红外表征Y分子筛
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© 1988 Butterworth Publishers
232
EXPERIMENTAL Sample preparation
The pure zeolite Y powder used in this study was obtained in its sodium form from the Wenzhou Catalyst Company. The zeolite was exchanged with NH4CI solution and then reacted with sufficient (NH,02SiF6 solution under pH = 6 at 70°C to prepare SY samples with different Si/AI ratios. EY samples were prepared by treating NaY with H,IEDTA in a Soxhlet extractor and then exchanging the product with NH.aC1. USY samples were obtained from the Wushun Institute of Petrochemical Engineering. The chemical analysis of the starting material and the samples is given in Table 1.
EXPERIMENTAL X-ray diffractometry (XRD)
X-ray diffractograms were recorded with a Rigaku D/Max-IIA diffractometer using CuK0c radiation. The relative crystallinity of the samples was determined by measuring the peak heights of 20 -- 15.7, 20.4, 23.8, 27.2, and 31.5 and then comparing them with the data of the starting zeolite Y. The u.c. size was estimated from the diffraction angles of (822,660), (751,555), (840), (664), (880), (1133), (10100), (997,1193), and (1533) peaks in the X-ray diffractogram of the samples using standard silicon powder as calibration material. An array of a0 were calculated for each sample from these diffrac-
ZEOLITES, 1988, Vol 8, May
Influence o f Si/AI ratio on silicon-enriched faujasites: G. Zi and 7". Yi
ຫໍສະໝຸດ Baidu
Table 1 Sample NHaY SY SY SY SY EY USY
Chemical analysis of the samples NaaO 1.57 1.21 1.37 0.82 0.69 1.19 0.16 AIzO3 (wt%) 16.11 13.12 11.44 10.28 8.58 10.79 17.30 SiO2 49.35 59.00 62.49 64.78 68.59 52.67 58.30 Si/AI 2.59 3.82 4.64 5.35 6.78 4.15 2.86 AI/(AI + Si) 0.279 0.207 0.177 0.157 0.129 0.194 0.259
Keywords:Zeolite Y; thermal stability; i.r. spectroscopy; surface acidity
Gao Zi and Tang Yi
INTRODUCTION
Faujasites have found wide acceptance as industrial cracking, isomerization, and reforming catalysts. It has been generally acknowledged that the thermal stability and catalytic performance of faujasites depend on the Si/A1 ratio. Increasing the relative silicon content of the framework will improve the stability of faujasites toward thermal and hydrothermal degradation and suppress hydrogen transfer reactions. However, most primary synthesis methods have failed to give highly siliceous faujasites. Hydrolysis by hydrothermal treatment of NH4Y zeolite I and extraction by EDTA complexation 2 are used commonly for the dealumination of zeolite Y. Because of the nature of these treatments, a substantial number of vacancies or defect sites and a certain amount of "debris" are eventually formed in the framework. Recently, a novel process, named secondary synthesis, has been developed for the substitution of silicon for aluminum in the zeolite framework using SiCl.l (Ref. 3) and (NH.I)2SiF6 (Ref. 4). Silicon-enriched zeolite Y essentially free of defect structure was reported to be prepared by reacting NH4Y with (NH.02SiF6 solution under mild conditions. "l The object of the present study was to examine by XRD, d.t.a., i.r. and t.p.d, methods the properties of zeolite Y samples with a Si/A1 ratio 2.6-6.8 prepared by secondary synthesis using (NH4)zSiF6 (SY) and to compare them with samples prepared by other methods, such as by EDTA extraction (EY) and hydrothermai treatment (USY).
Influence of Si/AI ratio on the properties of faujasites enriched in silicon
Department of Chemistry, Fudan University, Shanghai, People's Republic of China (Received 14July 1987)
Zeolite Y with different Si/AI ratios prepared by secondary synthesis with (NH4)2SiFs have been studied by XRD, d.t.a., i.r., and t.p.d, methods. In comparison with zeolite Y dealuminated by EDTA extraction and hydrothermal treatment, these zeolites have higher crystallinity and thermal stability. The relationships among crystal collapse temperature, unit cell size, i.r. asymmetric stretch frequency, defect factor, and AI/(AI + Si) ratio of these zeolites have been described. T.p.d. measurements showed that the number of acid sites on the zeolites equals approximately the number of tetrahedral aluminum sites in the framework and their acid strength is stronger than that of conventional zeolite Y. I.r. pyridine adsorption studies indicated that the acid sites are mainly Br6nsted-type acid sites.
RESULTS AND DISCUSSION
T h e physical properties o f the starting zeolite and all the samples are listed in Table 2. For all the SY samples p r e p a r e d by s e c o n d a r y synthesis with (NH4)2SiF6, X-ray crystallinity was maintained. No changes in the b a c k g r o u n d o f the X-ray diffractograms because o f the presence o f a m o r p h o u s material were observed. On the contrary, dealumination using E D T A complexation and h y d r o t h e r m a l treatm e n t resulted in a significant crystallinity loss o f the products. T h e i r relative crystallinity d r o p p e d to 84 and 68%, respectively. Isomorphic substitution o f silicon for aluminum causes a shrinkage o f the u.c. o f zeolite Y as shown in Table 2. T h e plot o f the u.c. size a0 o f the samples versus A1/A1 + Si) is shown in Figure 1, exhibiting a linear relationship for SY samples with Si/A1 ratios less than 5.4. T h e linear relation between ao and AI/(AI + Si) is as follows: a0 = 1.8192 A1 (A1 + Si) + 24.117
232
EXPERIMENTAL Sample preparation
The pure zeolite Y powder used in this study was obtained in its sodium form from the Wenzhou Catalyst Company. The zeolite was exchanged with NH4CI solution and then reacted with sufficient (NH,02SiF6 solution under pH = 6 at 70°C to prepare SY samples with different Si/AI ratios. EY samples were prepared by treating NaY with H,IEDTA in a Soxhlet extractor and then exchanging the product with NH.aC1. USY samples were obtained from the Wushun Institute of Petrochemical Engineering. The chemical analysis of the starting material and the samples is given in Table 1.
