Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and Their Structure-Dependent Catalytic Performance in the Conversion of Methanol to Olefins

Maria Castro, Stewart J. Warrender, Paul A. Wright, David C. Apperley, Youssef Belmabkhout, Gerhard Pirngruber, Hyung-Ki Min, Min Bum Park, Suk Bong Hong

Research output: Contribution to journalArticlepeer-review

35 Citations (Scopus)

Abstract

Details of the synthesis of the small-pore silicoaluminophosphate (SAPO) molecular sieves STA-7 (SAV) and STA-14 (KFI) prepared via a co-templating approach are described. STA-7 includes Si in the framework with Si/(Si+Al+P) ratios varying from 0.04 to 0.17, whereas STA-14 crystallizes from gels with a narrower compositional range (Si/(Si+Al+P) = 0.2-0.3). The main route to Si incorporation is by substitution for P, but in the presence of fluoride, aluminosilicate regions are formed in STA-7. Al-27 3Q MAS NMR studies enable resolution of tetrahedral Al in Al(OP)(4) and Al(OP3,OSi) environments in both as-prepared and calcined materials. For STA-7 with low Si content the presence of five- and six-fold Al coordinated with water molecules or fluoride or hydroxide ions has also been confirmed. Upon calcination, all Al adopts tetrahedral coordination. High pressure, CO2, CO, and CH4 adsorption at 100 degrees C indicates that the proton forms of the SAPO D6R zeotypes present effective polarities intermediate between cationic zeolites and pure silica chabazite. The methanol-to-olefin (MTO) performance of the SAPOs H-SAPO-34, H-STA-7, and H-STA-14 was found to depend oil both crystallite size and cage connectivity and topology. H-STA-7 with a crystal size of 2-3 mu m has MTO stability comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (>= 10 mu m) deactivate rapidly. Ex situ GC-MS analyses of the SAPO catalysts after MTO reaction demonstrate that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons and, hence, their MTO activity and deactivation behavior.

Original languageEnglish
Pages (from-to)15731-15741
Number of pages11
JournalJournal of Physical Chemistry C
Volume113
Issue number35
DOIs
Publication statusPublished - 3 Sept 2009

Keywords

  • CRYSTALLINE INORGANIC SOLIDS
  • MONTE-CARLO SIMULATIONS
  • QUANTUM MAS NMR
  • REACTION-MECHANISM
  • HYDROCARBON FORMATION
  • SILICON INCORPORATION
  • ADSORPTION MECHANISM
  • QUADRUPOLAR NUCLEI
  • MTO REACTION
  • CO-REACTION

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