Effect of 13X Zeolite Modified with CuCl2 and ZnCl2 for Removing

Jan 12, 2016 - ... Zeolite Modified with CuCl2 and ZnCl2 for Removing Phosphine from Circular Hydrogen of a Polysilicon Chemical Vapor Deposition Stov...
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Effect of 13X Zeolite Modified with CuCl2 and ZnCl2 for Removing Phosphine from Circular Hydrogen of a Polysilicon Chemical Vapor Deposition Stove Xuanwen Xu and Guoqiang Huang* School of Chemical Engineering and Technology, Tianjin University, Tianjin China ABSTRACT: Silicon materials used in semiconductor and photovoltaic products have strict purity requirements. The quality of materials will be affected severely if even trace amounts of PH3 are contained in the hydrogen of the chemical vapor deposition (CVD) stove. Thus, it is critical that the content of PH3 be controlled under a certain level. In contrast to series of metal oxide adsorbents reported in other research, 13X zeolite modified with ZnCl2 (Zn-13X) and CuCl2 (Cu-13X) was prepared to adsorb trace PH3 in this study. Breakthrough curves and adsorption capacities at different temperatures from −15 to 50 °C were investigated to determine the performance of the adsorbents. X-ray diffraction and surface analysis were carried out to characterize the adsorbents and the adsorption mechanism. The results showed that CuCl2 was dispersed on 13X in monolayer or submonolayer form, and ZnCl2 would change the structure of 13X under the experimental condition. New diffraction peaks at 35.6° and 38.8° appeared after loading CuCl2 in the X-ray diffraction pattern of Cu-13X, and the peak of Zn-13X at 10° almost disappeared. The half-pore width after modification was mainly centralized at 0.5 nm for each of them, and the specific surface areas of Cu-13X and Zn-13X were 245.2 and 19.2 m2/g, respectively. The breakthrough time for Cu-13X was always more than 600 min, whereas it sharply decreased from 350 to 20 min for Zn-13X when the temperature increased to 50 °C from −15 °C. The static adsorption capacity decreased from 106.5 mg to 67.2 mg and from 88.3 mg to 36.1 mg of PH3, respectively, for pergram Cu-13X and Zn-13X as the adsorption temperature changed in the chosen range. The working life of Cu-13X could be prolonged by N2 purging at room temperature according to experimental results because the rate of chemical reaction coupled with the adsorption between PH3 and the active components was much slower than that of its physical adsorption, but a higher temperature was needed for Zn-13X.

1. INTRODUCTION With the increasing demand for photovoltaic and semiconductor products, silicon materials with high purity are playing an important role in the social economy. The Siemens method is a common technology for producing silicon in this field. In this method, SiHCl3 is reduced to pure polysilicon by H2 in a chemical vapor deposition (CVD) stove. The limitation of impurity content in SiHCl3 and H2 is critical to ensure the quality of silicon products. The hydrogen in a CVD stove comes from two parts: fresh H2 produced by the water electrolytic system and recycled H2 from the stove. In this paper, we addressed the purification of the recycled H2 before it returns into the stove. The impurities contained in this hydrogen include chlorosilane, HCl, trace PH3, and B2H6. The quality of the products is insensitive to chlorosilane and HCl, and they are easy to separate from hydrogen by condensation, absorption, or other methods. However, the trace PH3 is difficult to remove, and the process may be harmed by its accumulation in the recycling process. Thus, it is important to remove it for quality control. Chemical absorption1−3 can remove PH3 via the presence of solutes with strong oxidizability. However, the treated gas will carry large amounts of water, which is extremely reactive to SiHCl3. Thus, another process for the removal of water is needed if solution absorption is used. In the CVD system, a dry treating process for the removal of PH3 is required. Carbon masks are considered as great adsorbents for the elimination of many toxic gases. Activated © XXXX American Chemical Society

