Volume 1, Year 2014 - Pages 43-47

DOI: 10.11159/jffhmt.2014.007

### Characterization of Inorganic Silicalite-1 Membrane to be used for the Separation of Greenhouse Gases

David Carter, Dean Kennedy, F. Handan Tezel, Boguslaw Kruczek

University of Ottawa, Department of Chemical and Biological Engineering, 161 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5

dcart028@uottawa.ca, dkenn022@uottawa.ca, handan.tezel@uottawa.ca, bkruczek@uottawa.ca

**Abstract** - *Defect free MFI type silicalite-1 membranes have been fabricated inside tubular TiO _{2}
ceramic supports to be used for the separation of greenhouse gases. In this
work, single gas permeance experiments have been conducted. The results of
which in conjunction with adsorption isotherms for each gas investigated have
been used to calculate the effective diffusivities of N_{2}, CH_{4},
and CO_{2}. The permeances at 22 �C and 1 atm of pressure differential
for He, N_{2}, CO_{2 }and CH_{4}, were found to be 4.38
x 10^{-7}, 7.71 x 10^{-7}, 1.35 x 10^{-6}, and 1.64 x
10^{-6} mol/m^{2}sPa, respectively, giving permeance ratios for
the separation of CH_{4}/N_{2}, CO_{2}/N_{2},
and CH_{4}/CO_{2} equal to 2.13, 1.75, and 1.22 respectively.
These permeance ratios are comparable to those found in other studies reported
in the literature, whereas the permeances are typically 1 to 2 orders of
magnitude greater than those found in the literature at similar conditions*

** Keywords:** Inorganic
membranes, Silicalite, characterization, Diffusivity, Surface diffusion, Adsorption,
Carbon dioxide, Methane, Nitrogen.

© Copyright 2014 Authors - This is an Open Access article published under the Creative Commons Attribution License terms. Unrestricted use, distribution, and reproduction in any medium are permitted, provided the original work is properly cited.

Date Received: 2014-08-17

Date Accepted: 2014-10-16

Date Published: 2014-11-04

#### 1. Introduction

In
order to mitigate the effects of global warming, a variety of carbon capture
technologies are required which are economically favourable [1]. Inorganic
membranes and MFI type silicalite-1 membranes in particular can tolerate
extremely acidic conditions, and can withstand high pressures which make them
suitable for the separation of CO_{2} and CH_{4} from gaseous
process streams [2].

The
inorganic membranes used in this study are fabricated by impregnating
silicalite-1 crystals into the pores of TiO_{2} membrane supports up to
a depth of approximately 5 μm to create a thin selective layer. By
minimizing the thickness of selective layer, a membrane is produced which has a
high permeance, and is able to process large volumes. There exists a trade-off
between permeance and permselectivity, the preferential interaction of one
component with the membrane.� The trade-off can be modelled and used to design
membranes for specific processes by characterizing various inorganic membranes [3].
To this end, an understanding of the transport of gas molecules through MFI
type silicalite-1 membranes is required, which is investigated in this study.

#### 2. Theory

The
silicalite-1 crystals synthesized in this study have zeolite type structure.
That is, they contain a uniformly distributed network of pores that in this
case have a single pore size with no pore size distribution. This 3D lattice
structure results in a large internal surface area, with an internal pore size
of 5.5 Å. This pore size is less than twice the kinetic diameter of the gases
investigated in this study: 3.3 Å, 3.6 Å, and 3.8 Å for CO_{2},_{ }N_{2},
and CH_{4}, respectively, and so it is assumed in this study that
monolayer, and not multilayer adsorption is occurring, and that the main
mechanism of gas transport at room temperature is surface diffusion. By this
mechanism, gas molecules are adsorbed inside the pores of the silicalite-1
crystals, and move in the direction of decreasing surface occupancy.

For
monolayer adsorption of N_{2, }CH_{4}, and CO_{2 }into
active sites inside a single pore of silicalite-1, a randomly oriented
monolayer, similar to a single file system is generated, and in this case,
diffusion can be described by Fick's laws [4]. A version of Fick's law has
therefore been used to calculate the effective diffusivity using the following
equation for defect free membranes.

In this equation, *J* represents the
membrane flux [mol/m^{2}/s], *D _{e}*
is the effective diffusivity [m

^{2}/s], and

*A*is the constant parameter for a single membrane which takes into account the physical characteristics of the membrane including membrane thickness and tortuosity.

*q*represents the driving force, which has been calculated as the gradient of surface occupancy, or working capacity of silicalite-1 from the feed side membrane pressure to the permeate side membrane pressure. Given the dependence of

_{f}- q_{p}*q*and

_{f}*q*on temperature, volumetrically determined adsorption capacities found by Li & Tezel [5], [6] were modified using the Temperature Dependant Sips model as described by Ahmadpour et al. [7] for the estimation of the adsorption capacities at room temperature. The general equation for the Temperature Dependant Sips model is as follows:

_{p}In
this equation, *p* represents pressure [atm], *q _{s}* is the
adsorption capacity at saturation [mmol/g of adsorbent],

*q*is the adsorption capacity at a pressure

*p*[mmol/g of adsorbent], and

*b*and

*n*are constants which have units of [atm

^{-1}] and [dimensionless], respectively.

