Original paper_

†E-mail: hteo009@e.ntu.edu.sg Paper presented at ACRA2018, June 10-13, 2018, Sapporo, Japan

Transactions of the Japan Society of Refrigeration and Air Conditioning Engineers, Received date : September 3, 2018 ; J-STAGE Advance published date : December 15, 2018 doi: 10.11322/tjsrae.18-30AC_OA

Aluminium Based Zeolites and MOFs for Adsorption Cooling

How Wei Benjamin TEO* Anutosh CHAKRABORTY*

*School of Mechanical and Aerospace Engineering, Nanyang Technological University (50 Nanyang Avenue, Singapore 639798, Republic of Singapore)

Summary In this paper, AQSOA-Z01, AQSOA-Z02, AQSOA-Z05 zeolites, CAU-10 and Aluminium Fumarate metal organic

frameworks undergo N2 and water adsorption experiments to observe how variations of the material properties affect the water adsorption performances. The experimentally measured N2 isotherms data are used to calculate the BET surface area, pore volume and, most importantly, the pore size distribution of these adsorbent materials. The amount of water uptakes under static and dynamic conditions are measured by a thermo-gravimetric analyser, and these data are fitted with adsorption isotherms and kinetics models within acceptable uncertainties. Based on isotherms and kinetics data, the performances of adsorption cooling in terms of SCP (specific cooling capacity) and COP (coefficient of performances) are evaluated. It is shown the CAU-10 provides the highest SCP and COP as it has higher water uptake – offtake differences with fast kinetics.

Keywords: Water adsorption, Metal organic framework, AQSOA typed zeolites, Microporous materials

1. Introduction

Adsorption cooling system has attracted attentions in past decade with its unique process. The system1, 2) consists of an evaporator, a condenser and at least two thermal compressors as shown schematically in Fig.1. Thermal compression is required to generate cooling with the aid of solar heat or waste heat from a co-generation plant. The adsorption cooling system reduces harmful effects such as global warming and ozone depletion potentials as water is mainly used as the refrigerant3). However, the current adsorbent materials such as silica gels and zeolites2, 4-7), used for thermal compression have relatively lower water uptake – offtake difference per adsorption and desorption periods. Hence, a large amount of adsorbent is required to be installed in the bed for adsorption purposes, making the cooling system huge and bulky.

Fig.1 Schematic of adsorption cooling system

Adsorbents that are providing ‘S’ – shaped isotherms with water/ethanol are effective for designing adsorption cooling system as they have potential to achieve higher cooling capacity and COP while reducing the amount of space required for installation7). The functional adsorbents such as AQSOA type zeolites

(types AFI and CHA) are the current adsorbent materials that adopt the ‘S’ – shaped water adsorption isotherms8, 9). On the other hand, metal organic frameworks (MOFs) have been drawing much attentions in adsorption cooling and heat pump technologies due to their structure tuning capabilities with high micro-porosity and decent hydrophobicity10). For example, aluminium based MOF such as Aluminium Fumarate (Al Fum) and CAU-10 has great potential towards the cooling and heat pump applications11).

To understand the behaviours of adsorbent + water systems, this article presents the surface characterisation and water adsorption experiments for wide ranges of pressures and temperatures. Employing isotherms and kinetics data, the SCP and COP of Al – MOFs based adsorption chiller are evaluated and presented in terms of various cycle times with the heat source and sink temperatures of 60°C and 30°C respectively.

2. Experiments

The adsorbents used in this paper are AQSOA-Z01, AQSOA-Z02, AQSOA-Z05, Al Fum and CAU-10. These materials, except for AQSOA typed zeolites, are synthesized. The surface of the materials will be characterized using N2 isotherm fitted with computed BET isotherm model to achieve the surface area, pore volume and pore size distributions (PSD curves).

Secondly, a thermo-gravimetric analyser is used to measure the water uptakes for all Al based adsorbent materials. Hence the hydrophobic length and highest water uptakes can be observed from all isotherms graphs.

