CO 2 Adsorption by Y-Type Zeolite Impregnated with Amines in Indoor Air

The capture and concentration of CO2 by cyclic adsorption–desorption processes in indoor air environments are studied. A synthesized Y-type zeolite was impregnated with amines used to capture CO2 to maintain good indoor air quality (IAQ). Three kinds of amines, including monoethanol amine (MEA), isopropanol amine (IPA), and tetraethylenepent amine (TEPA), were selected to study the performance of CO2 adsorption capacity at a fixed-bed in an 80-L air chamber. These Y8 loaded with amines were examined with a Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Infrared Spectroscope (IR) and Thermogravimetric Analyzer (TGA) to determine their textural properties before and after amines-impregnation. The CO2 adsorption capacities of modified Y8 was substantially higher than that of the original Y8 adsorbent, and the highest capacity was up to 158 mg/g-adsorbent-hr. This study shows that the order of adsorption capacity for the three types of amines was TEPA > MEA > IPA. The first-order kinetic model was best fitted to adsorb CO2. The rate constant (k) of the first-order adsorption model can be further used for the design and application of a CO2 control system in IAQ. The results of this study show that amines functionalized Y8 have excellent adsorption potential for removing indoor CO2.


INTRODUCTION
The issue of indoor air quality (IAQ) in offices, hospitals or public spaces has been receiving great consideration for recent years.The exposure to hazardous pollutants indoors, compared with those of outdoors, can have a great impact on human health (Ohura et al., 2005).The impact of low ventilation rates inside buildings result in increasing CO 2 concentrations leading to IAQ complaints and health symptoms.It has been found that high CO 2 concentrations in office buildings are associated with the increasing health symptoms (Erdmann et al., 2002).Indoor air pollutants (IAPs) include Total Volatile Organic Compounds (TVOC), Formaldehyde, Carbon Dioxide (CO 2 ), Bacteria, and Fungi etc.Among IAPs, CO 2 is the representative pollutant of indoor air quality and its concentration is associated with human activity.The outdoor concentrations of carbon dioxide range from 350-450 ppm.Since carbon dioxide is a product of human or animal respiration, carbon dioxide concentrations increase as the ventilation rate decreases.The indoor air temperature and humidity are usually maintained by air-conditioning systems (ACS); however ACS ignores the problem of CO 2 accumulation due to inadequate ventilation (Fig. 1).The time from 11:45 to 14:09 shown in Fig. 1 is lunch time, so there are fewer people causing CO 2 concentrations to go down at that time.
Amine based chemical absorption has been used commercially for CO 2 removal in industry (Chew et al., 2010).However, the liquid amine based process may have many problems due to high regeneration energy, large equipment size, solvent leakage and corrosion issues (Veawab et al., 1999).Thus recent studies have explored a capable technique which is the integration of organic amines into porous supports for CO 2 adsorption (Zelenak et al., 2008;Yan et al., 2011).This research studies the modification of porous materials by grafting amine functional groups to the surface of a solid zeolite.Amines (MEA, IPA, and TEPA) have been selected as modification materials for microporous support.Previous studies, as shown in Table 1, show that amine active sites bonded on the surface of porous materials can promote the capacity of carbon dioxide capture (Chatti et al., 2009;Su et al., 2010).The objectives of this study are to explore the kinetic and adsorption capacity of three amines on Y8 zeolite and to analyze the microstructures and properties of modified zeolite.

Synthesis of Sorbents
Y-type zeolite was synthesized with a Si and Al molar  ratio of 8.2 (abbreviated as Y8), then Y8 was impregnated by amines selected as the sorbent for CO 2 capture in indoor environment.First, Y8 was synthesized by mixed SiO 2 together with AlNaO 6 Si 2 prepared for use as a support material.Alk-solution NaOH (30.0 g) dissolved in 1 L of distillated water was added to form zeolite colloid and stirred until extrusion was complete.Thereafter, the zeolite was extruded with a fixed-shape and then drying at 105°C for 8 hrs.Finally, the fixed-shape of the solid was calcined at 450°C for 8 hrs.The physical properties of Y8 indicated that the surface area and average pore diameter of Y8 were 402 m 2 /g and 2 nm as micropore, respectively.