EXPERIMENTAL X-ray diffractometry (XRD)
X-ray diffractograms were recorded with a Rigaku D/Max-IIA diffractometer using CuK0c radiation. The relative crystallinity of the samples was determined by measuring the peak heights of 20 -- 15.7, 20.4, 23.8, 27.2, and 31.5 and then comparing them with the data of the starting zeolite Y. The u.c. size was estimated from the diffraction angles of (822,660), (751,555), (840), (664), (880), (1133), (10100), (997,1193), and (1533) peaks in the X-ray diffractogram of the samples using standard silicon powder as calibration material. An array of a0 were calculated for each sample from these diffrac-
ZEOLITES, 1988, Vol 8, May
Influence o f Si/AI ratio on silicon-enriched faujasites: G. Zi and 7". Yi
ຫໍສະໝຸດ Baidu
Table 1 Sample NHaY SY SY SY SY EY USY
Chemical analysis of the samples NaaO 1.57 1.21 1.37 0.82 0.69 1.19 0.16 AIzO3 (wt%) 16.11 13.12 11.44 10.28 8.58 10.79 17.30 SiO2 49.35 59.00 62.49 64.78 68.59 52.67 58.30 Si/AI 2.59 3.82 4.64 5.35 6.78 4.15 2.86 AI/(AI + Si) 0.279 0.207 0.177 0.157 0.129 0.194 0.259
Keywords:Zeolite Y; thermal stability; i.r. spectroscopy; surface acidity
Gao Zi and Tang Yi
INTRODUCTION
Faujasites have found wide acceptance as industrial cracking, isomerization, and reforming catalysts. It has been generally acknowledged that the thermal stability and catalytic performance of faujasites depend on the Si/A1 ratio. Increasing the relative silicon content of the framework will improve the stability of faujasites toward thermal and hydrothermal degradation and suppress hydrogen transfer reactions. However, most primary synthesis methods have failed to give highly siliceous faujasites. Hydrolysis by hydrothermal treatment of NH4Y zeolite I and extraction by EDTA complexation 2 are used commonly for the dealumination of zeolite Y. Because of the nature of these treatments, a substantial number of vacancies or defect sites and a certain amount of "debris" are eventually formed in the framework. Recently, a novel process, named secondary synthesis, has been developed for the substitution of silicon for aluminum in the zeolite framework using SiCl.l (Ref. 3) and (NH.I)2SiF6 (Ref. 4). Silicon-enriched zeolite Y essentially free of defect structure was reported to be prepared by reacting NH4Y with (NH.02SiF6 solution under mild conditions. "l The object of the present study was to examine by XRD, d.t.a., i.r. and t.p.d, methods the properties of zeolite Y samples with a Si/A1 ratio 2.6-6.8 prepared by secondary synthesis using (NH4)zSiF6 (SY) and to compare them with samples prepared by other methods, such as by EDTA extraction (EY) and hydrothermai treatment (USY).
Influence of Si/AI ratio on the properties of faujasites enriched in silicon
Department of Chemistry, Fudan University, Shanghai, People's Republic of China (Received 14July 1987)
Zeolite Y with different Si/AI ratios prepared by secondary synthesis with (NH4)2SiFs have been studied by XRD, d.t.a., i.r., and t.p.d, methods. In comparison with zeolite Y dealuminated by EDTA extraction and hydrothermal treatment, these zeolites have higher crystallinity and thermal stability. The relationships among crystal collapse temperature, unit cell size, i.r. asymmetric stretch frequency, defect factor, and AI/(AI + Si) ratio of these zeolites have been described. T.p.d. measurements showed that the number of acid sites on the zeolites equals approximately the number of tetrahedral aluminum sites in the framework and their acid strength is stronger than that of conventional zeolite Y. I.r. pyridine adsorption studies indicated that the acid sites are mainly Br6nsted-type acid sites.
RESULTS AND DISCUSSION
T h e physical properties o f the starting zeolite and all the samples are listed in Table 2. For all the SY samples p r e p a r e d by s e c o n d a r y synthesis with (NH4)2SiF6, X-ray crystallinity was maintained. No changes in the b a c k g r o u n d o f the X-ray diffractograms because o f the presence o f a m o r p h o u s material were observed. On the contrary, dealumination using E D T A complexation and h y d r o t h e r m a l treatm e n t resulted in a significant crystallinity loss o f the products. T h e i r relative crystallinity d r o p p e d to 84 and 68%, respectively. Isomorphic substitution o f silicon for aluminum causes a shrinkage o f the u.c. o f zeolite Y as shown in Table 2. T h e plot o f the u.c. size a0 o f the samples versus A1/A1 + Si) is shown in Figure 1, exhibiting a linear relationship for SY samples with Si/A1 ratios less than 5.4. T h e linear relation between ao and AI/(AI + Si) is as follows: a0 = 1.8192 A1 (A1 + Si) + 24.117