纳米沸石的模板可控去除及分子扩散限制

doi:10.7503/cjcu20150001

摘要/Abstract

摘要：

采用微波水热合成纳米β沸石, 并利用微波辅助的类芬顿法可控去除纳米β沸石孔道中的模板剂, 使其具备一定的孔道深度、比表面积以及可接近酸性位. 以果糖酸催化转化为5-羟甲基糠醛(5-HMF)为探针反应, 探索了果糖分子在不同孔道深度和活性位的纳米β沸石中的扩散行为对催化效果的影响. 研究发现, 酸性位相同的条件下, 纳米沸石孔道深度越深扩散限制越显著, 反应选择性越差. 即使在催化剂加入量相同即可接近酸性位不同时, 果糖分子在孔道中的扩散限制对催化反应的影响也显著于酸量的影响, 充分说明了微孔催化剂中反应物/产物分子扩散限制对于催化反应影响的重要性, 使得所制备催化剂的表面酸性位和适宜的孔道深度共同对反应产生有利的影响.

关键词: 纳米β沸石, 可控去模板, 果糖脱水, 扩散限制

Abstract:

Zeolites are microporous crystalline materials with large surface area and high hydrothermal stability, which have been widely applied in areas like petrochemical industry, fine chemistry and environmental protection. However, their small and long micropore channels result in larger diffusion resistance in catalytic reaction and consequent coke deposition and fast deactivation of catalysts. Here, β nanozeolites were hydrothermally synthesized under microwave irradiation, followed with a microwave-assisted Fenton-like oxidation step to rapidly and controllably remove the template of as-prepared β nanozeolites. Consequently, a series of β nanozeolites with different pore depths and available acid sites was obtained. Still, dehydration of fructose into 5-HMF(5-hydroxymethylfurfural) catalyzed by acid catalyst was used to study the influence of different pore depths and acid sites of β nanozeolites on their catalytic performances. It is found that diffusing limitation displays more important effects than those of the active sites numbers in the reactions catalyzed by microporous zeolites, which further implies the significance of diffusing limitation of reactants/products in catalytic reactions with microporous catalysts, and the necessity of preparing catalysts with hierarchical pores and short pore length, which will make favorable effects on the reaction results by both the influence of acid sites and micropore depths for as-prepared catalyst.

Key words: βNanozeolites, Controllable detemplation, Fructose dehydration, Diffusing limitation

中图分类号:

O643.36

TrendMD:

郭肖, 林少颖, 秦恺, 张亚红, 唐颐. 纳米沸石的模板可控去除及分子扩散限制. 高等学校化学学报, 2015, 36(4): 713.

GUO Xiao, LIN Shaoying, QIN Kai, ZHANG Yahong, TANG Yi. Controllable Detemplation of Nanozeolites and Research of Molecular Diffusing Limitation^†. Chem. J. Chinese Universities, 2015, 36(4): 713.

图/表 10

Fig.1 Illustration of controllable Fenton-like oxidation of nanozeolites

Fig.2 SEM images of β nanozeolites with different micropore depths before/after Fenton-like oxidation (A) β-24; (B) β-12; (C) β-6; (D) β-unoxidized.

Fig.3 XRD patterns of β nanozeolites with different micropore depths before/after Fenton-like oxidation a. β-unoxidized; b. β-6; c. β-12; d. β-24.

Fig.4 TGA curves of β nanozeolites with different micropore depths before/after Fenton-like oxidation a. β-unoxidized; b. β-6; c. β-12; d. β-24.

Fig.5 N2 adsorption-desorption isotherms of β nanozeolites with different micropore depths before/after Fenton-like oxidation a. β-unoxidized; b. β-6, +150; c. β-12, +350; d. β-24, +500.

Table 1 Micropore information for β nanozeolites with different micropore depths

Sample	Micropore area/(m²·g^-1)	Micropore volume/(cm³·g^-1)
β-24	358.2	0.134
β-12	263.8	0.077
β-6	136.6	0.053
β-unoxidized	2.0	0

Fig.6 Total acidity measurements of β nanozeolites with different micropore depths by potentiometric titration in acetonitrile using n-butylamine a. β-unoxidized; b. β-6; c. β-12; d. β-24.

