Alkali Treatment of Commercial Silicoaluminophosphate Molecular Sieves (SAPO-34) Enhances the Water Adsorption and Desorption Properties

Commercial silicoaluminophosphate molecular sieves (SAPO-34) received alkali treatment with either NaOH (0.2, 0.01, 0.005, or 0.001 M) or NH4OH (0.005 M). Treatment with NaOH (0.005 M) increased the water adsorption initial rate of SAPO-34 by 1.4-fold. The alkali treatment introduced Na adsorption sites into the SAPO-34. The desorption ratio (adsorption at 30 ̊C and desorption at 100 ̊C) was 88.2% higher than the original rate (84.3%). On the other hand, after alkali treatment of SAPO-34 using NH4OH (0.005 M), calcination resulted in the highest desorption ratio at 91.3%. When combined with calcination, alkali treatment with NH4OH introduced H adsorption sites into SAPO-34, H adsorption sites feature low levels of interaction with water, which enhanced the desorption ratio, but decreased the initial adsorption rate. These results indicate that treating commercial SAPO-34 with 0.005 M NaOH enhances both the adsorption and desorption behaviors.


Introduction
In 1984, a novel class of oxide molecular sieves with a crystalline microporous framework, silicoaluminophosphates, was synthesized [1] [2]. These new materials possess properties of both zeolites and aluminophosphetes, which makes How to cite this paper: Katoh, M., Horiuchi, K., Satoh, A., Aoyagi, K. and Sugiyama, S. (2019) Alkali Treatment of Commercial Silicoaluminophosphate Molecular Sieves (SAPO-34) Enhances the Water Adsorption and Desorption Properties. Journal of Encapsulation and Adsorption Sciences, 9, 149-158. https://doi.org/10.4236/jeas.2019.94008 Journal of Encapsulation and Adsorption Sciences them unique in many ways. The present study was focused on commercial silicoaluminophosphate molecular sieves (SAPO-34 (AQSOA-Z02,)) that were supplied by the Mitsubishi Chemical Corp. The water adsorption isotherm of AQSOA-Z02 revealed interesting behaviors [3]. The adsorbed amount of water increased rapidly at 10% relative humidity and the adsorbed amount of water corresponded to the amount that would saturate a Y type zeolite. The results of alkali treatment for both NaY zeolites [4] and ZSM-5 type zeolites [5] were reported previously. Water diffusivity was enhanced for NaY zeolites via alkali treatment with 1 M of a NaOH solution. This concentration was too high, however, for the alkali treatment of ZSM-5 zeolites, and the concentration was decreased to an optimum of 0.2 M.
In the present study, SAPO-34 was treated with either NaOH (0.2, 0.01, 0.005, or 0.001 M) [4] [5] or NH 4 OH (0.005 M) with the goal of achieving a higher rate of water diffusivity. The modified SAPO-34 was characterized by XRD, SEM, XRF, IR, and nitrogen adsorption. The amount of water adsorbed onto zeolites under the recycled temperature conditions was estimated using the IR method [6]. The IR spectra of water adsorbed onto zeolites were measured under differential temperatures at a constant pressure. The water adsorption behavior was estimated by measuring the change of pressure under a constant volume [4] [5] [7].

Experimental Methods
All experiments were performed at Tokushima University from April, 2015 through September, 2019.

Materials
The SAPO-34 (AQSOA-Z02) used in this study was supplied by the Mitsubishi Chemical Corp. The Alkali treatment of AQSOA-Z02 was performed within an aqueous solution of either NaOH (0.2, 0.01, 0.005, or 0.001 M) or NH 4 OH (0.005 M). Then, 1.0 g of AQSOA-Z02 was placed into a 100 mL of each aqueous solution and was kept at 95˚C for 1 h. After a period of 1 h, the slurry was filtered using a filter media made of glass fiber with 0.3 μm pores. The filtered cake was dried overnight at 100˚C in an air oven. After drying, the cake was crushed into powder, and the powder was rinsed with hot distilled water at 80˚C for 2 h. After rinsing, the powder was filtered and dried to obtain either Z02-NaOH(X M) (X denoted the concentration of NaOH aqueous solution) or Z02-NH 4 OH (0.005 M). As a reference, Z02-NH 4 OH-C was obtained by calcination of Z02-NH 4 OH MicrotracBEL Corp.). The micropore and mesopore volumes were evaluated using the MP and BJH method, respectively.