carbon (AC) modified with alkaline or acid compounds such as sodium hypochlorite,4 sodium hydroxide,4 sodium carbonate5 or HCl5,6 have been reported, but these adsorbents cannot be regenerated like metal oxides. Copper-loaded carbon masks called whetlerite were first manufactured during World War I, after which improved carbon masks modified with more transition metals such as ASC whetlerite were also developed.7 Transition metal and rare earth oxides such as copper(II), zinc(II), cobalt(III), cerium(III), and lanthanum(III) indeed have been proven to be excellent active agents for the elimination of PH3. Ning et al. tested the performance of copper−zinc−(cerium, lanthanum)−AC8,9 and copper−iron− (cerium, lanthanum)−AC10 adsorbents with O2 as a reactant for the removal of PH3. Hsu and co-workers.11 compared the PH3 purification performance of ZSM-5 and Y zeolite without the presence of O2, which were both loaded by copper(II), zinc(II), or manganese(IV) oxides, and they found that copper oxides had the best effect. In other studies, sol−gel-derived TiO212 and sol−gel-derived γ-Al2O3,13 which were both modified with copper(II) oxides, were developed to remove PH3 from dry or humidified air, and a detailed mechanism of adsorption was proposed. Analogous research was also carried out by Brinen and co-workers14 and Watanabe et al.15 In their Received: September 16, 2015 Revised: January 5, 2016 Accepted: January 12, 2016

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DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research works, AC modified with copper(II), chromium(III) oxides,14 and copper oxide pellets15 were used in to remove AsH3, which is somewhat similar to PH3 in some chemical properties. Metal copper also has been studied for AsH3 adsorption,16−18 but it was proven to be less effective in purification than its oxides.16 Metal oxides such as copper(II) would be reduced by PH3 in the adsorption process according to these reports, and oxidizing them again by blowing air seemed to be a common method to regenerate adsorbents.11,13,14 Regeneration by pure N2 also was tested in the data source of ref 9, and it was proven that the effect of this method improved as regeneration temperature increased. All active components in these studies consisted of metal oxides, and they showed excellent performance in the removal of PH3. However, H2O might be generated by the reaction between metal oxides and PH3,16 residual HCl or H2 when used in the treatment of recycled gas from the CVD stove. This would be a severe threat to the system because of its high sensitivity to chlorosilane, and the silicon formed previously also could be corroded if the oxygen were taken as a reactant in the adsorption or regeneration process. In this paper, we concentrated on developing a new adsorption process that could meet the requirement of the CVD system. Adsorbents that would not generate water were prepared by directly loading CuCl2 or ZnCl2 onto 13X to remove PH3, and their performance in adsorption and regeneration were also investigated under the condition of no oxygen.

The adsorption experiments were carried out in the range of −15 to 50 °C, and the volumetric flow rate of feed gas was controlled at 50 mL/min. The column and 5 m of gas preheating pipe were both placed into a thermostatic bath to control the adsorption temperature. PH3 concentration at the column outlet was examined by a PH3 detector from Yuante Technology Co., Ltd. of Shenzheng, China. The minimum detection limit was 1 ppb. Static adsorption capacities were determined at 1 atm by the volumetric method, in which adsorbents and 20 mL of PH3 (99%) were placed together in a piston syringe of stainless steel that could be easily pushed by the pressure difference between the inside of the syringe and the atmosphere. The volumes of PH3 (99%) before and after adsorption were both calibrated at 0 °C, and then were converted into mass (mg) by the formula of PV = nRT. The adsorption system is shown in Figure 1.

Figure 1. System of adsorption: (1) PH3 (99%); (2) N2; (3) pressure reducing valve; (4) valve; (5) pressure gauge; (6) cylinder; (7) volumetric flowmeter; (8) preheating pipe; (9) adsorption column; (10) thermostatic bath; (11) PH3 detector.