Table 2 Acid strengths and numbers of β nanozeolites with different micropore depths

Sample	E_i/mV	E(≥100 mV)/(mmol·g^-1)	Acid number^*/(μmol·m^-2)
β-24	603.8	0.425	1.187
β-12	579.3	0.335	1.269
β-6	563.0	0.194	1.420
β-unoxidized	-79.4

Table 3 Used amount and micropore properties of β nanozeolites with different micropore depths in the two experiments designs*

Catalyst	Micropore area/(m²·g^-1)	Mass of used β nanozeolites/mg	Total micropore area of used β nanozeolites/m²
β-24	358.2(358.2)	6.29(12.5)	2.25(4.48)
β-12	263.8(263.8)	8.54(12.5)	2.25(3.30)
β-6	136.6(136.6)	16.49(12.5)	2.25(1.71)
β-unoxidized	2.0(2.0)	－ (12.5)	－ (0.025)

Fig.7 Conversions of 5%(mass fraction) fructose catalyzed by β nanozeolites with different micropore depths but the same acid sites numbers(A) or the same catalyst amounts(B) in water under microwave irradiation at 195 ℃ for 24 min

参考文献 21

[1]	Cundy C. S., Cox P. A., Chem. Rev., 2003, 103, 663—701
[2]	Tanabe K., Hölderich W. F., Appl. Catal. A,1999, 181, 399—434
[3]	Möller K., Bein T., Chem. Soc. Rev., 2013, 42, 3689—3707
[4]	Valtchev V., Tosheva L., Chem. Mater., 2005, 17, 2494—2513
[5]	Schoeman B. J., Sterte J., Otterstedt J. E., Zeolites,1994, 14, 110—116
[6]	Muller M., Harvey G., Prins R., Microporous Mesoporous Mater., 2000, 34, 135—140
[7]	Masuda T., Otani S., Tsuji T., Kitamura M., Mukai S., Sep. Purif. Technol., 2003, 32, 181—187
[8]	Heng S., Lau P., Yeung K., Djafer M., Schrotter J., J. Membrane Sci., 2004, 243, 69—78
[9]	Kuhn J., Motegh M., Gross J., Kapteijn F., Microporous Mesoporous Mater., 2009, 120, 35—38
[10]	Parikh A., Navrotsky A., Li Q., Yee C., Amweg M., Corma A., Microporous Mesoporous Mater., 2004, 76, 17—22
[11]	Hu Y. Y., Zhang Y. H., Tang Y., RSC Adv., 2012, 2, 6036—6041
[12]	Maesen T., Kouwenhoven H., Bekkum H., Sulikowski B., Klinowski J., J. Chem. Soc. Faraday Trans., 1990, 86, 3967—3970
[13]	Hu Y. Y., Liu C., Zhang Y. H., Ren N., Tang Y., Microporous Mesoporous Mater., 2009, 119, 306—314
[14]	Xia Y. D., Mokaya R., J. Phys. Chem. B,2006, 110, 9122—9131
[15]	Melian-Cabrera I., Kapteijn F., Moulijn J. A., Chem. Commun., 2005, 21, 2744—2746
[16]	Maurya M. R., Titinchi S. J. J., Chand S., Mishra I. M., J. Mol. Catal. A: Chem., 2002, 18, 201—212
[17]	Rivas F. J., Beltran F. J., Gimeno O., Frades J., J. Agric. Food Chem., 2001, 49, 1873—1880
[18]	Cabrera I.M., Kapteijn F., Moulijn J. A.,Chem. Commun., 2005, 2178—2180
[19]	RaoK. N., Reddy K. M., Lingaiah N., Suryanarayana I., Prasad P. S., Appl. Catal. A: Gen., 2006, 300, 139—146
[20]	KhderA. S., Ahmed A. I., Appl. Catal. A: Gen., 2009, 354, 153—160
[21]	CidR., Pecci G., Appl. Catal. A: Gen., 1985, 14, 15—21