Control of Water-Adsorption Properties of Mesoporous Silica and MOF by Ion Exchange and Salt Impregnation

양이온 교환 및 염 함침을 통한 메조다공성 실리카와 유기-금속 구조체의 수분 흡착 특성 조절

The adsorbent used in water-adsorption cooling system utilizing low-temperature heat of below $90^{\circ}C$ is required to exhibit high water uptake capacity at a relative humidity ($P/P_0$) between 0.1 and 0.3. Mesoporous silica (MCM-41) and MOF(MIL-101) exhibit quite large water adsorption capacity under saturated water vapor at $35^{\circ}C$. However, these adsorbents show small water adsorption capacity ($0.027{g_{water}\;g_{ads}}^{-1}$, $0.074{g_{water}\;g_{ads}}^{-1}$, respectively) in the relative humidity ($P/P_0$) range of 0.1 to 0.3. In this study, the surface properties of mesoporous silica and MOF were modified by simple methods to develop an adsorbent having a higher water uptake than the conventional water adsorbents at a relative humidity ($P/P_0$) of 0.1 ~ 0.3. In the case of mesoporous silica (MCM-41) exhibiting mainly water adsorption at $P/P_0=0.5{\sim}0.7$, aluminum species was functionalized on the mesopore walls and then cations existing near the aluminum were exchanged with various cations (e.g., $Na^+$, ${NH_4}^+$, and $(C_2H_5)_4N^+$). In addition, 20 wt% (to total weight of the composites) of hygroscopic inorganic salt ($CaCl_2$) was impregnated on the MCM-41. In the case of the MIL-101 (MOF), 20 wt% of hygroscopic inorganic salt ($CaCl_2$) was impregnated on the MIL-101. The MCM-41 which was ion-exchanged with various cations has main adsorption branch around 0.5 of $P/P_0$ which was slightly shifted with low-pressure direction in comparison with pristine MCM-41. However, tiny increases were observed on the adsorption in the range of $P/P_0$ between 0.1 and 0.3. After salt impregnation on the MCM-41, the adsorption capacity under $P/P_0=0.1{\sim}0.3$ at $35^{\circ}C$ was increased from $0.027{g_{water}\;g_{ads}}^{-1}$ to $0.152{g_{water}\;g_{ads}}^{-1}$. In the case of MIL-101, the amount of water adsorption at $35^{\circ}C$ under $P/P_0=0.1{\sim}0.3$ was increased from $0.074{g_{water}\;g_{ads}}^{-1}$ to $0.330{g_{water}\;g_{ads}}^{-1}$ after the salt impregnation.

$90^{\circ}C$ 이하의 저온열원 구동 수분 흡착식 냉방 시스템에 사용되는 흡착제는 효과적인 냉열 생산을 위해서 상대습도($P/P_0$) 0.1 ~ 0.3에서 높은 수분 흡-탈착량 차를 보이는 것이 좋다. 메조다공성 실리카(MCM-41)와 다공성 유기-금속 구조체(MIL-101) 의 경우 최대 수분 흡착량은 많지만 상대습도($P/P_0$) 0.1 ~ 0.3 구간에서 각각 $0.027{g_{water}\;g_{ads}}^{-1}$, $0.074{g_{water}\;g_{ads}}^{-1}$의 낮은 수분 흡-탈착량 차를 갖는다. 이 연구에서는 메조다공성 실리카와 다공성 유기-금속 구조체의 표면 성질을 조절하여 상대습도($P/P_0$) 0.1 ~ 0.3에서 수분 흡-탈착량 차를 증가시켰다. 주로 수분 흡착이 상대습도($P/P_0$) 0.5 ~ 0.7에서 일어나는 메조 다공성 실리카의 경우 알루미늄을 관능화 시킨 후에 염기도가 다른 여러 양이온($Na^+$, ${NH_4}^+$, $(C_2H_5)_4N^+$)들로 교환하거나 염($CaCl_2$)을 20 wt% 함침하여 각각의 흡착제들에 대해 $35^{\circ}C$에서 수분 흡착 등온선을 측정하였다. 양이온 교환 후 수분 흡착이 주로 일어나는 구간이 상대습도($P/P_0$) 0.5 부근으로 이동하였으나 여전히 상대습도($P/P_0$) 0.1 ~ 0.3에서 낮은 수분 흡-탈착량 차를 보였다. 하지만 흡습성을 갖는 염($CaCl_2$)을 20 wt% 함침한 메조다공성 실리카는 상대습도($P/P_0$) 0.1 ~ 0.3에서 수분 흡-탈착량 차가 $0.027{g_{water}\;g_{ads}}^{-1}$에서 $0.152{g_{water}\;g_{ads}}^{-1}$으로 증가하였다. 수분 흡착이 상대습도($P/P_0$) 0.3 ~ 0.5에서 주로 일어나는 다공성 유기-금속 구조체에도 염($CaCl_2$)을 20 wt% 함침하였더니 상대습도($P/P_0$) 0.1 ~ 0.3에서 수분 흡-탈착량 차가 $0.330{g_{water}\;g_{ads}}^{-1}$까지 증가하였다.