2. EXPERIMENTAL SECTION 2.1. Adsorbents Preparation. Commercial zeolite 13X (diameter: 0.5−1 mm) was used as a support of active components. CuCl2·2H2O (99%) and ZnCl2 (99%) were supplied by Guangfu Reagents Company of Tianjin, China. In the preparation, the virgin zeolite was washed twice by distilled water and then heated at 250 °C for 10 h in muffle furnace to remove possible impurities. Subsequently, virgin zeolite treated by the previous step was impregnated at 90 °C for 12 h under the condition of reflux condensation by an aqueous solution of CuCl2 or ZnCl2, both of whose concentrations were 0.2 mol/L. The impregnated samples were dried at 110 °C for 24 h to remove main water and then heated at 350 °C for 24 h to activate before being conserved under a no-water condition. Finally, two adsorbent samples of Cu-13X and Zn-13X, which were modified with CuCl2 and ZnCl2, respectively, were prepared. The mass content of active element (Cu2+ or Zn2+) for each of them was 10%. 2.2. Characterization of Adsorbents. The pore size, pore volume, and specific surface area of adsorbents were measured by nitrogen adsorption carried out at 77.4 K with a surface analyzer (Quantachrome NOVA). BET and DFT method were used to calculate the specific surface area and the pore size distribution, respectively. The crystal structures of used and fresh adsorbents were determined by X-ray diffraction (D/ MAX-2500, Rigaku Corporation of Japan). The X-ray was produced by Cu Kα at the scanning speed of 1°/min, and the range of 2θ was 5° to 90°. 2.3. PH3 Adsorption. PH3 (99%) and nitrogen (99.9999%) were supplied by Nanjing Specialty Gases Company of China and Jinxi Huanda Gases Company of Tianjin, China, respectively. In this study, 50 ppm of PH3 was prepared as feed gas by N2 dilution in a cylinder, and 0.5 g of adsorbents were filled into an adsorption column (4 mm in diameter and 50 mm in length).

2.4. Pilot Scale Experiments. Pilot scale experiments were carried out at the Reduction & Hydrogenation Workshop of Xinjiang Daqo New Energy Co., Ltd. Circular hydrogen from a polysilicon CVD stove was treated at room temperature by a Zn-column (an adsorption column containing Zn-13X adsorbents) 800 mm in diameter and 2000 mm in length. The volumetric flow rate of hydrogen and the operating pressure were 1700 N m3/h and 1.4 MPa, respectively. 2.5. Intermittent Breakthrough Experiments. To research the mechanism of the adsorption, intermittent breakthrough experiments on Cu-13X and Zn-13X were designed by the following method. The adsorption was stopped by cutting off the feed gas and the column outlet when the adsorbents were broken through, and this state was maintained for a certain period of time. After the pause, the supply of feed gas was restarted to continue the adsorption under the same conditions. The performance of these adsorbents in the process of the second adsorptive operation, which was restarted after different pause times, was investigated via breakthrough curves. All of these operations were carried out at 20 °C, and the operational conditions of adsorption remained the same as described in chapter 2.3 of this paper, except that the PH3 concentration of the feed gas for Cu-13X was increased to 500 ppm in these experiments.

3. RESULTS AND DISCUSSIONS 3.1. Characterization of Blank 13X, Zn-13X, and Cu13X Adsorbents. Crystal structures of blank 13X, Zn-13X, and Cu-13X were measured and compared by XRD in the 2θ range of 5−55° to analyze the effect of ZnCl2 and CuCl2 on adsorbents. The XRD patterns of fresh adsorbent samples including all main characteristic peaks are shown in Figure 2. B

DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Half-pore size distribution of fresh blank 13X, Cu-13X, and Zn-13X: (a) blank 13X, (b) Cu-13X, (c) Zn-13X.

