Synthesis of High Performance SAPO-34 Zeolite Membrane for CO2/CH4 Separation

Synthesis of high performance 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).

Membrane treatment	CO2 Permeance mol/(Pa m2 s)	CO2/CH4 Selectivity
1wt% NaNO3 in H2O	2.1°Á10-7	75
1wt% LiNO3 in Acetone	3.9°Á10-7	70
1 wt% NaNO3 in H2O	2.6°Á10-7	81
1 wt% KNO3 in H2O	2.8°Á10-7	73
Eight membranes calcined directly	3.0°Á10-7	52

Eight SAPO-34 membranes were prepared and calcined directly in vacuum to remove the template. The average CO₂ permeance and CO₂/CH₄ selectivity are 2.8°Á10^-7 mol/(Pa m² s) and 52 respectively (feed pressure 4.6MPa 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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