Modification of Sorbent
After preparation of support materials, a modification step was conducted by loading three types of amines (MEA, IPA, TEPA) individually into the adsorbents.Amines were introduced into the zeolite prepared by wet impregnation (Yue et al., 2008).In a typical preparation of 20 and 40 (wt%) modifier, 5, 13.3 (g) amines were dissolved in methanol, respectively.Then at least 20 g of Y8 was dried at 150°C for 3 hrs after impregnation with different contents of the amines solution.Finally, the mixture was dried by vacuum oven at 105°C for 3 hrs.Water vapor improves CO 2 adsorption on the amine functional groups which increases CO 2 adsorption capacity (Donaldson and Nguyen, 1980).Previous studies have also suggested that the CO 2amine bonding is enhanced when water vapor is present during the adsorption (Gray, 2005;Zhou et al., 2005).The reaction of CO 2 with amine generation during the synthesis and modification processes resulted in the reduction of total adsorption capacity due to the vapor effect.Therefore, moisture removal of the adsorption material is a very important step before adsorption testing to avoid reaction during preparation.

Chamber Adsorption Study
The 80 L chamber had two ports, an inlet and an outlet, for air circulation and a fan for uniform mixing.The 20 g of amines-Y8 was filled at a fixed-bed in the chamber and then the inside air was circulated by a pump (Fig. 2).In accordance with the provisions of the building code for maintaining good IAQ, the ventilation rate (or air exchange rate) ranges from 3 and 6 times per hour (times/hr) (C.P.A.M., Taiwan, R.O.C. 2005).In this study the ventilation rate was set to 6 (times/hr) and the calculated flow rate in the fixed-bed was 8 L/min.The initial CO 2 concentration was controlled at approximately 1500 ppm at 25°C in the chamber study.

The Microstructures Analysis of Y8
The CO 2 concentration was determined using a CO 2 meter (HAL-HCO201, Hal Technology, USA).The microstructures of sorbents was analyzed by a powder Xray diffractometer (XRD, Bruker D8 Advanc, Germany).The thermal stability of sorbents in the air was determined by a thermogravimetric analyzer at a heating rate of 5°C per minute at 0-800°C (TGA, Pyris 1, PerkinElmer, UK).The surface functional groups of sorbents were examined by an Infrared Spectroscopy (IR, PerkinElmer, UK).

Adsorption Behaviors
The CO 2 adsorption behaviors of Y8 with two contents (20 wt% and 40 wt%) and three amines (TEPA, MEA, and  3.The CO 2 adsorption capacities of modified-Y8 were substantially higher than that of original Y8.The higher weight loss of TEPA showed higher content of TEPA in Y8 based on the TGA test.However, the 40 wt% TEPA in Y8 resulted in lower CO 2 absorption capacity due to the pores being filled.Therefore, the amount of CO 2 absorbed in Y8 conducted in the 1-hr experiments were reduced because lower CO 2 gas passed through the pores.Overall, the saturation adsorption capacity of amines-Y8 indicated that the total CO 2 adsorption capacity of the 40 wt% of TEPA was better than that of the 20 wt% in cumulative adsorption studies.Additionally, the higher MEA-Y8 and IPA-Y8 content (40 wt%) provided better adsorption capacity than that of low content (20 wt%).The 20 wt% of IPA-Y8 had the lowest adsorption capacity.
Fig. 4 illustrates the saturation adsorption capacity of amines-Y8 by the cumulative adsorption process since the initial CO 2 concentration of 1500 ppm was not sufficient for the material to reach a condition of equilibrium.Thus the cumulative adsorption experiments were conducted to determine the CO 2 adsorption capacities.The adsorption studies were consecutively performed 27 times and each time they were conducted for one hour.Therefore, the order of adsorption capacities were obtained as 40 wt% of TEPA > 20 wt% of TEPA ≈ 40 wt% of MEA > 40 wt% of IPA > 20 wt% of MEA.The total CO 2 adsorption capacity on TEPA 20, 40 (wt%), MEA 20,40 (wt%) and IPA 40 wt% were 84, 158, 51, 87, 71 mg/g-adsorbent, respectively.In aqueous solutions of the primary and secondary alkyl amines, the following reactions with CO 2 occur (Donaldson and Nguyen, 1980;Blauwhoff et al., 1984): The MEA and IPA had a functional group of -NH 2 that are considered to be the primary amines that occurs in the chemical reaction between the amines and the carbon dioxide.Amines provide the catalytic function for the adsorption of carbon dioxide by amine-modified silica materials, yielding carbamate in anhydrous conditions by reactions 1 (Zelenak et al., 2008).The TEPA contains both the primary amine (R 1 NH 2 ) and secondary amine (R 1 R 2 NH), and both amines can react with CO 2 and lead to the formation of one carbamate ion as reactions (1) and (2).Thus the CO 2 adsorption capacity of the TEPA is better than MEA and IPA.

