liquids, CtL process). Due to the limited availability of local biomass, the size of the BtL plants should be downscaled compared to that of a GtL or CtL plant. Since the feasibility of the XtL (X refers to any energy source that can be converted to liquid, including coal, NG, biomass, municipal solid waste, etc.) processes is strongly influenced by the economies of scale, the viability of small-scale BtL plants can be compromised. An interesting approach to overcome this issue is to increase the productivity of the FTS process by developing reactors and catalysts with higher productivities to generate the desired product fraction. Recently, by integrating membrane reactors with the FTS process the gas feeding and separation unit have been demonstrated in a single reactor. In this review, the most significant achievements in the field of catalytic membrane reactors for the FTS process will be discussed. Different types of membranes and configurations of membrane reactors, including H2O separation and H2-feed distribution, among others, will be analyzed.

A Bibliometric Survey of Paraffin/Olefin Separation Using Membranes

Bibliometric studies allow to collect, organize and process information that can be used to guide the development of research and innovation and to provide basis for decision-making. Paraffin/olefin separations constitute an important industrial issue because cryogenic separation methods are frequently needed in industrial sites and are very expensive. As a consequence, the use of membrane separation processes has been extensively encouraged and has become an attractive alternative for commercial separation processes, as this may lead to reduction of production costs, equipment size, energy consumption and waste generation. For these reasons, a bibliometric survey of paraffin/olefin membrane separation processes is carried out in the present study in order to evaluate the maturity of the technology for this specific application. Although different studies have proposed the use of distinct alternatives for olefin/paraffin separations, the present work makes clear that consensus has yet to be reached among researchers and technicians regarding the specific membranes and operation conditions that will make these processes scalable for large-scale commercial applications.