The
pressure gradient normalized flux is referred to as permeance, and has units of
[mol/m^{2}sPa].

#### 3. Experimental

3.1. Membrane Fabrication

The
interrupted pore plugging method was used to synthesise defect free MFI type
silicalite-1 membranes on TiO_{2} ceramic membrane supports as reported
by Miachon et al. [8]. Some modifications to the reported procedure were made,
and are as follows:

- Following centrifugation of the precursor solution, supernatant liquid is added to the Teflon beaker, and topped up after a period of 3 hours instead of 10 minutes in order to ensure that the membranes' supports have absorbed as much precursor solution as possible at room temperature and pressure.
- For hydrothermal synthesis, the oven was not preheated, and was instead heated up to 170 °C at a rate of 1 °C/min with the autoclave module inside the oven from the start.
- The reported standard calcination procedure was used with a heating up rate of 1 °C/min instead of 1.7 °C/min.

3.2. Single Gas Permeation Experiments

Before
commencing each single gas permeation experiment, the membrane was purged with
He at a flow rate of 100 cc(STP)/min and a feed side pressure of 2 atm for 16
hours. Each gas was then investigated in turn for successively greater feed
pressures between 1 atm and 5 atm. The gases of interest were tested in order
of increasing adsorption capacity on silicalite-1 as found by Li & Tezel [5],
[6], which is N_{2}, CH_{4}, and CO_{2}.

#### 4. Results and Discussion

4.1 Adsorption Isotherms
for N_{2, }CH_{4}, and CO_{2}

As described in the previous section, adsorption
isotherms were generated using the Temperature Dependant Sips model which is
based on Equation 2 for N_{2, }CH_{4}, and CO_{2} on
silicalite-1 at a temperature of T = 22 °C.

Figure 1 shows the expected order of increasing adsorption capacity q for the gases studied in this paper. He gas is assumed to be not adsorbed in silicalite-1. The reported isotherms from which these isotherm curves were generated are for particles and not membranes, however, the constant A in Equation 2 is assumed to account for this variation.

4.2. Single Gas Permeances
for He, N_{2, }CH_{4}, and CO_{2}

The single gas permeances for He, N_{2, }CH_{4},
and CO_{2} were found for a silicalite-1 membrane on a 0.8 μm TiO_{2}
support and are shown in Figure 2.

Defects
are confirmed to be absent from the membrane by analysing the permeance of He
and N_{2} as a function of pressure differential, which is shown in
Figure 2. In the presence of defects, the contribution of surface diffusion to
gas transport becomes negligible, and either viscous flow or Knudsen diffusion
dominates. In the case of gas transport due to viscous flow, permeance is
directly proportional to the average pore pressure, which is not the case as we
see that permeance of He and N_{2} is independent of pressure. Knudsen
diffusion however is independent of pressure, and could be considered present
if only He or N_{2} permeance is given. It is for this reason that the
permeance of both He and N_{2} have been measured. In the case where
Knudsen diffusion dominates, the effects of surface diffusion are negligible
and we would expect to see the He permeance to be greater than that of N_{2}.
This is not the case, which shows that Knudsen diffusion is not significant,
and so the membranes are considered to be defect free. The He and N_{2}
permeance has been measured at 3 pressure differentials including the extreme
pressures of interest as this number of points is sufficient to prove that the
membrane is defect free, and verify the trend in permeance for both He and N_{2}.
In the case of CH_{4} and CO_{2}, more points are needed to
verify the trend in permeance, and so more intermediate pressure differentials
were investigated.

The decreasing permeance trends shown by CO_{2}
and CH_{4} in Figure 2 can be explained in terms of the shape of their
respective isotherms shown in Figure 1. More specifically, in the range of the
experimental feed and permeate pressures, the CO_{2} and CH_{4}
isotherms are convex (in contrast to the N_{2} isotherm which is
approximately linear). Thus, the adsorption capacity gradient, which represents
the driving force for surface diffusion, is not directly proportional to the
pressure gradient. In turn, since the permeance is the pressure gradient normalized
flux, in the absence of defects in the membrane structure, permeances of CO_{2}
and CH_{4} are expected to decrease with increasing feed pressure. In
other words, the fact that experimentally observed permeances of CO_{2}
and CH_{4} decrease with feed pressure provides additional confirmation
that our membranes are defect free. Using the same argument, since the N_{2}
isotherm is approximately linear in the range of the feed and permeate
pressures, its permeance was expected to be independent of the feed pressure.