Microstructures Analysis of Y8 and after Amines Loadings
SEM pictures of Y8, TEPA-Y8, MEA-Y8 and IPA-Y8 .The result of SEM confirmed that the amines-Y8 was grafted successfully by the amine groups.The Y8 after amines modification still had many available pores.TEPA-zeolite, MEA-zeolite and IPA-zeolite had a little amine adhered to Y8 in Fig. 5.The comparison of XRD patterns for Y8 before and after loading of three types of amines (TEPA-Y8, MEA-Y8 and IPA-Y8) are shown in Fig. 6.The Y8 consistent diffraction patterns were modified by different amines.The sample of the diffraction peaks located at 2θ = 6.0-6.5°can clearly be observed, as the higher concentration of amines showed higher intensity of diffraction patterns of amines in Y8.The intensity of the diffraction peak did not vary after the modification of the amines implying that the pore structure order was not affected by amines.These changes were caused by the pores being filled in the Y8 channels and the amines coating on the outer surface of the Y8 crystals.These results are consistent with Xu et al. (2005).Fig. 7 presents the IR spectra of the amine groups for zeolite after modification.The IR used to examine nitrogen surface functional groups exhibiting modification in amines-Y8.The amines-Y8 differed from the pure Y8 in that it exhibited several peaks.After the Y8 was modified, the adsorption peaks at 1530-1560 cm -1 and 3300-3500 cm -1 were associated with the stretching vibrations of -NH 2 .The peak at 1630 cm -1 corresponded to -NH 3 + O-Si/-NH 2 + O-Si, while the absorption banding at 2900-2971 cm -1 represented stretching of CH from the CH 2 CH 2 CH 2 -NH 2 groups.Thus, the IR spectra of amine-modified Y8 confirmed   the integration of amine inside of the Y8's channels.Fig. 8 shows the TGA of the Y8 loading with different kinds and amounts of amines.We used nitrogen as a carrier gas in this study.The sample was heated from 30°C to 800°C by 4°C per minute.The weight lost that occurred near 100°C and a sharp weight loss appeared at 150°C indicated the desorption of moisture.The heating of pure Y8 from 150 to 800°C did lose much weight loss indicating the high thermal stability of Y8.The TEPA, MEA and IPA had flash points of 163°C, 85°C and 71°C as shown in Table 2 which indicated that TEPA-Y8 had two weight losses at 100°C and 150°C, and that the first loss was moisture that had evaporated and second loss resulted from desorption of -NH 2 (Su et al., 2010).Both MEA-Y8 and IPA-Y8 had only one weight loss prior to reaching 100°C.This was because the amines flash points were both lower than 100°C.Thus, the weight of Y8 with amines subtracting that of the pure Y8 was the amount of amines impregnated into Y8.The TEPA 20, 40 (wt%), MEA 20, 40 (wt%) and IPA 20, 40 (wt%) weight losses were about 16, 40, 10, 38, 20, 38 (%), respectively.

Kinetic Model
The CO 2 adsorption was further used to estimate an adsorption rate constant (k 1 ), where n is reaction order, C 0 is the initial concentration of CO 2 (ppm), C A is the concentration of CO 2 (ppm) at time t, and t is the adsorption time (min.).Eq. ( 3) is a first-order reaction function as n = 1.Eq. ( 4) derived from eq. ( 1) that can be used to determine the reaction rate constant (Lagergren, 1898).Amines-Y8 with different modifier concentrations was best fitted to the first-order kinetic model as shown in Table 3.The R 2 of TEPA-Y8 and MEA-Y8 were greater than 0.94 that was suitable for application of the first-order kinetic model.The 40 wt% of IPA-Y8 R 2 value was close to 1; but the 20 wt% of IPA-Y8 R 2 was 0.39, and the results demonstrate that the higher IPA concentration was better for fitted in the first-order kinetic model.Comparing different amines with different modifier concentrations, the higher the k value is suggests better adsorption capacity.The results of k were TEPA > MEA > and that conformed to the experiments conducted in this study.

CONCLUSIONS
A amines-modified Y8 adsorbent was successfully developed for indoor CO 2 capture.Y8's microstructures were enhanced after being modified with amines.Amines Y8 showed superior CO 2 adsorption capacity.Formation of amine functional groups in Y8 was determined by IR confirming that CO 2 adsorption associated with the functional groups (primary, secondary and tertiary amines) had better adsorption behaviors.The material thermal stability in connection with the flash point showed the Y8 has high thermal stability.The cumulative adsorption capacity demonstrated that the 40 wt% of TEPA had the highest capacity and could capture up to 158 mg/g-adsorbent-hr.Simulation of the amines-Y8 kinetic model can be used to assess CO 2 removal in indoor air quality control.

Table 1 .
Porous materials with different amines adsorption CO 2 .

Table 2 .
Physicochemical properties of three amines.

Table 3 .
First-order kinetic model fitted to Y8-modified adsorption behavior.