Synthesis and Characterization of Magnesium Substituted Aluminophosphate Molecular Sieves with AEL Structure

Transcription

1 Journal of Natural Gas Chemistry 13(2004) Synthesis and Characterization of Magnesium Substituted Aluminophosphate Molecular Sieves with AEL Structure Benjing Xu, Ling Qian, Xinmei Liu, Chunmin Song, Zifeng Yan State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, University of Petroleum, Dongying , China [Manuscript received May 12, 2004; revised July 08, 2004] Abstract: MAPO-11 molecular sieves were synthesized by hydrothermal methods. The influence of precursor of magnesium, Mg/Al ratio, synthesis temperature, synthesis time and the type of template on the formation and properties of MAPO-11 molecular sieves was examined. The samples were characterized by the techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric/differential thermogravimetric analysis (TG-DGA), etc. The results show that the shape and size of crystal were influenced by the precursor of Mg, the Mg/Al ratio and the type of template, and the TG-DGA analysis shows that MAPO-11 molecular sieves as-synthesized have poor thermal stability. Key words: MAPO-11 molecular sieves, Mg/Al ratio, synthesis temperature, synthesis time 1. Introduction Aluminophosphate (AlPOs) molecular sieves are an important family of molecular sieve materials [1]. But AlPOs possess an electronically neutral framework owing to the strict alternation of AlO 4 ( ) and PO 4 (+) tetrahedral units in their structure and the resultant leads to little acidity, this limits their application as a potential selective adsorbents [2]. Upon incorporation of various metal elements into the framework of aluminophosphate molecular sieves, metaloaluminophosphates (MeAPOs) can be obtained which have acid sites or redox-active sites due to the replacement of P 5+ or both P 5+ and Al 3+ by Me 4+ ions such as Si 4+, Ti 4+, etc, or the replacement of Al 3+ by Me 2+ ions such as Co 2+, Mg 2+, etc [3]. MAPO-11 molecular sieves have been known by researchers since 1986, however, the systematic study on this molecular sieve has not been made. The synthesis and characterization of magnesium substituted AlPOs with AEL type structure (MAPO-11s) are described in this paper. And the influence of some important synthesis parameters such as temperature, template, Mg/Al ratio and conditions of gel synthesis were examined. 2. Experimental 2.1. Synthesis of MAPO-11 molecular sieves MAPO-11 molecular sieves were synthesized by hydrothermal methods. The synthesis procedure was as follows: the required amount of aluminum isopropoxide (99%, Aldrich), phosphoric acid (85%, Aldrich) and deionized water were mixed together. This mixture was stirred for a while before adding magnesium chloride (98%, Aldrich) or magnesium acetate (99%, Aldrich), and then organic templates (din-propylamine (DPA) (99%, Aldrich) and diisopropylamine (DIPA) (98%, Aldrich)) were added. The final gel molar composition was: 0.9Al 2 O 3 :P 2 O 5 :R: xmgo:50h 2 O (R represents organic template). After the ph was adjusted to the suitable value and the mixture was stirred for another hour or so, it Corresponding author. zfyancat@hdpu.edu.cn

