(631g) Shape-Selective Adsorption Separation of C4 Olefins in Gallate-Based Metal-Organic Frameworks

C₄
olefins including 1,3-butadiene (C₄H₆), 1-butene (n-C₄H₈),
iso-butene (iso-C₄H₈), are widely used in the
production of synthetic rubbers and varieties of chemicals. C₄
olefins are generally obtained from C₄ hydrocarbon mixtures produced
in petrochemical plants. High-purity individual C₄ olefin is critical
for its downstream processes. However, these C₄ hydrocarbons have
similar molecular sizes, shapes and physical properties, such as close boiling
points and polarizability, making it difficult to separate. Currently,
industrial separation of C₄ hydrocarbons is performed by extractive
distillations, which are energy-intensive and cost-inefficient. A promising
energy-efficient purification method is physisorption with porous materials
that can separate gas mixtures by distinguishing the differences in molecular
sizes, shapes, polarities, polarizabilities, coordination abilities, and so forth.
Metal-organic frameworks (MOFs), emerging as a new class of porous materials, have
showed great potential in adsorptive separation of C₄ hydrocarbons.

Herein we report
gallate-based metal-organic frameworks, M-gallate (M=Mg, Co, Ni), realized
highly efficient separation of C₄ olefins into individual components
by shape-size sieving (Scheme 1). Single-component adsorption isotherms of C₄
olefins on Mg-gallate revealed that the uptake of C₄H₆ can
reach as high as 1.96 mmol/g (higher than DD3R of 0.83 mmol/g at 303 K and 1
bar), while the uptakes of n-C₄H₈ and iso-C₄H₈
were as low as 1.46 mmol/g and 0.13 mmol/g, respectively (Figure 1), affording high
selectivities of C₄H₆/n-C₄H₈,
C₄H₆/iso-C₄H₈ and n-
C₄H₈/iso-C₄H₈. DFT modelling
was performed to
gain a better insigh into the adsorption mechanism (Figure 2). Fixed bed
breakthrough experiments using a ternary C₄
mixture was conducted to simulate the practical industrial separation processes
at 298 K and 1.01 bar on Mg-gallate (Figure 3). iso-C₄H₈ broke
through the bed of Mg-gallate almost immediately after feeding while n-C₄H₈ and C₄H₆ began to
breakthrough at approximately 20 min and 38 min. This work demonstrated that M-gallate
was one of top-performing MOF materials for adsorption separation of C₄H₆, n-C₄H₈ and iso-C₄H₈.

Scheme 1. Schematic
illustration of the adsorption separation of C₄ olefins on M-gallate
(M=Mg, Co, Ni).

Figure 1. Adsorption
isotherms of C₄H₆(red), n-C₄H₈(blue),
iso-C₄H₈(dark green) on Mg-gallate at 298K.

Figure 2. The
adsorption binding sites of C₄H₆ in Mg-gallate calculated
by DFT method.

Figure 3. Ternary
mixture breakthrough curves on Mg-gallate for C₄H₆/n-C₄H₈/iso-C₄H₈/He
(20/10/10/60) at 298 K, with a flow rate of 0.5 mL/min.

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