Due
to large energy requirements of the traditional gas separation
processes, novel and less energy-intensive technologies, such as adsorption-
and membrane-based ones, are anticipated to play major role in future
industrial separations. Thus, finding new means for economical fabrication
of materials related to these processes is of significant importance
to facilitate their implementation in large-scale operations. In this
work, we synthesized high-quality activated porous carbons (AC) and
carbon nanotube (CNT) membranes using asphaltene, an abundant waste
of the petroleum industry. The resulting materials were tested for
CO2 separation in adsorption and membrane modes. Among
the various porous carbons produced, AC from raw asphaltene reached
a CO2 sorption capacity of 7.56 mmol/g at 4 bar and 25
°C with a relatively low heat of adsorption (up to 23 kJ/mol)
implying low energy requirement for regeneration. The versatility
of the asphaltene precursors in the formation of carbon nanomaterials
was also demonstrated by growing, for the first time, CNT membranes
via template-based, catalyst-free carbonization of asphaltene inside
the pores of anodized alumina. The resulting CNT membranes attained
a promising separation performance with permeability ratios exceeding
the respective Knudsen values for H2/CO2, N2/CO2, N2/CH4, and H2/CH4 gas pairs.
Membranes
consisting of ultrathin, oriented, single-wall carbon
nanotube (SWCNT) micropores with a diameter of ∼4 Å were
developed. c-Oriented AFI-type aluminophosphate (AlPO)
films (AlPO4-5 and CoAPO-5), consisting of parallel channels
7.3 Å in diameter, were first fabricated by seeded growth on
macroporous alumina supports, and used as templates for synthesis
of CNTs inside the zeolitic channels by thermal treatment, utilizing
the structure directing agent (amine) occluded in the channels as
carbon source. Incorporation of CNTs inside the AFI channels altered
the transport mechanism of all permeating gases tested, and imposed
a substantial increase in their permeation rates, in comparison to
the AlPO4-5 membrane, despite the pore size reduction due
to nanotube growth. The enhancement of the permeation rates is attributed
to repulsive potentials between gas molecules and occluded nanotubes,
which limit adsorption strength and enhance diffusivity, coupled to
the smooth SWCNT surface that enables fast diffusion through the nanotube
interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking
medium on both AlPO and CNT/AlPO membranes and was preserved after
CNT growth.
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