377

Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers, Vol.35,No.4(2018),pp.377-382, doi: 10.11322/tjsrae.18-30AC_OAReceived date: September 3, 2018; J-STAGE Advance publishied date: December 15, 2018

Original Paper

－101－

2.1 Synthesis At first, Al Fum12) was synthesized by ambient

pressure reaction method. 2.45g of aluminium chloride hexahydrate and 1.4g of fumaric acid were added into a beaker of DMF (50ml). The suspension was stirred at the temperature of 130°C for 4 days under ambient conditions. A centrifugal spinning machine separates the mixture after the reaction. Finally, white, synthesized adsorbent was retrieved and rinsed with acetone and methanol for two times. Then, the synthesized material was heated at 80°C for drying.

Secondly, CAU-10 is prepared by hydrothermal reaction method. 1g of isophthalic acid (1, 3-H2BDC) were dissolved in 5ml of DMF and 4g of aluminium sulphate hydrate were dissolved in 20ml of water. The solutions were mixed into an autoclave reactor and it was placed in an oven of 135°C for 12 hours. The mixture was separated by using a centrifugal spinning machine. The synthesized material was rinse with water via sonication for three times. Then, the material was heated at 80°C for drying. 2.2 Adsorption experiment

The specific surface area, pore volume and pore size of the samples can be achieved by N2 adsorption isotherms at 77.4K. The adsorbents are degassed at 120°C for 3 hours before N2 adsorption. The measured isotherms data were equipped with BET (Brunuer-Emmet-Teller) equation to determine the BET surface area. The isotherms data can also estimate the pore size distribution and the pore volume of the material using pore filling concept.

The water adsorption isotherm and kinetics results of the samples will be measured by a gravimetric analyzer at temperatures ranging from 298K to 333K. During experimentation, the samples were heated at 120°C for 2 hours to remove water molecules from the adsorbents. The samples were held by a microbalance in the chamber and the adsorption chamber was free from the mixture of dry N2 and water vapour. The relative pressure of the chamber is controlled by maintaining the flow of N2 gas and water vapour generated from the humidifier. As the samples adsorb water molecules, the mass of the adsorbed sample is increased. Thus, the water adsorption isotherm is measured based on the amount of mass increases from the sample’s dry mass at different relative pressures.

3. Theoretical Modelling

The water isotherm of the adsorbent materials is an important factor towards the size and performance of an adsorption cooling system. Furthermore, the adsorption kinetics ensures that the equilibrium uptake is achieved

at the cycle time during adsorption process. Hence, the following isotherm model13) is adopted to understand the shapes of adsorption isotherms.

θ = K(P/Ps)m/[1+(K-1)(P/Ps)m], (1)

where K is the isotherm coefficient, θ is the surface coverage and m defines the adsorbent surface heterogeneity. Hence θ = q/qo, q is the amount of water uptake and qo defines the limiting uptake. Ps indicates the saturation pressure.

Based on the Langmuiran analogy, the constant, K, determines the adsorption/desorption coefficient rates for kinetics analysis, i.e K = kads/kdes. Hence, the dynamic uptake of the adsorbent materials can be defined as

dq/dt = qm(kads) (P/Ps)m – q(1/τ) (2)

where 1/τ = kads(P/Ps)m + kdes[1-(P/Ps)m] represents the time constant14). The parameters determined from these theoretical models are used to simulate the performance of an adsorption cooling system using the energy balance equations of a conventional two-bed system15).

4. Results and Discussion

An example of illustration of zeolite and MOF adsorbents are shown in Fig. 2. The adsorbents particles are relatively 1μm in size and the shape is determined by the assembly of the metal nodes and ligands, which resemble the shape of the molecular models constructed.

Fig. 2 SEM of (a) AQSOA-Z01 and (b) CAU-10 MOF

Fig. 3 illustrates the N2 isotherms of the adsorbents.

The N2 isotherms of Al based MOFs are shown on the top and those of AQSOA type zeolites are on the bottom for comparison purposes. Al Fum has the highest uptake among the samples and the uptake increases tremendously at the saturated pressure region. This shows a large existence of macro pores in Al Fum. The N2 isotherms determine the surface area and pore volumes of the adsorbent materials. Table 1 shows the values of the BET surface area, pore volume and average pore size of the samples. Al Fum has higher average pore size than the other samples (Table 1). Further comparisons show that AQSOA-Z02 has higher BET surface area and pore volume than those of CAU-

(a) (b)

378

－102－

10 MOF. The average pore size of AQSOA-Z02, however, is smaller than CAU-10. It is noted that CHA zeolites have smaller pores, hence this may contribute to the faster kinetics. On the other hand, AQSOA-Z02 containing Si ions in the material structure may be the element that enhance the surface area and pore volume.

