(58d) Synthesis of Ion-Exchanged SAPO-34 Zeolite Membrane for CO2/CH4 Separation

Synthesis of ion-exchanged SAPO-34
zeolite membrane for CO₂/CH₄
separation

Yanfeng Zhang¹,Meng Li¹, Yuhan Sun¹

¹Shanghai Advanced Research
Institute, Chinese Academy of Sciences, 100 Haike Rd, Pudong New District, Shanghai 201203, P.R China

Introduction

Natural gas usually is contaminated with CO₂,
which must be removed to meet pipeline requirement. The presence of CO₂
in natural gas decreases the energy content of the gas, and causes pipeline corrosion
in the presence of water. Amine adsorption is a mature technology for CO₂
removal from natural gas, but it suffers from high energy cost for solvent
regeneration, especially for natural gas with high CO₂ content.
Membrane separation has many advantages, such as low energy cost, low capital
investment, flexible size, etc, and polymer membranes have been used for
natural gas sweetening at commercial scale. However, polymer membranes cannot
treat natural gas with high CO₂ content (>20%), since plasticization
effect decreases membrane selectivity significantly. Zeolite
membranes have great chemical and thermal stability, and their uniform pore
size. Zeolite membranes[1¨C4]
have potential for CO₂/CH₄ separation owing to their great
thermal, mechanical, and chemical stability, and stability at high CO₂
pressures. SAPO-34, a silicoaluminophosphate with chabazite (CHA) type framework having pore diameter 0.38 nm,
is perfect for CO₂/CH₄ separation because CO₂
(0.33 nm kinetic diameter) and CH₄ (0.38 nm) can be separated by
molecular sieving mechanism. The selective adsorption of CO₂ in
SAPO-34 pores also favours the separation of CO₂. Many factors
affect the performance of SAPO-34 membrane, including feed pressure [1,2], Si/Al
ratios [3,4], seed size [5], template types [5], membrane thickness [4], cation forms [6], CO₂/CH₄ feed ratio,
support properties [7], concentration polarization [8] and template residue [9].
Mei et al prepared ion-exchanged SAPO-34 zeolite
membrane and CO₂/CH₄ selectivity was increased by 30%.
Here we report a simple one step method of preparing ion-exchanged SAPO-34
membrane by combining template removal and ion-exchange in to one step.
Obtained Na, K and Li-SAPO-34 membranes have improved separation performance.

Experimental

Detailed information about synthesis, calcination and gas separation test of SAPO-34 membrane can
be found in ref 9. Uncalcined SAPO-34 membrane was
soaked in 1wt% NaNO₃, KNO₃ or LiNO₃ solution
for 5 min and dried at 383K for 2 h. Then the SAPO-34 membrane was calcined in vacuum at 673 K for 4 h with 1K/min heating and
cooling rates. For comparison, blank SAPO-34 membranes were also calcined in vacuum at the same condition.

Results
and discussion

Fig 1 shows the SEM images of SAPO-34 membrane.
Typical cubic crystals with perfect intergrowth were found in Fig 1a and
membrane thickness is ~ 5 µm (Fig 1b). SAPO-34
crystals with thin plate morphology, as shown in Fig 1c, were used as seed for
membrane synthesis. XRD patterns indicate that the obtained crystals and
membranes have CHA structure (not shown here).

(a)  
                                      (b)

(c)

Fig 1
SEM images of SAPO-34 membrane and SAPO-34 seed, (a)top
view and (b)cross section view and (c)SAPO-34 seed.

Table 1. Separation performance of SAPO-34 membranes (50/50 molar CO₂/CH₄
mixtures, room temperature, 4.0 MPa feed pressure).

Eight SAPO-34 membranes were prepared and calcined directly in vacuum to remove the template. The
average CO₂permeance and CO₂/CH₄
selectivity are 2.8°Á10^-7 mol/(Pa m² s) and 52 respectively (feed
pressure 4.0MPa and 50/50 CO₂-CH₄ mixture). Four
membranes were soaked in 1wt% salt solution for 5 min and dried at 383K. Then
these membranes were calcined at 673K in vacuum to
remove template. The melting points of NaNO₃, KNO₃ and
LiNO₃ are 581, 607 and 528K respectively, all lower than calcination temperature of 673K. This indicates that the
deposited salt on the membrane surface will be melted during calcination. After template removal, the as prepared
H-SAPO-34 membrane will be exchanged to Na, K and Li-SAPO-34 membranes. All
four membranes exhibits 40 to 60% increase in CO₂/CH₄
selectivity, compared with control experiment. Ion-exchange of H-SAPO-34 increases
basicity of SAPO-34 crystals, which favours the adsorption
of CO₂. Ion-exchange may also reduce pore size of SAPO-34 crystals, which
might limit the diffusion of CH₄. This result is consistent with Mei's
result [6].

Conclusions

A simple method was developed to prepare
ion-exchanged SAPO-34 membranes in one step. By combining template removal and
ion-exchange into one step, Na, K and Li-SAPO-34 membranes were prepared. The
obtained SAPO-34 membranes have much higher CO₂/CH₄
selectivity, which is the result of increased basicity
from ion-exchange. The combination of template removal and ion-exchange
eliminate one step and greatly reduce synthesis cost. This method can also be
applied to other zeolite membranes.

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