2 232 Benjing Xu et al./ Journal of Natural Gas Chemistry Vol. 13 No was transferred into a stainless-steel autoclave lined with polytetrafluoroethylene and heated in an oven at K for h. The product was filtered off, washed with deionized water and dried at 373 K Characterization methods The X-ray diffraction (XRD) patterns were obtained using a diffractometer D/max-3A (Rigaku Co., Japan) with Cu K α radiation. The scanning electron micrographs were made on a JEM-5410 LV (Jeol Co. Japan) electronic mirror with accelerated volt of 20 kv and magnified times of TGA- DGA (thermal gravimetric analysis and differential gravimetric analysis) measurements were obtained on a TA Instruments (Beijing Analyzing Co., China) in flowing nitrogen with a temperature ramp of 10 K/min to 1273 K. 3. Results and discussion 3.1. Inf luence of magnesium precursors on the formation of MAPO-11s Figure 1 indicates that the three samples have the same crystalline structure as that of standard AELtype molecular sieves [4]. It also shows that the main XRD peak intensities of MAPO-11 molecular sieves are lower than that of AlPO-11 molecular sieve. Figure 1. XRD patterns of MAPO-11 molecular sieves synthesized with different Mg precursors (a) AlPO-11, (b) MAPO-11(MgCl 2 ), (c) MAPO-11(Mg(Ac) 2 ) Probably, (1) the existence of Mg may alter the growth directions of crystal surface; and/or (2) the introduction of Mg to the framework may result in the formation of non-framework species, which counteract the growth of different crystal surfaces as explained in Ref [5]. Anyway, Figure 1 clearly demonstrated that Mg can be incorporated into the framework of AlPO-11 molecular sieves, but not all of them can be incorporated. On the other hand, it also shows that for MAPO-11 synthesized by using MgCl 2 as the magnesium precursor, the intensity of the main peak at 2θ=21.3 o, corresponding to d(002), was extremely reduced. This is different from those of as-synthesized AlPO-11 and MAPO-11 molecular sieves synthesized by using the Mg(CH 3 COO) 2 as the magnesium precursor. Usually, anions are considered to have so little effect on the formation of zeolites that they are always ignored. However, haloid ions (Cl, Br, I ) exert an influence on the synthesis of X zeolites [6,7]. Therefore, it is thought that the existence of Cl in the sol brings about the observed difference which should be studied in more detail. Figure 2 shows that the morphology of MAPO- 11 molecular sieve synthesized by using Mg(Ac) 2 as the Mg precursor is similar to that synthesized by using MgCl 2 as the Mg precursor, but the dimensions of these two samples are different, which may be attributed to the different reaction ability of MgCl 2 and Mg(Ac) 2 during the crystallization. Hereafter, the MAPO-11 molecular sieves were all prepared by using MgCl 2 as the Mg precursor at 453 K for 24 h, if not mentioned specially.

3 Journal of Natural Gas Chemistry Vol. 13 No Figure 2. SEM images of MAPO-11 molecular sieves samples synthesized with different Mg precursors (a) Mg(Ac) 2, (b) MgCl Influence of Mg/Al ratio on the formation of MAPO-11 Figure 3 indicates the AEL structure can be obtained from Mg/Al=0 to Mg/Al=1/3 when the original mixture was heated at 453 K for 24 h and the mixture of DPA and DIPA were used as the template. The degree of crystallinity decreases with the increase of Mg/Al ratio. Meanwhile, higher ratios of Mg/Al results in the formation of other crystal phase which is mainly MAPO-5 with AFI structure, and the amount of it increases with the increase of Mg/Al ratio. At the same time, we can see that the relative XRD intensities of different peaks change with the increase of Mg/Al ratio, which may be caused by the formation of non-framework species which counteract the growth of different crystal surfaces, and this function is different in our present three cases. Figure 3. XRD patterns of MAPO-11 with different Mg/Al ratios (a) Mg/Al=1/9, (b) Mg/Al=2/9, (c) Mg/Al=1/3 Figure 4 shows that the morphology of these assynthesized samples varies with Mg/Al ratios, from special shape to disorder which is in accord with the results of XRD analysis. The amorphous species are mainly non-framework species. The formation of nonframework species results from the saturation of negative charge caused by Mg substitution of Al in the framework.

4 234 Benjing Xu et al./ Journal of Natural Gas Chemistry Vol. 13 No Figure 4. SEM images of MAPO-11 samples with different Mg/Al ratios (a) Mg/Al=1/9, (b) Mg/Al=2/9, (c) Mg/Al=1/ Inf luence of temperature on the formation of MAPO-11 The different growing surfaces of a crystal have different activating energy. Therefore, the crystalline temperature affects on the growing velocity of various crystalline surfaces via influencing their activating energy [6]. XRD patterns of MAPO-11 synthesized at different crystalline temperature are shown in Figure 5. The decrease in the intensity of peaks displays that the degree of crystallinity decreases with the decrease in crystalline temperature. At the same time, with the decrease in crystalline temperature the intensity of the peaks related to other phase (MAPO-5 with AFI structure) increased as shown in Figure 5. Figure 5. XRD patterns of MAPO-11 with different crystalline temperatures (a) 453 K, (b) 433 K, (c) 413 K