Fig. 4 shows the pore size distribution graph of the adsorbent materials computed from the N2 isotherm using DFT model. Al Fum has a high distribution of micropores (~6Å) available in the structure. This is due to fumaric acid produce smaller linker. However, it is observed that AQSOA-Z02 and CAU-10 provide the smallest pores available (~5Å) in the structure despite having similar distribution trend. It is believed that the addition of Si ions (for AQSOA-Z02) and isophthalic acid structure (for CAU-10) formed these small pores with the bond of Al3+ ions. The AFI zeolites, on the other hand, has larger micropore (~7Å) distribution. Comparing the results (Figure 2) with the average pore size as furnished in Table 1, the MOFs has higher average pore size due to a wide distribution of micropores while the AQSOA type zeolites have limited micro and meso-pore distribution.

Fig. 3 N2 Adsorption Isotherms for adsorbent samples

Fig. 4 Pore Size Distribution of Adsorbent materials

Fig. 5 shows the water adsorption uptake at 25°C. It is shown that Al Fum has higher water uptake since it has wide pore distribution and higher surface area. It is observed that AQSOA-Z05 has longer hydrophobic length while Al Fum, CAU-10 and AQSOA-Z01 have lower hydrophobic length and AQSOA-Z02 has almost no hydrophobic properties. Comparing the results with

Figure 2, the distribution of micropores at a size of ~5Å is ideal for short hydrophobic length.

It is observed that CAU-10 has similar hydrophobic length as AQSOA-Z01 but the water uptake is almost similar to those of AQSOA-Z02. In Fig. 2, AQSOA-Z02 with higher micropore distributions of ~5Å, and with Si ions shows good affinity with water molecules, this material has very low hydrophobic length with higher water uptake. Secondly, AQSOA-Z01 having lower distribution of micropores (around ~7Å size), has low water uptake and short hydrophobic length due Fe ions in the material structure and provides relatively good affinity with water molecules. Thirdly, CAU-10 has a smaller distribution of micropores of ~5Å to provide short hydrophobic length. Another lower micro-pore distribution of (~12Å) provides the higher uptake for water adsorption. Moreover, CAU-10 is reviewed as a breathing MOF11), where pore size increases at certain relative pressure. Hence, these properties of CAU-10 provide the distinctive water isotherms of these adsorbent materials.

Fig. 5 Water Adsorption Isotherms for various

adsorbents

Table 1. Properties of zeolite and MOF samples Samples Surface

Area (m2/g) Pore Vol.(cm3/g)

Pore Size (Å)

Al Fum 792.26 0.926 10.91 CAU-10 553.32 0.25 10.82 Z01 152.95 0.076 10.76 Z02 763.62 0.281 10.66 Z05 242.47 0.095 10.63 By fitting the theoretical models (Equation (1) and

(2)) with the experimental data, the properties of these adsorbent towards water vapor are achieved to simulate the performances of the adsorption cooling system. Fig. 6 shows the adsorption isotherms of the various adsorbents at temperature ranging from 25°C to 60°C and pressures up to 6kPa. The experimental data are fitted with Equation (1). The cooling capacity of an adsorption cooling system relates to the water uptake – offtake difference (Δq) between adsorption and

379

－103－

desorption temperature with working pressures between 1kPa and 5kPa. Hence, Figure 4 shows the Δq between the temperature of 25°C and 60°C to estimate the cooling performance of a low heat transmitted adsorption cooling system. It is found that conventional material (AQSOA zeolites) has much lower Δq than the MOFs. AQSOA-Z01 has a Δq of 0.15g/g, which is the highest among AQSOA zeolites. The Δq for the MOFs are presented at 0.35g/g (Al Fum) and 0.24g/g (CAU-10), which shows that MOFs have better potential in achieving better cooling performance than the AQSOA zeolites with low regeneration heat. Thermodynamics parameters of zeolites + water systems and MOFs + water systems are estimated by the fitting of Equation (1) and are shown in Table 2.

