Hits:

Indexed by:期刊论文

Date of Publication:2016-05-01

Journal:MICROPOROUS AND MESOPOROUS MATERIALS

Included Journals:SCIE、EI

Volume:225

Page Number:74-87

ISSN No.:1387-1811

Key Words:SAPO-34; Orientated attachment crystallization; Hierarchical structure; Chloromethane conversion; Light olefins

Abstract:Understanding silicoaluminophosphate formation mechanism lays the foundation for their structure manipulation via crystallization process control. Crystallization of SAPO-34 from a dry gel using tetraethyl ammonium hydroxide as structure-directing agent was monitored to unravel the formation mechanism. The initial gel was found to form a lamellar precursor first, which subsequently underwent phase transformation to discrete SAPO-34 nanocrystallites. The nanocrystallites thereafter mutually aligned with neighboring ones via a non-classic oriented attachment growth mechanism, affording large crystals as a result of grain boundary elimination. A new protocol to prepare hierarchical SAPO-34 was designed by hindering the aggregation of primary nanocrystallites with a growth inhibitor 1,2,3-hexanetriol. The structure of hierarchical SAPO-34 was characterized by XRD, N-2 physisorption, mercury intrusion, SEM, TEM, as well as Al-27, Si-29, P-31 MAS NMR spectra and compared with a conventional SAPO-34. More Si islands were formed via combined SM3 (Al+P pairs substitution by 2Si) and SM2 (P substitution by Si) mechanism for hierarchical SAPO-34 as Si was not fully incorporated into the precursor lamellar phase. NH3-TPD showed that hierarchical SAPO-34 has comparable acidic strength to conventional SAPO-34. The obtained hierarchical SAPO-34 is comprised of <100 nm crystallites and possesses well-connected mesopores, both factors are crucial to mass transfer in zeotype materials. Hierarchical SAPO-34 exhibited a 1.5 times lifetime increase in catalytic chloromethane to olefin conversion with respect to a conventional counterpart. (C) 2015 Elsevier Inc. All rights reserved.