Insights into the effective removal of volatile organic compounds based on adsorptive and catalytic approaches

Abstract

It is well known that water molecules are omnipresent in polluted air and industrial gas streams, and often have negative effects on the removal of VOCs. The remediation of VOCs through adsorption and photocatalysis by various materials have been well examined by many researchers. Meanwhile, the performance evaluations of VOCs based on ambient conditions are strongly required instead of unrealistically high pressure regions. AC has been carried out for the treatment of gaseous benzene in a long-time history. Meanwhile, porous metal-organic frameworks (MOFs) have paved a new route to build an efficient platform for air/gas purification against various organic (or inorganic) impurities in vapor phase via adsorption and/or catalysis. Nevertheless, the performances of adsorption and catalysis technologies are expected to be suppressed or enhanced by many variables (e.g., presence of water vapor and rising temperature) under the real-world conditions. Thus, it is very meaningful to study the moisture effect on both of adsorption and catalysis techniques onto the adsorbents and catalysts so as to widen the usefulness of these two technologies. In chapter 2, the effects of moisture on the adsorption of microporous activated carbon (AC) and metal organic frameworks (MOFs: MOF-199 and UiO-66-NH2) were investigated using benzene as a model VOC. The breakthrough volume (BTV) of the AC and MOFs was examined as a function of benzene concentration (10 to 200 ppm) and relative humidity (RH of 0 to 100%). For AC: at lower benzene levels (<100 ppm), the BTV was affected sensitively by RH to reduce the maximum adsorption capacity by 27 to 65% at 100% RH (relative to 0% RH). In contrast, at higher benzene levels (~200 ppm), the comparable RH effect was not evident only to reduce less than 5.3% of capacity. In case of lower benzene, the effect of pore filling by water condensate is apparent. For higher benzene, the capacity vs. outlet stream benzene (2 to 20 Pa) profile followed Henry’s law at 0% RH (7.5 mg g-1 Pa-1, R2 = 0.997). In three adsorption-desorption cycles, the maximum capacity for 50 ppm benzene decreased at the last cycle by 12% at continuous zero RH, and it also decreased 39.6% at continuous 100% RH. Comparatively, for MOFs: the adsorption breakthrough (BT) behavior of the two strong sorbent MOFs (i.e., MOF-199 (Cu-MOF) and UiO-66-NH2 (Zr-MOF)) were also investigated against gaseous benzene to assess the combinatorial effect of the two key variables: (i) the inlet benzene vapor concentrations (10 to 200-ppm) and (ii) relative humidity (RH = 0 to 100 %). Based on the BT volume (BTV at 10% BT), the adsorption performance of MOF-199 showed a 6-fold enhancement over UiO-66-NH2 at 10-ppm benzene and zero RH. The adsorption capability of both MOFs declined dramatically with an increase of RH levels (e.g., in reference to RH= zero), which was more apparent at lower (≤ 50-ppm) than at higher benzene levels (≥100-ppm). More importantly, the BT behavior of benzene (at RH levels of 20 to 100%) was seen to be suppressed more sensitively on MOF-199 than on UiO-66-NH2 (e.g., by the preferential occupation of the unsaturated copper sites with water molecules in the former). The experimental adsorption data of both MOFs fitted best with Freundlich isotherm at the zero RH. As a comparison, dynamic fitting results were observed to fit with all isotherms for the two MOFs at various RH conditions. At low RH (0 ~ 20%) conditions, benzene adsorption onto MOFs showcased good fitting with both of pseudo-first (PFO) and -second-order (PSO) kinetics. The experimental data at high RHs (> 50%) nonetheless failed to fit with both kinetics with the low R2 values (0 < R2 ≤ 0.87). The Bohart-Adams, Thomas, and Yoon-Nelson predictions were all in very good agreement with the experimental results. As a whole, although MOF-199 exhibited best benzene adsorption performance in terms of the maximum capacities, especially at higher benzene concentrations (e.g., 50 - 200 ppm). The slight influence of moisture effect on AC at all concentration levels demonstrated AC is a far better adsorbent compared to other two MOFs in real applications. In Chapter 3, the formed Schiff base complex (termed as imine) between functional amine-functionalized metal-organic frameworks (MOFs: e.g., UiO-66-NH2) and gaseous formaldehyde (FA) is expected to restrict its regeneration/reusability for practical adsorption removal of airborne aldehyde pollutants. Because of this constraint, a series of experiments were conducted to investigate the effect of relative humidity (RH: 20 – 100%) on the adsorption/desorption recovery of gaseous FA (50 and 200 ppm) from UiO-66-NH2 sorbent. Although AC (Duksan) was verified to be excellent for the remediation of gaseous benzene in Chapter 2, even in the moisture environment, the low adsorption performance of AC towards FA elucidated AC was not a good option for removing FA gas and it was not necessary to investigate the recovery of AC towards FA. The results revealed the higher chemisorption affinity of UiO-66-NH2 for FA with the increase of gaseous FA loading from 50 to 200 ppm. More importantly, such chemically adsorbed FA molecules were quickly recovered by the desorption at low RH conditions (≤ 60%) for low FA partial pressure compared to high pressure, with FA recovery of 41.9 ~ 83.4% (50 ppm) and 12.9 ~ 80.5% (200 ppm), respectively. Further, the complete recovery of 50 ppm FA (≥ 99%) from exhausted UiO-66-NH2 was achieved with increases in RHs levels up to 70 ~100%. The recyclability potential of UiO-66-NH2, if examined through the repeated adsorption-desorption cycle of gaseous FA, can be explained by the competing relationship between FA and H2O molecules. Overall, the obtained data verified the reversibility of imine-bonds between aminated MOFs and gaseous aldehyde could effectively be regulated by controlling RH conditions during adsorption/desorption processes. In Chapter 4, the catalytic removal of gaseous formaldehyde was studied using Pt/TiO2 catalysts under varying operation conditions (e.g., changes in Pt content (0, 0.25, 0.5, and 1 wt.%), presence of moisture levels, and reduction (R) treatment). For higher loading of Pt in Pt/TiO2, higher oxidative removal of formaldehyde was achieved, while the presence of moisture further promoted its oxidation. The combined effects of UV light and water vapor were evident even at the lower loadings of Pt (e.g., 0.25%). However, the UV light exhibited negligible effect on the oxidation of FA under 100% RH with higher Pt contents (≥ 0.5%). For the pre-treated Pt/TiO2 (Pt/TiO2(R): R as reduction treatment with H2 (10%) in He gas), the enhancement in oxidative removal of formaldehyde was distinctively recognized. Further, 1%-Pt/TiO2(R) achieved 82.4% of removal efficiency (RE) of formaldehyde at dry condition, while it recorded the nearly full oxidative removal (i.e., RE: 96%) at 20% RH under UV light. It was striking to note that the complete removal of FA was achieved at 100% RH without the help of UV light. Thus, Pt/TiO2(R) catalysts exhibited the great potential for the oxidative removal of gaseous formaldehyde as either photocatalyst or thermocatalyst depending on humidity conditions. Overall, the adsorptive removal of gaseous benzene using two types of adsorbents (i.e., AC and MOFs (MOF-199 and UiO-66-NH2)) with the effect of relative humidity was studied firstly in this thesis. The water vapor showed the slightest effect on the benzene adsorption onto AC compared to the other two adsorbents. Comparatively, MOF-199 was only considered as an excellent for the capture of gaseous benzene at the dry condition, and UiO-66-NH2 was not a promising adsorbent even at the dry state. Thus, the conventional adsorbent (e.g., AC) exhibited better performance compared to the advanced functional materials (e.g., MOFs), and the latter still needs to be modified so as to expand their applications. Secondly, the water vapor was observed to help the desorption of amine-functionalized adsorbents. The introduction of 70% RH almost resulted in the complete recovery for the spent UiO-66-NH2 (exposed with 50 ppm of FA), and the highest recovery of nearly 90% can also be achieved at RH ≥ 70% for 200 ppm concentration level of FA. Then, the regeneration study elucidated that UiO-66-NH2 exhibited an excellent recovery efficiency because it maintained a high adsorption capacity after six adsorption-desorption cycles. Finally, water vapor was also found to help the oxidative removal of gaseous formaldehyde over Pt/TiO2 catalysts. The co-presence of 1%-Pt/TiO2(R) and RH (i.e., ≥ 50%) dramatically promoted the complete removal of formaldehyde vapor with or without the help of UV light.