Figure 2. XRD patterns of fresh blank 13X, Cu-13X, and Zn-13X. The black asterisks denote the characteristic peaks of blank 13X, the red asterisks denote characteristic peaks of copper(II) oxides: (a) blank 13X, (b) Cu-13X, (c) Zn-13X.

centralized between 0.5 and 0.7 nm after loading with either CuCl2 or ZnCl2. The specific surface area and total pore volume of Cu-13X decreased by approximately 60% and 27%, respectively. The reason was that CuCl2 had coated the inner surface of the 13X and filled the pores in monolayer or submonolayer form. Moreover, the structural change of Zn-13X was further reflected by the information on its total pore volume and specific surface area, which coincided with the disappearance of characteristic peaks to some degree. 3.2. Adsorption of PH3. Comparative adsorption experiments among blank 13X, Cu-13X, and Zn-13X were carried out at −15, 5, 20, 35, and 50 °C to investigate the effects of CuCl2 and ZnCl2 on removing PH3. It was proven that both of them, especially CuCl2, would significantly improve the purification performance of 13X. As observed from Figure 4, the breakthrough curves of blank 13X, Cu-13X, and Zn-13X at −15 °C showed no significant

As observed, Cu-13X still retained the characteristic peaks of blank 13X, which means that the main structure of blank 13X was not damaged after loading CuCl2. However, the relative height among the three main peaks, in the range of 23−33°, had apparently changed, which showed that CuCl2 had been successfully loaded onto the surface of the channel of the 13X and affected the diffraction in a certain degree. Peaks of CuCl2 were not found on the XRD pattern of Cu-13X. This indicated that CuCl2 had not formed a new crystal phase but was dispersed well on the inner surface of the 13X in monolayer or submonolayer form. Some researchers19−23 have found that the dispersion of CuCl2 on supports such as zeolite is a spontaneous monolayer distribution process if its load does not reach the threshold value, and it consequently would not be examined by XRD. Moreover, we can find that new peaks at 35.6° and 38.8°, which belonged to copper(II) oxides, appeared on the pattern of Cu-13X, which resulted from the hydrolysis of trace CuCl 2 or ion exchange between Cu2+ and the exchangeable Na+ of 13X. On the pattern of Zn-13X, peaks at 6.2° and 10° decreased significantly compared with other characteristic peaks, whereas two new peaks appeared at 17.4° and 31.6°. This means that loading ZnCl2 greatly changed the crystal structure of 13X. Moreover, a dealuminated process, in which Zn2+ entered the basic structure of 13X and formed a new crystal face with Si and O, could have happened in the process of impregnation. The parameters of fresh adsorbents in terms of the NLDFT pore radius, the total pore volume, and the BET surface area are listed in Table 1. In detail, Figure 3 shows the change of 13X in terms of pore size distribution after loading CuCl 2 and ZnCl 2 (the distribution pattern of Zn-13X was amplified in the appended drawing). There was a widespread distribution in the range of 0.6−2 nm around the pore radius of blank 13X, but it was

Figure 4. Comparison of breakthrough curves of blank 13X, Cu-13X, and Zn-13X at −15 °C: (a) Cu-13X at −15 °C, (b) blank 13X at −15 °C, (c) Zn-13X at −15 °C.

differences until 200 min, and the efficient adsorption time of Cu-13X and Zn-13X were both more than 350 min if 1 ppm of PH3 at the column outlet was taken as the breakthrough point. All of them displayed excellent adsorption performance at this temperature. Figure 5 and Figure 6 indicate that apparent decreases both for blank 13X and Zn-13X occurred when the adsorptions were carried out at 5, 20, 35, and 50 °C, at which the ability of Zn-13X in purification was lost gradually but at a slower rate than that of blank 13X. Moreover, we can see that Cu-13X still performed exceptionally well as the adsorption temperature increased to 50 °C. The static adsorption capacities of adsorbents at different temperatures are shown in Figure 7, from which we can find that the capacity of 13X for the removal of PH3 was

Table 1. Specific Surface Area, Total Pore Volume, and NLDFT Pore Radius of Fresh Blank 13X, Zn-13X, and Cu13X adsorbent

specific surface area (m2/g)

total pore volume (mL/g)

NLDFT pore radius (nm)

blank 13X Zn-13X Cu-13X

606.5 19.21 245.2

0.5058 0.0394 0.3694

1.776 0.584 0.637 C

DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Table 2. Resistivity of Silicon, Content of Phosphorus and Boron in Silicon without and with Treatment by Zn-Column

ordinary CVD CVD with Zn-column

resistivity of silicon (Ω·cm)

content of phosphorus (ppt)

content of boron (ppt)