Table 2: Thermodynamic parameters of adsorbents Samples ���∗

(kJ/kg) m α qm

(g/g) Z01 3030 4.9 9.0 × 10-7 0.215 Z02 4560 1.01 7.0 × 10-6 0.29 Z05 2800 6.55 6.77 × 10-5 0.22 Al Fum 2780 6.5 6.78 × 10-4 0.407 CAU-10 3030 5 9.0 × 10-7 0.298

The adsorption kinetics for zeolites + water systems

and MOFs + water systems show the duration needed for water adsorption to reach equilibrium state at given condition. The kinetics data determines the cycle time for adsorption cooling system. Fig. 7 shows the water adsorption kinetics of the zeolites and MOFs up to 3000s. At 25°C, the AFI AQSOA zeolites have faster kinetics behaviour than the rest of the adsorbents as the dynamic uptake is above Δθt/Δθ = 0.5 at 3000s. The MOFs and AQSOA-Z02, on the other hand, is shown to have faster kinetics behaviour at 60°C. The respective pore size distribution in Fig. 3 shows that larger pore achieves faster kinetics at room temperature. It is also noted that higher pore distribution increases the kinetics behaviour, which explains Al Fum has higher dynamic uptake than AQSOA-Z02 and CAU-10. The experimental data is fitted with Equation (2) to estimate the adsorption kinetics parameters of the zeolites + water systems and MOFs + water systems, and it is shown in Table 3.

Fig. 6: Water adsorption isotherm of (a) AQSOA-Z01, (b) AQSOA-Z02, (c) AQSOA-Z05, (d) Al Fum and (e)

CAU-10 at temperature ranging 25°C to 60°C

380

－104－

Fig. 7: Water adsorption kinetics of (a) AQSOA-Z01,

(b) AQSOA-Z02, (c) AQSOA-Z05, (d) Al Fum and (e) CAU-10 at temperature of 25°C and 60°C

Using the parameters from Table 2 and 3, the

cooling performance of the adsorption cooling system is simulated by Thermodynamic Framework adopted in

previous work15). Fig. 8 shows the COP and SCP of the cooling system adopting the analyzed adsorbents presented herein. The hot water inlet temperature is set at 60°C with the cooling water and the chilled water inlet temperatures of 25°C and 14.8°C respectively. The switching time is set at 25s. It is observed that the adsorption chiller adopting MOF performs better in terms of COP and SCP. The SCP for CAU-10 is simulated to be the highest due to its decent hydrophobic length to achieve high water uptake – offtake differences. It is noted that the performance for Al Fum increases with higher cycle time. This is due to the fact that Al Fum requires longer time to achieve decent uptake – offtake differences despite having high uptake – offtake differences between the isotherms of 25°C and 60°C. Nonetheless, CAU-10 shows great potential to be adopted as the adsorbent material for adsorption cooling system given its performance at lower cycle time.

Fig. 8 (a) COP and (b) SCP of adsorption chiller

Table 3: Kinetics parameters of zeolites and MOFs Samples kads (1/s) Ea (J/mol) Z01

RTE

TP ao exp.81.3526 33,480

Z02

RTE

TP ao exp.5.12993

52,250

Z05

RT

ETP ao exp.12.3471

38,700

Al Fum

RTE

TP ao exp.93.14705

36,660

CAU-10

RTE

TP ao exp.73.10270

36,720

381

－105－

[Original paper] Trans. of the JSRAE, Advanced Publication, published online on December 15, 2018

5. Conclusions

Al based adsorbents were experimented through N2 and water adsorption tests. The N2 isotherms showed that the large number of macro pores can be found in Al Fum surface. It is also estimated that Al Fum has better surface properties than CAU-10 and AQSOA-Z01. A pore size distribution graph showed that the MOF materials have two distinct distribution peaks while zeolites have only one distribution peak. Water adsorption isotherms showed that smaller distribution of pores can be a factor to reduce the hydrophobic length of the adsorbent. It also concludes that two distribution peaks at the micropore region will achieve higher uptakes. A decent water uptake – offtake difference, such as Al Fum and CAU-10, can achieve better performance for adsorption cooling system. Hence, Al based MOFs adopted as the potential adsorbent material for adsorption cooling system.

