Controllable Detemplation of Nanozeolites and Research of Molecular Diffusing Limitation†

doi:10.7503/cjcu20150001

Abstract

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

CLC Number:

O643.36

TrendMD:

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

Figures/Tables 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

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