471 766

731 514

95 95

experiments. This proved that Zn-13X had excellent adsorption selectivity between phosphorus and boron. 3.4. Mechanism of Adsorption. The XRD patterns of fresh adsorbents and used adsorbents are shown in Figure 8 Figure 5. Comparison of breakthrough curves of blank 13X, Cu-13X, and Zn-13X at 5 °C: (a) blank 13X at 5 °C, (b) Cu-13X at 5 °C, (c) Zn-13X at 5 °C.

Figure 8. Comparison of XRD patterns for fresh Zn-13X and used Zn13X: (a) Zn-13X, (b) used Zn-13X. Figure 6. Comparison of breakthrough curves of blank 13X, Cu-13X, and Zn-13X at 20, 35, and 50 °C: (a) blank 13X at 20 °C, (b) Zn-13X at 20 °C, (c) Cu-13X at 20 °C, (d) blank 13X at 35 °C, (e) Zn-13X at 35 °C, (f) Cu-13X at 35 °C, (g) blank 13X at 50 °C, (h) Zn-13X at 50 °C, (i) Cu-13X at 50 °C.

Figure 9. Comparison of XRD patterns for fresh Cu-13X and used Cu-13X: (a) Cu-13X, (b) used Cu-13X. Figure 7. Static adsorption capacity of blank 13X, Cu-13X and Zn-13X at different temperatures. a: Cu-13X, b: Zn-13X, c: blank 13X.

and Figure 9. We can see that the peaks of copper(II) oxides almost disappear in Figure 9, whereas the pattern of Zn-13X is intact. This means that copper(II) oxides reacted with PH3, which has been reported by many other studies.8−15 The common opinions in these reports were that copper(II) oxides could convert PH3 into phosphorus or oxides. However, its actual effect for purification was very weak in this study because it was not the main existing species of copper. The performance of Cu-13X and Zn-13X in the process of intermittent experiments are displayed in Figure 10. We can see that there was a notable recovery in the adsorptive abilities of Cu-13X in the second operation, which was restarted after a 60 min pause, whereas it seemed to be negligible for Zn-13X. Figure 11 displays the efficient time of Cu-13X and Zn-13X in the second operations with different pause periods. Taking 5%

significantly promoted by loading CuCl2 and ZnCl2. Moreover, a fact that deserves attention is that by comparing Figure 6 and Figure 7, marked disparity of the capacities between Zn-13X and blank 13X remain even at high temperatures, in contrast to their breakthrough curves under the same conditions. This means that the adsorption rate of Zn-13X for low concentrations of PH3 was as slow as blank 13X at high temperatures, even though its capacities were still much higher. 3.3. Pilot Scale Experiments. The results of the pilot scale experiments are shown in Table 2. The resistivity of silicon was increased by 62.6% owing to the 29.7% decrease in the content of phosphorus from Table 2, whereas the boron content remained unchanged in the D

DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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confirmatory experiments. However, the rate might slow down significantly if the content of CuCl2 dispersed onto the 13X was not sufficient to form a crystal phase. This might be because the interaction between the monolayer CuCl2 and the surface of the 13X had greatly reduced its chemical activity to PH3. Thus, we considered the fact that the chemical conversion rate of PH3 adsorbed by Cu-13X could not instantaneously break the adsorptive balance when the adsorbents were physically saturated, but the breakage of this balance could be achieved by a certain period of pause for the adsorptive operation because PH3 would have sufficient time to be chemically converted by CuCl2. This fact resulted in the performance recovery of Cu-13X in the second adsorptive operation. From Figure 9, we can find that copper(II) oxides were approximately exhausted by only one adsorptive operation. This was because it had a very minute content and had been formed into a crystal phase in the hydrolytic process of CuCl2, which caused it to have good chemical activity with PH3. In Figure 11, we can see that Zn-13X continued its inability to control the PH3 concentration at the column outlet under the breakthrough point even though the pause time was extended to 8 h. This could be because main zinc had been formed in the structure of 13X, which caused its inability to react with PH3 and break the adsorptive balance. Thus, physical adsorption played the primary role in removing PH3 for Zn13X. However, we can still find a weak recovery in the adsorptive abilities of Zn-13X from Figure 10. The cause of this phenomenon might be that there was a small amount of ZnCl2 that did not become a part of the framework of 13X and was dispersed on the surface. The trace residual ZnCl2 would react with PH3 via 3ZnCl2 + 2PH3 → Zn3P2 + 6HCl and have a slight effect on the recovery of adsorptive abilities.