Measurement of the Thermal Behavior of Water Adsorbed Onto Samples
The infrared (IR) spectra of the water-alkali treated SAPO-34 adsorption systems were obtained using the IR cell reports from other papers [4]- [10]. A sample disc that was 13 mm in diameter, approximately 8.5 mg in weight, and approximately 60 µm in thickness was pretreated in the cell at 350˚C and 4.0 × 10 −4 Torr for 2 h. The spectra were recorded on a FTX3000MX Bio-Rad Laboratories, Inc. spectrometer with a mercury-cadmium-telluride (MCT) detector at a resolution of 4 cm −1 . The spectra were recorded by increasing the temperature from 30˚C to 200˚C and under water pressure of 10 Torr. The IR integrated intensity of the peak around 1650 cm −1 , which was assigned to the bending vibration of adsorbed water, was normalized by the weight and diameter of the sample disc.
The IR normalized intensity was equivalent to the amount of adsorbed water at a given temperature.

Measurement of the Water Adsorption and Desorption Behavior of Several Samples
Water adsorption was measured using a volumetric adsorption apparatus assembled in our laboratory [4] [5] [7]. The adsorbent was initially outgassed in a sample room at 350˚C and 4.0 × 10 −4 Torr for 2 h. Water was introduced to the sample room at a saturated water pressure and at room temperature. When water was adsorbed onto the samples in the sample room, the pressure was decreased by increasing the time under this constant volume. The amount of water adsorbed onto several samples was calculated from the pressure profile. This method revealed the behavior of water adsorption on the stronger adsorption sites, because the amount of water introduced into the sample room was limited.
Only the slope of the adsorption initial behavior is discussed here.    in NaOH treatments [4]. The Na content was increased with an increase in the NaOH concentration. These results indicated that the H + was changed by Na + .

Framework Structures and Nitrogen Adsorption Isotherms of Several Samples
These results were supported by IR measurement (Figure 3).       Figure 5 shows the normalized IR integrated intensity for several samples. The IR integrated intensity at 30˚C and the desorption ratios (adsorption at 30˚C and desorption at 80˚C or 100˚C) are summarized in Table 3. The temperature behavior was changed by the alkali treatment. After NaOH treatment, Z02-NaOH (0.005 M) had the highest IR integrated intensity of 30˚C. The best concentration for NaOH treatment was 0.005 M. These results were supported by the enhancement of N 2 adsorption properties. Alkali treatment with higher concentrations of NaOH solution tended to damage the pore structure, but treatment with a lower concentration did not sufficiently affect the pore structure to increase micropore volume. Also, introducing the optimum amount of Na + exchanged by alkali treatment was also important. The desorption ratio of Z02-NaOH (0.01 M) was smaller than that of Z02-NaOH (0.005 M). These results indicated that although a higher NaOH concentration can introduce a sufficient amount of Na + into SAPO-34, the desorption ratios (adsorption at 30˚C and desorption at 80˚C or 100˚C) were decreased. Strong interaction between Na + and water prevented the release of water from SAPO-34. On the other hand, with NH 4 OH treatment, Z02-NH 4 OH had a higher IR integrated intensity at 30˚C and Z02-NH 4 OH-C Figure 5. IR integrated intensity of adsorbed water at 10 Torr of several samples. NH or H + decreased the amount of water adsorbed at higher temperatures. Figure 6 showed the adsorption behavior of water. The slope of the adsorption initial behavior determined the adsorption initial rate [4], which is summarized in Table 4. Z02-NaOH (0.005 M) had the highest rate of water adsorption.

Water Adsorption and Desorption Behavior
The increase in the micropore volume and introduction of an optimum amount of Na + sites induced the highest rate of water adsorption. These results indicated

Conclusions
The present study examined the effects that alkali treatment (NaOH or NH 4 OH) exerts on commercial silicoaluminophosphate molecular sieves (SAPO-34).
1) XRD patterns showed that, except for 0.2 M NaOH treatment, the other alkali treatments produced effects to SAPO-34 that were identical to that of CHA.  2) SEM images of all treated materials showed mainly in a cubic structure.
Obvious changes to the surface morphology of SAPO-34 were induced only by a relatively higher NaOH concentration (0.2 M).
3) SAPO-34 samples treated with 0.005 M NaOH showed the highest amount of adsorbed water at 30˚C as well as the highest adsorption capacities (adsorption at 30˚C and desorption at 80 or 100˚C). 4 OH treated samples, the amount of adsorbed water at 30˚C and the effective adsorption capacities (adsorption at 30˚C and desorption at 80˚C or 100˚C) were higher than for NaOH-treated samples, but the adsorption initial rate was less than half that of the NaOH treated samples due to the low interaction between water and either + 4

4) For NH
NH or H + . 5) When SAPO-34 was treated with 0.005 M NaOH, the initial rate pf water adsorption at 30˚C was about 1.4 times that of the original rate, which represented a higher amount of adsorbed water and a higher effective adsorption capacity than that of other alkali treatments of SAPO-34 by NaOH solutions. These results indicated that treatment of commercial SAPO-34 with 0.005 M NaOH produced the most advantageous conditions for water adsorbancy.