Acknowledgement

The authors acknowledge the financing support from Ministry of Education, Singapore (grant no. MOE 2014-T2-2-061)

Nomenclature

K: Langmuir coefficient kads: adsorption coefficient rate, g/g s-1

kdes: desorption coefficient rate, g/g s-1

m: heterogeneity factor P: pressure, kPa Ps: saturated pressure, kPa q: gravimetric uptake, g/g qm: limiting uptake, g/g t: time, s Greek Symbols θ: adsorption uptake fraction τ: time constant, s

References

1) Chakraborty, A., Leong, K. C., Thu, K., Saha, B. B., and Ng, K. C., Theoretical insight of adsorption cooling, Applied Physics Letters, vol. 98, p. 221910, 2011.

2) Saha, B. B., Akisawa, A., and Kashiwagi, T., Silica gel water advanced adsorption refrigeration cycle, Energy, vol. 22, pp. 437-447, 1997.

3) Jeremias, F., Fröhlich, D., Janiak, C., and Henninger, S. K., Water and methanol adsorption on MOFs for

cycling heat transformation processes, New Journal of Chemistry, vol. 38, p. 1846, 2014.

4) Anyanwu, E. E. and Ogueke, N. V., Thermodynamic design procedure for solid adsorption solar refrigerator, Renewable Energy, vol. 30, pp. 81-96, 2005.

5) Chua, H. T., Ng, K. C., Chakraborty, A., Oo, N. M., and Othman, M. A., Adsorption Characteristics of Silica Gel + Water Systems, Journal of Chemical and Engineering Data, vol. 47, pp. 1177-1181, 2002.

6) Thu, K., Chakraborty, A., Saha, B. B., and Ng, K. C., Thermo-physical properties of silica gel for adsorption desalination cycle, Applied Thermal Engineering, vol. 50, pp. 1596-1602, 2013.

7) Myat, A., Kim Choon, N., Thu, K., and Kim, Y.-D., Experimental investigation on the optimal performance of Zeolite–water adsorption chiller, Applied Energy, vol. 102, pp. 582-590, 2013.

8) Goldsworthy, M. J., Measurements of water vapour sorption isotherms for RD silica gel, AQSOA-Z01, AQSOA-Z02, AQSOA-Z05 and CECA zeolite 3A, Microporous and Mesoporous Materials, vol. 196, pp. 59-67, 2014.

9) Kayal, S., Baichuan, S., and Saha, B. B., Adsorption characteristics of AQSOA zeolites and water for adsorption chillers, International Journal of Heat and Mass Transfer, vol. 92, pp. 1120-1127, 2016.

10) Henninger, S. K., Habib, H. A., and Janiak, C., MOFs as Adsorbents for Low Temperature Heating and Cooling Applications, J Am Chem Soc, vol. 131, pp. 2776-2777, 2009.

11) Henninger, S. K., Ernst, S.-J., Gordeeva, L., Bendix, P., Fröhlich, D., Grekova, A. D., Bonaccorsi, L., Aristov, Y., and Jaenchen, J., New materials for adsorption heat transformation and storage, Renewable Energy, 2016.

12) Kiener, C., Muller, U., and Schubert, M., Organometallic Aluminum Fumarate Backbone Material, USA Patent US20090092818A1, 2009.

13) Sun, B. and Chakraborty, A., Thermodynamic formalism of water uptakes on solid porous adsorbents for adsorption cooling applications, Applied Physics Letters, vol. 104, p. 201901, 2014.

14) Sun, B. and Chakraborty, A., Thermodynamic frameworks of adsorption kinetics modeling: Dynamic water uptakes on silica gel for adsorption cooling applications, Energy, vol. 84, pp. 296-302, 2015.

15) Teo, H. W. B., Chakraborty, A., and Han, B., Water adsorption on CHA and AFI types zeolites: Modelling and investigation of adsorption chiller under static and dynamic conditions, Applied Thermal Engineering, vol. 127, pp. 35-45, 2017.

382

－106－