AFPM-EXAM-2-Zeolites-VS-MOFs-Gas-Seperation

What is a zeolite

crystalline aluminosilicate or silica polymorphs based on corner sharing
tetrahedra forming three-dimensional frameworks with uniform sized micropores

what is the main difference between zeolites and MOFs

zeolites: Strong Si-O covalent bonds with “rigid and uniform micropores”
MOFs: high degree of framework flexibility
(breathing or gate opening effect)

Why zeolites as membranes for CO2
separation

Molecular sieving (rigid pore size allows for “sharp” size exclusion)
Adsorption (preferential interaction of CO2 with zeolite
framework extra-framework cations due to electronic properties)
Thermal, mechanical, chemical stability

Why MOFs as membranes for CO2
separation

Chemical stability in the presence of water and some hydrocarbons
Exceptional uptake capacities for CO
Open porous framework structure with large accessible pore volumes
Pore sizes in the range of the kinetic diameter of several relevant gas molecules

Why is ZIF-8 highly attractive for CO2 separation

CH4 pore size 3.8 A, CO2 Pore size 3.3 A, N2 pore Size 3.6 A
ZIF-8 pore size: 3.4 A
Preferential CO2 adsorption because of polar walls caused by uncoordinated nitrogen

Selectivity vs permeance for CO2/CH4 separation e zeolite vs MOF membranes

Zeolite: High selectivity. Low to moderate permeances
MOF: Low selectivity. Moderate to high permeances

Selectivity vs permeance for CO2/N2 separation e zeolite vs MOF membranes

Zeolite: High selectivity. Low to moderate permeances
MOF: Low selectivity. Moderate to high permeances

What zeolite membranes would be used for CH4/CO2 separation

SAPO-34 (CHA)
Zeolite Y (FAU)

What MOF membranes would be used for CH4/CO2 separation

MMOF (0.35 nm)
BioMOF-1 (0.8 nm)
ZIF-90 (0.35 nm)
ZIF-8 (0.34 nm)

zeolite vs MOF membranes for Kr/Xe separation

ZIF-8: separation selectivities as high as 16.1, and Kr
permeances ~51 GPU.
SAPO-34 separation selectivities of 35 and
Kr permeances ~360 GPU (for molar
compositions close to air composition)

what made SAPO-34 better for Kr/Xe separation

sharp molecular sieving
thinner membranes for higher permeance
(Free energy profiles support higher selectivity for SAPO-34)

What was Kr permeance correlated with for SAPO-34

membrane thickness

How were SAPO-34 membranes thickness controlled

With different molar ratios of water

How were SAPO-34 membranes synthesized

by secondary seeded growth on porous tubular alpha-Al2O3 supports.

what are zeolite membranes advantages

Sharp molecular sieving (“rigid pore size” leading to high CO2eparation selectivity)
High chemical and thermal stability (strong Si-O covalent bonds)
Scalability demonstrated (industrially: removal of water from ethanol; pilot plant: CO2/CH4)
Well-developed synthesis approaches (in situ growth, secondary seeded growth)
Lower synthesis costs (SDA and typically water)

what are MOF membranes advantages

High porosity and surface areas (lead to moderate to high fluxes)
Wide range of pore sizes (infinite number of structures with flexible pore sizes, structures)
Unprecedented high CO2 uptakes (may result in high CO2 ideal selectivity over light gases)
Functionalization of the organic linker (pore size control, adsorption properties)

Challenges in the lab

Membrane reproducibility
Synthesis routes to prepare crystals with different sizes and morphologies
Effect of seeding techniques to improve membrane intergrowth: physical, chemical, thermal approaches.
Membrane –support interaction (charge matching or other chemical bond).
Effective control over membrane thickness
Other methods to prepare MOFs: electrochemical, EISA, polymer-modified..…
Sharp molecular sieving effect needs to be demonstrated for MOFs
Long term stability
Estimation (quantification of defects)

Challenges for practical membrane application

Reproducibility
Demonstrate scalability
Stability under real industrial-like conditions
Economically feasible