The permeances shown in Figure 2 at a pressure
differential of 1 atm for He, N_{2}, CO_{2}, and CH_{4}
are 4.38 x 10^{-7} ± 1.0 x 10^{-9}, 7.71 x 10^{-7} ±
2.8 x 10^{-9}, 1.35 x 10^{-6} ± 5.5 x 10^{-9}, and 1.64
x 10^{-6} ± 5.1 x 10^{-9} mol/m^{2}sPa respectively.
These values are greater than the single gas permeances reported in other
studies for silicalite-1 membranes at 25°C. For He, N_{2}, CO_{2},
and CH_{4}, at 25 °C, pressure unspecified, Soydaz et al. [9] found
values of 0.75 x 10^{-7}, 2.1 x 10^{-7}, 3.5 x 10^{-7},
and 4.8 x 10^{-7} mol/m^{2}sPa respectively. For N_{2}
and CH_{4}, Lovallo et al. [10] found values of 5.0 x 10^{-8}
and 7.0 x 10^{-8} mol/m^{2}sPa respectively. For N_{2},
Miachon et al. [8] found a value of 2 x 10^{-8} mol/m^{2}sPa.
Higher single gas permeances for N_{2}, CO_{2}, and CH_{4}
were found by Algieri et al. [11], at a pressure differential of 40 kPa, and
are 4.6 x 10^{-6}, 4.3 x 10^{-6}, and 6.4 x 10^{-6}
mol/m^{2}sPa. Wirawan et al. [12] was also able to find a greater
permeance, but only for CO_{2} at a lower pressure differential. For He
and CO_{2} at a pressure differential of 80 kPa, values of 3.06 x 10^{-8},
and 1.25 x 10^{-5} mol/m^{2}sPa were found.

From the permeances shown in Figure 2, the
permeance ratios for CH_{4}/N_{2}, CO_{2}/N_{2},
and CH_{4}/CO_{2} at a pressure differential of 1 atm are 2.13,
1.75, and 1.22. These permeance ratios are within 50% of the permeance ratios
found by the authors of the studies mentioned previously. The separation of CH_{4}
and N_{2} is a particularly difficult separation that is relevant to a
range of natural gas upgrading processes, and many research teams are
investigating membranes to be used for this separation process [13], [14]. The
combination of this membrane's comparatively high permeance ratio, and high
permeance are therefore indicative that this membrane should be further
investigated for this purpose.

4.3. Effective Diffusivities
for N_{2, }CH_{4}, and CO_{2}

The effective diffusivities have been calculated as a function of the membrane specific constants A using the trends found from the single gas permeances shown in Figure 2, and using Equation 1.

By using the adsorption capacities shown in Figure
1, Equation 1 was used to calculate the effective diffusivities of CO_{2},
N_{2}, and CH_{4} as a function of the physical properties of
the membrane, represented by A. The effective diffusivity ratios for the
separation of CH_{4}/CO_{2}, N_{2}/CO_{2 }and
CH_{4}/N_{2} are calculated, and decrease from a feed side
pressure of 1 atm to 5 atm as follows, 1.54 - 1.38 ± 0.007, 1.44 - 1.32 ±
0.007, and 1.08 - 1.05 ± 0.004 respectively. The shape of the effective
diffusivity curves agree with Figure 1 in that as the pressure increases,
surface occupancy increases, and so the number of interactions between adjacent
molecules increases too. Due to the increased number of interactions, it
follows that diffusivity would also increase as shown in Figure 3.

Interestingly,
the order of increasing effective diffusivities, CO_{2}, N_{2},
and CH_{4} is not the same as the order of increasing permeance which
is N_{2}, CO_{2}, and CH_{4}. According to Equation 1,
flux is directly proportional to both the effective diffusivity and the working
capacity. In other words, the permeance does not solely depend on the effective
diffusivity or the working capacity, but is a combination of both. It should be
emphasized that unlike polymer membranes, single gas permeances of adsorbent
membranes cannot be used to accurately predict the mixed gas permeation
performance. With the limited number of adsorption sites, the molecules having
a greater affinity towards the membrane may block access to the pores of those
molecules having lower affinity towards the membrane. This blocking effect has
been reported in the literature previously for binary mixtures involving CO_{2}
[15]. To this end, mixed gas permeation experiments will be conducted for
membranes of the type used in this study, and their performance will be
reported in the near future.

#### 6. Conclusions

Single gas permeance experiments for He, N_{2},
CH_{4}, and CO_{2} were carried out with a defect free
fabricated silicalite-1 membrane inside a tubular TiO_{2} ceramic
support. Their permeances were found to be 4.38 x 10^{-7}, 7.71 x 10^{-7},
1.35 x 10^{-6}, and 1.64 x 10^{-6} mol/m^{2}sPa,
respectively at a pressure differential of 1 atm and a temperature of 22°C.
Given the pore size of silicalite-1, and the lack of membrane defects as
evidenced by the greater permeance of N_{2} over He, and that none of
the permeances increase as a function of pressure, surface diffusion is
considered to be the mechanism for gas transport through this silicalite-1
membrane. Effective diffusivities were also calculated for the membrane based
on the adsorption capacity of each gas on silicalite-1, and the experimentally
determined single gas permeances. For the separation of CH_{4}/N_{2},
CO_{2}/N_{2}, and CH_{4}/CO_{2} the permeance
ratios were found to be 2.13, 1.75, and 1.22 respectively. In order to better
understand this behaviour and characterize membranes accordingly, future
experiments will be conducted using binary gas mixtures.

#### Acknowledgements

The authors would like to acknowledge the financial support received from NSERC (Natural Sciences and Engineering Research Council) and NRCan (Natural Resources Canada).

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