Figure 10. Breakthrough curves in intermittent experiments of Cu13X and Zn-13X at 20 °C when the pause time was 60 min: (a) first for Zn-13X, (b) 60 min pause for Zn-13X, (c) first for Cu-13X, (d) 60 min pause for Cu-13X.

Figure 11. Breakthrough time of Cu-13X and Zn-13X at 20 °C in second adsorptive operation with different pause times: (a) Cu-13X, (b) Zn-13X.

4. REGENERATION OF MODIFIED ADSORBENTS Huge industrial costs and after-treatment problems would be incurred if exhausted adsorbents were thrown away. Thus, it was important to attempt to extend the working life of adsorbents. In this study, we researched the regenerable abilities of blank 13X, Cu-13X, and Zn-13X by blowing pure N2 through the column after saturated adsorption. The performance of the adsorbents regenerated by N2 purging were determined by breakthrough curves, and PH3 concentration at the column outlet during regeneration was also followed. The volumetric flow rate of N2 for regeneration was 200 mL/min, and the purging operations were conducted at 20 °C for Cu13X and at 20, 90, 130, and 160 °C for Zn-13X. Figure 12 shows the adsorptive abilities of Cu-13X after regeneration with N2 purging and after the adsorption pause without N2 purging at 20 °C. Its breakthrough time, which was treated by N2 purging, was approximately twice as long as that which was not in the second adsorptive operation. A higher temperature was needed in the regeneration of Zn-13X based on Figure 13. Under N2 blowing for 4 h, the adsorptive ability had recovered by approximately 40% when the regeneration temperature reached 90 °C. The performance of Zn-13X almost achieved the initial level when the temperature was increased to 160 °C. The reason for this phenomenon might be that more layers of PH3 were adsorbed by Cu-13X than by Zn-13X, and some PH3 of the external adsorption layers was weakly bound on Cu-13X. Moreover, the reactive rate between adsorbed PH3 and loaded CuCl2 was slow, and PH3 could not be chemically converted immediately. Thus, it was possible for this PH3 to be purged by

of PH3 input concentration as the breakthrough point, we can find that the breakthrough time of Cu-13X in the second operation would increase with the growth of the pause time between two operations and then be approximately stable at 400 min. However, Zn-13X still showed no adsorptive abilities when the pause time was extended to 8 h. The significant recovery of the performance indicated that it was a process of physical adsorption coupled with chemical reaction for Cu-13X, in which PH3 was first adsorbed on the surface at a great rate and then underwent a slow chemical conversion. Many mathematical models were developed to describe the mechanisms of adsorption in previous research. Loureiro and Soares et al.24 used the method of characteristics to analyze the propagation of concentration waves in fixed-bed adsorptive reactors. Some other mathematical model studies7,25,26 of instantaneous nonlinear adsorption coupled with finite zero-, first-, and second-order irreversible reaction were also reported by Loureiro et al. They provided detailed descriptions of the influences of the mathematical model and operating parameters on the breakthrough time of adsorptive reactors. Moreover, Friday and co-workers also did many experimental researches about the adsorption equilibrium,27 predicting breakthrough of various challenge chemicals28 and axial dispersion29 of adsorption beds. In this paper, we discuss the PH3 adsorption mechanisms of Cu-13X and Zn-13X with the methods of experiments and XRD characterizations. PH3 could react with CuCl2 via 3CuCl2 + 2PH3 → Cu3P2 + 6HCl, and the reactive rate between them would be very quick if CuCl2 existed in the form of crystal phase according to our E

DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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of fresh blank 13X when the temperature was 50 °C. The performances of three adsorbents in different adsorption cycles are shown in Figure 15. We can find that both Zn-13X and

Figure 12. Breakthrough curves of fresh Cu-13X at 20 °C, Cu-13X regenerated by 60 min N2 purging at 20 °C, and Cu-13X after 60 min operation pause at 20 °C and regeneration curve (RC) of Cu-13X: (a) first for Cu-13X, (b) 60 min pause for Cu-13X, (c) 60 min N2 purge for Cu-13X, (d) RC for Cu-13X.

Figure 15. Breakthrough time of blank 13X (be regenerated at 50 °C), Cu-13X without N2 purging, Cu-13X with N2 purging regeneration at 20 °C and of Zn-13X (be regenerated at 160 °C), in different adsorption cycles: (a) blank 13X at −15 °C, (b) Zn-13X at 20 °C, (c) Cu-13X without N2 purge at 20 °C, (d) Cu-13X with N2 purge at 20 °C.

blank 13X could nearly retain the initial level of PH3 removal if proper regeneration methods were carried out. For Cu-13X, the performance would decrease with decreasing adsorption cycles owing to the consumption of the active component. However, the decreasing trend could be decelerated by N2 purging during regeneration, as shown in the figure, which indicated that N2 purging could really prolong the working life of Cu-13X.

Figure 13. Breakthrough curves of fresh Zn-13X at 20 °C and of Zn13X regenerated by N2 purging at 20, 90, 130, and 160 °C, and regeneration curves (RC) at 20, 90, 130, and 160 °C: (a) RC at 20 °C, (b) RC at 90 °C, (c) RC at 130 °C, (d) RC at 160 °C, (e) first for Zn13X, (f) after regeneration (aR) at 20 °C, (g) aR at 90 °C, (h) aR at 130 °C, (i) aR at 160 °C.

5. CONCLUSIONS The specific surface area, pore size, and pore volume of 13X would decrease after being modified with either CuCl2 or ZnCl2. For Cu-13X, CuCl2 was dispersed well on the surface of 13X in monolayer or submonolayer form, and CuCl2 loading had no marked effect on the crystal structure of 13X. The removal of PH3 was a process of instantaneous adsorption coupled with finite reaction. The adsorption performance of Cu-13X was always outstanding in the temperature range of −15−50 °C, and the static adsorption capacities were much larger than those of Zn-13X and blank 13X. For Zn-13X, a significant change in the structure of 13X zeolite would be caused by ZnCl2 loading during impregnation. Physical adsorption was considered as the main mechanism of Zn-13X in removing PH3, and a low adsorption temperature would be needed to make Zn-13X perform well in PH3 removal according to our research. N2 purging could achieve the reuse of Zn-13X and blank 13X at the required temperature and could decelerate the decreasing trend of Cu-13X in the adsorption cycles because some PH3 in the external adsorption layers was weakly bound with adsorbents and had a slow chemical conversion rate.

N2. Compared with Cu-13X, the adsorption layers of Zn-13X might be much thinner and closer to the surface of adsorbents, so it was difficult to purge PH3 by N2 at moderate temperatures. In Figure 14, we can see that blank 13X that was saturated at −15 °C could be regenerated well at room temperature, and the performance almost could reach the level

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Figure 14. Breakthrough curves at −15 °C of fresh blank 13X and of blank 13X regenerated by N2 purging at 20 and 50 °C, and regeneration curves (RC) at 20 and 50 °C: (a) RC at 20 °C, (b) RC at 50 °C, (c) first for blank 13X, (d) after regeneration (aR) at 20 °C, (e) aR at 50 °C.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b03458 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX