(814c) Multiwalled Carbon Nanotube Mixed-Matrix Membranes Containing Amines for Acid Gas Removal From Natural Gas

Ansaloni, L., Alma Mater Studiorum - Università di Bologna
Zhao, Y., The Ohio State University
Ramasubramanian, K., The Ohio State University
Ho, W. S. W., The Ohio State University
Giacinti Baschetti, M., University of Bologna

Multiwalled Carbon Nanotube Mixed-Matrix
Membranes Containing Amines for Acid Gas Removal from Natural Gas

Luca Ansaloni1, Yanan Zhao2,
Kartik Ramasubramanian2, Marco Giacinti Baschetti1, W.S.
Winston Ho2,3


1Dipartimento di
Ingegneria Civile, Chimica, Ambientale e dei Materiali, Alma Mater Studiorum -
Università di Bologna, Bologna, Italy

2William G. Lowrie Department of
Chemical and Biomolecular Engineering, The Ohio State University, Columbus,
Ohio 43210, United States

3Department of Materials Science
and Engineering, The Ohio State University, Columbus, Ohio 43210, United States

natural gas is an important source for energy production.  Depending on the
original source, raw natural gas contains various impurities in composition,
which must be removed before delivery to the pipelines.  Removal of acid gases,
namely, CO2 and H2S, from natural gas is therefore an
important industrial gas separation process.

For CO2
removal from natural gas, polymeric membranes compete against conventional separation
processes, such as amine absorption, due to superior advantages of lower costs,
lower energy consumption, and lower waste production.1  Several
attempts have been made to develop CO2/CH4 separation membranes
that match the pipeline specifications.  Glassy polymers, in particular
polyimides, have been reported to show the best performance for this specific
separation.2,3  The higher
condensability and smaller kinetic size make CO2 molecules more
soluble and diffuse faster in the membrane matrix than methane, resulting in
good selectivity and permeability.  Recently, polyimide-based polymer with
intrinsic microporosity (PIM)4 and
cyclodextrine grafted co-polyimides5 demonstrated
very high CO2 permeability at high pressure (10 atm) but low CO2/CH4
selectivity, less than 25.  In fact, at high pressures, these membranes suffer considerably
from the plasticization problem caused by high concentration of the acid gas,
resulting in decreasing membrane selectivity significantly.  Crosslinking can
help to minimize the CO2-induced swelling; however, an increase in
separation factor of a solution-diffusion polymeric membrane always accompanies
a reduction in the penetrant flow due to the well-known trade-off relationship.6  

transport membranes hold great potential for natural gas CO2 removal.
 This type of membrane is able to achieve high CO2 permeability
accompanied with high selectivities vs. CH4, H2, or N2
due to the specific, prompt reaction between amine carriers and CO2
molecules.7,8  This work
exploited the superlative mechanical property of multi-walled carbon nanotubes
(MWNTs) to reinforce the mechanical strength of amine facilitated transport
membranes, from which we successfully synthesized nanostructured
polymer-MWNT membranes by incorporating MWNTs (about 18 nm diameter by 180 nm)
into the crosslinked polyvinylalcohol-polysiloxane network containing amine carriers.

The synthesized
membranes inherited the facilitated transport mechanism and thus achieved high
CO2 permeability and selectivity vs. CH4 at high
pressures of at least 15 atm and high temperatures of more than 100°C.  At 15
atm and 107°C using a feed gas of 50% CH4 and 50% CO2,
which simulated the natural gas composition, the membrane showed a high CO2/CH4
selectivity of about 100 accompanied with a CO2 permeability of
about 350 Barrers.  This lower permeability was due to the higher degree of
carrier saturation at the higher CO2 concentration.  It should be
noted that this permeability is still higher than that of commercial membrane,
e.g., cellulose acetate (about 5 Barrers).

In this work, we
also synthesized acid-treated MWNTs and amino-functionalized MWNTs as advanced nanofillers
for amine facilitated transport membranes.  The acid-treated MWNTs were
functionalized with hydrophilic groups including carboxylic acid and hydroxyl
groups by strong acid oxidation.  The amino-functionalized MWNTs were
chemically modified with 3-aminopropyltriethoxysilane.  The presence of these
hydrophilic groups was confirmed by FTIR spectroscopy.  The membranes
incorporated with the modified MWNTs demonstrated significant improvement in CO2/CH4
separation performance, especially in term of separation factor.  A CO2/CH4
selectivity of 180 along with CO2 permeability of around 1000 Barrers
have been obtained at 15 atm and 107°C for a feed gas composition of 59.8% CH4,
20.2% H2 and 20% CO2.  These improvements are attributed
to the modification of MWNTs.  With the hydrophilic surface groups, MWNTs were compatible
with the network of crosslinked polyvinylalcohol and polyamines.  Thereby, the
MWNTs were able to insert between polymer chains and disrupted polymer chain
packing more effectively, resulting in increased free volumes between polymer
chains.  In addition, the membrane performance at higher feed pressure up to 60
atm as well as the effects of membrane composition, temperature and membrane thickness,
has been investigated.

multiwalled carbon nanotube; mixed matrix membrane; facilitated transport;
carbon dioxide separation; natural gas.



1.     Baker, R. W. & Lokhandwala, K. Natural Gas Processing with Membranes:  An Overview. Industrial
& Engineering Chemistry Research
47, 2109?2121 (2008).

2.     Wang, L. et al. Gas
transport properties of 6FDA-TMPDA/MOCA copolyimides. European Polymer
44, 225?232 (2008).

3.     Wind, J. D., Paul, D. R.
& Koros, W. J. Natural gas permeation in polyimide membranes. Journal of
Membrane Science
228, 227?236 (2004).

4.     Ghanem, B. S., McKeown, N.
B., Budd, P. M., Selbie, J. D. & Fritsch, D. High-Performance Membranes
from Polyimides with Intrinsic Microporosity. Advanced Materials 20,
2766?2771 (2008).

5.     Askari, M., Xiao, Y., Li, P.
& Chung, T.-S. Natural gas purification and olefin/paraffin separation
using cross-linkable 6FDA-Durene/DABA co-polyimides grafted with α,
β, and γ-cyclodextrin. Journal of Membrane Science 390-391,
141?151 (2012).

6.     Robeson, L. M. The upper
bound revisited. Journal of Membrane Science 320, 390?400 (2008).

7.     Bai, H. & Ho, W. S. W.
Carbon dioxide-selective membranes for high-pressure synthesis gas
purification. Industrial & Engineering Chemistry Research 50,
12152?12161 (2011).

8.     Xing, R. & Ho, W. S. W.
Crosslinked polyvinylalcohol?polysiloxane/fumed silica mixed matrix membranes
containing amines for CO2/H2 separation. Journal of Membrane Science 367,
91?102 (2011).