Regeneration Process of Ammonia-Absorbed Zirconium Phosphate to Zirconium Phosphate.

Zirconium phosphate [Zr(HPO4)2·H2O] absorbs 2 mol(NH3)/mol[Zr(HPO4)2·H2O] with a low equilibrium plateau ammonia concentration of around 1 ppm in water. In this study, in order to investigate the regeneration process of ammonia-absorbed zirconium phosphate [Zr(NH4PO4)2·H2O], Zr(NH4PO4)2·H2O was heat-treated above 353 K under an inert gas. Then, the structures of the heat-treated samples were evaluated using powder X-ray diffraction and thermogravimetry-mass spectrometry measurements. Zr(NH4PO4)2·H2O started to desorb ammonia and the crystal water at 353 K. Then, Zr(NH4PO4)2·H2O was changed to the anhydrous monoammoniate [Zr(NH4PO4)(HPO4)] at 473 K and formed anhydrous zirconium phosphate [Zr(HPO4)2] at 673 K. The anhydrous zirconium phosphate and the anhydrous monoammoniate reabsorbed ammonia in ammonia water. Those initial absorption rates were small compared with Zr(HPO4)2·H2O. The slow kinetics of the anhydrous zirconium phosphate corresponded to the small interlayer distances. The ammonia concentration composition isotherms indicated that the anhydrous zirconium phosphate and anhydrous monoammoniate have a low ammonia equilibrium plateau concentration of around 1 ppm in ammonia water. Zr(NH4PO4)2·H2O is formed from Zr(NH4PO4)(HPO4) by the reabsorption of ammonia and water after 1-10 cycles. We found that zirconium phosphate is an ammonia remover which can be used repeatedly at 473 K.

Introduction

Reactive nitrogen is the next major environmental problem.[1−3] In 1913, the Haber–Bosch process was developed, and the technology to convert atmospheric nitrogen into ammonia was established. The amount of reactive nitrogen remaining in the environment has more than doubled over the past 100 years.[4,5] Ammonia is the main substance of reactive nitrogen released and remaining in the environment, and it is also a factor causing environmental pollution such as PM 2.5 and red tide.[1−6] However, ammonia is currently used in various fields such as fertilizers,[1−6] semiconductor materials,[7] and spacecraft coolants.[8] In addition, demand for ammonia is expected to increase for use as a fuel (energy carrier) and a hydrogen carrier.[9−11] Currently, water is used as a removal agent when ammonia leaks from ammonia production plants or power plants. Since water used as a removal agent contains a large amount of ammonia, it is necessary to remove the ammonia before releasing it into the environment.[12] So far, the stripping method has been used to remove ammonia from ammonia water. However, this method requires large-scale facilities and is not suitable for emergency use. Therefore, a method that can remove ammonia from ammonia water without requiring special facilities is needed.

In our previous studies,[13,14] we have reported that zirconium phosphate [Zr(HPO4)2·H2O] has a high ammonia absorption capacity of 10.2 wt % with the low equilibrium concentration in ammonia water [6.7 mmol (NH3)/g, ammonia equilibrium concentration: below 1 ppm at about 298 K]. Zr(HPO4)2·H2O showed the greatest amount of ammonia absorption among insoluble ammonia absorbing materials such as Prussian blue.[13,15,16] However, there is no research on recycling back to Zr(HPO4)2·H2O after regeneration of [Zr(NH4PO4)2·H2O].

In this report, we have studied a heat treatment method under an inert atmosphere to regenerate zirconium phosphate from ammonia-absorbed Zr(HPO4)2·H2O [Zr(NH4PO4)2·H2O]. The ammonia ab/desorption characteristics of heat-treated Zr(NH4PO4)2·H2O were investigated using the ammonia concentration composition isotherm (CCI), powder X-ray diffraction (PXRD), and thermogravimetry–mass spectrometry (TG–MS) measurements.