5 Journal of Natural Gas Chemistry Vol. 13 No Inf luence of synthesis time on the formation of MAPO-11 Because of the low stability of molecular sieves, one crystal phase can be changed to other crystal phases. Therefore synthesis time is a very significant factor in the synthesis of molecular sieves. From Figure 6, it can be seen that the degree of crystallinity varied with the time going on and it is found that there were no formation of MAPO-5 phase when synthesis time was 24 h or 72 h. When the synthesis time was 12 h, MAPO-5 is formed heavily, which may imply that MAPO-5 phase is the precursor of MAPO-11 phase. All the above results indicate that MAPO-11 and MAPO-5 can be transformed from one phase to the other as the synthesis time is increased. Figure 6. XRD patterns of MAPO-11 with different synthesis time (a) 12 h, (b) 24 h, (c) 48 h, (d) 72 h, (e) 96 h 3.5. Influence of template on the formation of MAPO-11 SEM photos of MAPO-11 synthesized by using different organic templates are shown in Figure 7. Although the MAPO-11s synthesized by using diisopropylamine and the mixture of diisopropylamine and dipropylamine (molar ratio 1:1) as the template respectively are very similar in their crystalline morphologies, the particle size of these two samples are different. This difference is resulted from the own crystalline structure of MAPO-11, and the different structures and spacial dimensions of diisopropylamine and dipropylamine [8].

6 236 Benjing Xu et al./ Journal of Natural Gas Chemistry Vol. 13 No Figure 7. SEM images of MAPO-11 synthesized by using different organic templates (a) diisopropylamine as template (b) the mixture of diisopropylamine and dipropylamine (molar ratio 1:1) as template 3.6. Thermogravimetric/dif ferential gravimetric analysis (TGA/DGA) of MAPO-11s Thermogravimetric/differential gravimetric analysis of MAPO-11 synthesized by Mg(Ac) 2 as the Mg precursor is shown in Figure 8. The results are similar to those reported in Ref [9]. It was reported that there are three steps of weight loss: (1) between K was attributed to the desorption of water, (2) between K was attributed to the desorption of physisorbed amine, (3) between K was attributed to the Hoffman degradation on protonated amine. These authors also suggested a weight loss of the third step will be increased with the increase in magnesium content. This is the evidence of magnesium isomorphous substitution in the AEL structure. Figure 8. TG and DGA curves of MAPO-11 molecular sieves under nitrogen atmosphere However, the thermogravimetry of as-synthesized samples under nitrogen atmosphere displays four steps of weight loss. The forth weight loss is suggested to be caused by the collapse of framework. The framework of MAPO-11 molecular sieves can no longer be detected when the temperature goes up to 1073 K as we can see from Figure 9.

7 Journal of Natural Gas Chemistry Vol. 13 No Conclusions The results show that MAPO-11 molecular sieves can be prepared by hydrothermal methods using magnesium chloride or magnesium acetate as the magnesium precursor. The formation of MAPO-11 molecular sieves is affected by synthesis temperature, synthesis time, the Mg/Al ratio and the type of template, etc. And the best synthesis condition is that Mg/Al ratio is not over 2/9, synthesis temperature and synthesis time are 453 K and 24 h respectively. The assynthesized MAPO-11 molecular sieves exhibit poor thermal stability. References Figure 9. The XRD of MAPO-11 molecular sieves calcined at different temperatures (a) Not calcined, (b) Calcined at 823 K for 4 h, (c) Calcined at 1073 K for 4 h [1] Wilson S T, Lok B M, Messina C A et al. J Am Chem Soc, 1982, (104): 1146 [2] Qiu S, Tian W, Pang W et al. Zeolits, 1991, (11): 371 [3] Zhao X S, Lu G Q (Max). Microporous and Mesoporous Materials, 2001: 44 45, 185 [4] Stenphen T Wilson, Brent M Lok, Edith M Flanign. US [5] Wang Z M, Yan Z F. Acta Petrolei Sinica (Petroleum Processing Section), 2001, 17(5): 77 [6] Xu R R, Pang W Q. The Chemistry of Inorganic Synthesis and Preparation, Beijing: High Education Press, 2001: 431 [7] Cocks P A, Pope C G. Zeolites, 1995, (15): 701 [8] Liu Y M, Zhang F M, Shu X T. Acta Petrolei Sinica (Petroleum Processing Section), 2002, 18(6): 28 [9] Romilda Fernandez, Marcus V Giotto, Heloise O Pastore et al. Microporous and Mesoporous materials, 2002, (53): 135