Methods

Zirconium phosphate [Zr(HPO4)2·H2O, CZP-100] with an interlayer was purchased from Daiichi Kigenso Kagaku Kogyo Co., Ltd. Zr(HPO4)2·H2O was used as received without further purification. Various concentrations of ammonia water were prepared using 10 wt % ammonia water from KENEI Pharmaceutical Co., Ltd., diluting with ion exchange water.

The ammonia-adsorbed samples were heat-treated for 4 h after reaching 473 or 673 K at 5 K/min in an inert gas such as Ar or N2. We call these heat-treated samples HTS473K and HTS673K, respectively. This process was used to remove water and ammonia from the ammonia-absorbed zirconium phosphate [Zr(NH4PO4)2·H2O].

TG–MS measurements were carried out in order to know the desorbed gaseous species, the desorption temperatures, and the weight loss of the heat-treated samples and Zr(NH4PO4)2·H2O. TG–MS profiles were investigated by TG (Rigaku plus RS-8200 manufactured by Rigaku Co.) and MS (MQA200TS manufactured by Anelva Co.) in a flowing Ar gas (300 cm3/min) with a heating rate of 5 K/min. The maximum temperature was 723 K. Chemical formulas of HTS473K and HTS673K were calculated using the weight loss of Zr(NH4PO4)2·H2O.

PXRD measurements were carried out to characterize the structures of the HTS473K, ammonia-absorbed HTS473K, HTS673K, ammonia-absorbed HTS673K, Zr(HPO4)2·H2O, and Zr(NH4PO4)2·H2O. XRD patterns were recorded on a Bragg–Brentano diffractometer (Rigaku RINT-2500V manufactured by Rigaku Co.) and Cu Kα at a tube current of 200 mA and a tube voltage of 40 kV. Each sample was pressed at a constant load on a glass holder before the XRD measurements.

Ammonia CCI measurements of the heat-treated samples (HTS473K and HTS673K) and Zr(HPO4)2·H2O were performed to evaluate the ammonia absorption capacities and the ammonia equilibrium concentrations at 298 K. Ammonia water with various concentrations (100–2500 ppm) was prepared, and the temperature of the solution was controlled at 298 K. The ammonia concentration and the potential of hydrogen (pH) in ammonia water were measured using an ammonia meter (Orion Star A324 and Orion 9512 manufactured by Thermo Scientific Orion) and pH meter (CyberScan pH310 manufactured by EUTECH Ins.), respectively.[14] 0.5 g of each sample was added to the solution, and the NH3 concentration and pH were continuously measured at regular time intervals. Here, ammonia has two kinds of forms which are NH3 and NH4+ in ammonia water. The NH3 concentration [(NH3)] was directly measured using the ammonia meter. Then, the NH4+ concentration [(NH4+)] was calculated using following eq (17)where Kb is the base dissociation constant and pH is the potential of hydrogen. We assumed that Kb is constant (Kb = 1.8 × 10–5 at 298 K). Then, the ammonia absorption capacity (Cab) was calculated using following eq where [NH3]be and [NH3]af are the NH3 concentration before and after HTS473K, HTS673K, or Zr(HPO4)2·H2O is added, respectively, [NH4+]be and [NH4+]af are the NH4+ concentration before and after HTS473K, HTS673K, or Zr(HPO4)2·H2O is added, respectively, and L is the volume of ammonia water.

Results and Discussion

Heat Treatment of Ammonia-Absorbed Zirconium Phosphate [Zr(NH4PO4)2·H2O] at 473 and 673 K

In our previous work, it has been reported that Zr(HPO4)2·H2O absorbs 2 mol(NH3)/mol[Zr(HPO4)2·H2O] of ammonia at an equilibrium plateau concentration of around 1 ppm at about 298 K and has two equilibrium ammonia concentrations in ammonia water.[14] The ammonia-absorbed zirconium phosphate [Zr(NH4PO4)2·H2O] releases 1 mol(NH3) and 1 mol(H2O) under 473 K and releases another 1 mol(NH3) between 473 and 673 K. It is expected that the chemical formulas of HTS473K and HTS673K are Zr(NH4PO4)(HPO4) and Zr(HPO4)2, respectively.

Figure shows TG-MS curves of HTS473K and HTS673K and Zr(NH4PO4)2·H2O. Ammonia (5.8 wt %) is released from HTS473K. The calculated weight of NH3 divided by that of Zr(NH4PO4)(HPO4) is 5.7 wt %, which is very close to the experimental result. HTS673K does not release NH3 and H2O. We confirmed the chemical formulas of these heat-treated samples.

Figure 1

(a) TG curves of Zr(NH4PO4)2·H2O (black line),[14] HTS473K (red line), and HTS673K (blue line) and (b,c) mass spectra at m/z 18 and 17 of desorption gas from Zr(NH4PO4)2·H2O (black line),[14] HTS473K (red line), and HTS673K (blue line) in a flowing Ar gas (300 cm3/min) with a heating rate of 5 K/min.

Figure shows the XRD patterns of Zr(HPO4)2·H2O, Zr(NH4PO4)2·H2O, HTS473K, and HTS673K. The XRD peaks of HTS673K were consistent with the XRD peaks of Zr(HPO4)2. The difference in peak intensity between HTS673K and Zr(HPO4)2 is thought to be due to the fact that the peaks showing layering (2θ: 12.3°) are oriented, and their intensity increased due to pressing before the measurement. It has been reported that Zr(HPO4)2 has a layer structure.[18] It is indicated that the HTS673K is an anhydrous zirconium phosphate Zr(HPO4)2 having a layer structure. HTS473K [Zr(NH4PO4)(HPO4)] has different XRD patterns compared to Zr(HPO4)2·H2O and Zr(HPO4)2.

Figure 2

XRD patterns of Zr(HPO4)2·H2O, Zr(NH4PO4)2·H2O, HTS473K, ammonia-absorbed HTS473K, HTS673K, ammonia-absorbed HTS673K, Zr(HPO4)2·H2O (JCPDS: 00-033-1482), Zr(NH4PO4)2·H2O (JCPDS: 01-071-1633), and Zr(HPO4)2 (JCPDS: 00-032-1495).

Zr(HPO4)2·H2O, Zr(NH4PO4)2·H2O, and HTS673K [Zr(HPO4)2] have interlayer distances d(002) which are 7.6 Å (2θ: 11.6°),[19] 9.4 Å (2θ: 9.4°),[20] and 7.2 Å (2θ: 12.3°),[18] respectively. HTS473K was heated at a temperature lower than 673 K. Therefore, HTS473K is also considered to have a layered structure and interlayer distance which is 7.5 Å (2θ: 11.8°).

Ammonia Absorption Characteristics of HTS473K and HTS673K

Ammonia absorption properties of the heat-treated samples (HTS473K and HTS673K) in aqueous ammonia were evaluated using the pH meter and the ammonia meter. Figure shows the ammonia absorption capacities of Zr(HPO4)2·H2O, HTS473K, and HTS 673K estimated using eq as a function of time. These ammonia absorption capacities increase with time, approaching constant equilibrium values. Zr(HPO4)2·H2O shows an inflection point around 55 mg(NH3)/g[Zr(HPO4)2·H2O]. This value is consistent with about 1.0 mol(NH3)/mol[Zr(HPO4)2·H2O]. The slopes (ammonia absorption rates) before and after the inflection point are 11 and 5.0 mg/(g min), respectively.

Figure 3

Relation between ammonia absorption capacity and time [black circle: Zr(HPO4)2·H2O, red circle: HTS473K, and blue circle: HTS673K].

It has been reported that the interlayer distance of Zr(HPO4)2·H2O is similar to that of Zr(NH4PO4)(HPO4)·H2O[14] as shown in Table S1. When ammonia is absorbed above 1.0 mol(NH3)/mol[Zr(HPO4)2·H2O], the interlayer distance changes, which probably caused a change in the ammonia absorption rate.

The ammonia absorption capacity of HTS473K linearly increases with respect to time [rate: 5.3 mg/(g min)] as shown in Figure . The value is close to that of the slope of the second-step ammonia absorption of Zr(HPO4)2·H2O. After this experiment, HTS473K is changed from Zr(NH4PO4)(HPO4) to Zr(NH4PO4)2·H2O as shown by the XRD patterns in Figure .

The ammonia absorption rate of HTS673K is 1–5 mg/(g min) below the capacity of 55 mg/g and 5.2 mg/(g min) above the capacity of 55 mg/g. Therefore, the interlayer distance of zirconium phosphate is considered to change in two steps during the absorption process from ammonia water. This suggests that anhydrous zirconium phosphate Zr(HPO4)2 absorbs water and ammonia at the same time below 55 mg(NH3)/g(sample) to form Zr(NH4PO4)(HPO4)·H2O. Above the capacity of 55 mg(NH3)/g(sample), Zr(NH4PO4)2·H2O is formed (Figure ).

Figure shows ammonia CCIs of HTS473K, HTS673K, and Zr(HPO4)2·H2O at 298 K. The ammonia equilibrium plateau concentration in the heat-treated samples was about 1 ppm. As shown in Figure , the ammonia equilibrium concentration of HTS673K is lower than 0.01 ppm below the ammonia absorption capacity of 1 mol(NH3)/mol(sample) because of the minimum limit of detection. HTS673K [Zr(HPO4)2] has the ammonia equilibrium plateau concentration of about 1 ppm in the range from 1 to 2 mol(NH3)/mol(sample). In other words, it shows the same absorption characteristics as the ammonia CCI of Zr(HPO4)2·H2O. HTS473K [Zr(NH4PO4)(HPO4)] has the ammonia equilibrium plateau concentration of about 1 ppm in the range from 0 to 1 mol(NH3)/mol(sample). This profile is similar to that of Zr(HPO4)2·H2O, which absorbs 1–2 mol(NH3)/mol [Zr(HPO4)2·H2O].

Figure 4

Ammonia CCI plots of HTS473K [Zr(NH4PO4)(HPO4), red circle], HTS673K [Zr(HPO4)2, blue circle], and zirconium phosphate [Zr(HPO4)2·H2O, black circle] at 298 K.

According to the results of PXRD patterns and ammonia absorption characteristics, following reactions and 4 will exist because HTS473K [Zr(NH4PO4)(HPO4)] and HTS673K [Zr(HPO4)2] absorb and desorb ammonia reversibly.

We found that ammonia-absorbed zirconium phosphate is recycled back to anhydrous zirconium phosphate by the heat treatment at 473–673 K under an inert gas (Figure S1).

Cycling Characteristics

We evaluated the cycling characteristics of ammonia ab/desorption of HTS473K. Waste heat below 573 K constitutes over 60% of the total waste heat.[21,22] The heat treatment energy would be supplemented by waste heat. Figure shows the ammonia absorption and desorption cycling characteristics at 473 K. The ammonia concentration in the water was measured about 30 min after the sample was added.

Figure 5

Ammonia absorption and desorption cycling characteristics between HTS473K and ammonia-absorbed HTS473K.

The ammonia equilibrium plateau concentration is about 1 ppm from 1 to 10 cycles. Figure S2 shows XRD patterns of HTS473K after ammonia absorption and heat treatment. The XRD peaks of HTS473K after ammonia absorption and heat treatment did not change regardless of the cycle numbers. Therefore, zirconium phosphate can be used repeatedly as an ammonia removal agent.

Conclusions

Zr(NH4PO4)2·H2O desorbed ammonia by the heat treatment at 473 and 673 K under an inert gas. HTS473K [Zr(NH4PO4)(HPO4)] and HTS673K [Zr(HPO4)2] absorbed ammonia in ammonia water with hydration. The process of ammonia absorption of HTS473K and the desorption by the heat treatment at 473 K was repeated 10 times. HTS473K absorbed ammonia with a low equilibrium ammonia plateau concentration of around 1 ppm in water from 1 to 10 cycles.