Synthesis, characterization and catalytic properties of silicoaluminophosphate microporous molecular sieves
Date
2014-02-06
Authors
Ramaripa, Phuti Suzan
Journal Title
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Abstract
In this study, an attempt was made to synthesis SAPO-34 using piperidine as
structure directing agent/ template. The synthesis parameters such as crystallization
temperature, crystallization time, phosphorus content and silicon content, as well as
Na, Fe, Co and Ni impregnation were varied. The resulting materials were
characterized by a combination of physicochemical techniques such as XRD, SEM
spectroscopy, NH3-TPD and SS NMR spectroscopy. The peaks at 2θ = 9.9⁰, 13.0⁰,
15.0⁰, 18.0⁰, 21⁰, 26⁰, 32⁰ on XRD patterns confirmed the presence of SAPO-34
phase, extra peaks at 2θ = 7.9 ⁰, 20 ⁰, 23 ⁰and 26 of XRD patterns indicated the
existence of SAPO-5. Most of the samples of the molecular sieves comprised SAPO-
5 and SAPO-34. The SAPO-5 materials were the main domain and SAPO-34
materials were less. The two peaks that appeared in the temperature regions
between the ranges 190-250 ⁰C and 340-380 ⁰C that were assigned for weak and
medium BrØnsted acid site on NH3-TPD profiles. The SS NMR spectroscopy and
XRD patterns confirmed a SAPO-34 phase with amorphous phase in a sample
prepared with SiO2/Al2O3 = 2.4 molar ratio at 200 ⁰C for 5 days. The NH3-TPD profile
and SS NMR spectroscopy for this molecular sieve confirmed the presence of weak
and strong acid sites assigned to P-OH-Al and Si-OH-Al hydroxyl groups. All the
molecular sieves were used in MTO reaction. The selectivity to ethylene and
propylene on the molecular sieves prepared at 190 ⁰C for 5 days is better than the
selectivity to ethylene and propylene on the molecular sieve synthesized at 170 ⁰C,
200 ⁰C and 210 ⁰C for 5 days. Molecular sieve prepared at 200 ⁰C for 4 days was
better in ethylene and propylene selectivity than the molecular sieve prepared at 200
⁰C for 2-3 days, 5-6 days. Methane and DME were the main products on most of the
molecular sieves. Methane, ethylene and propylene and some impurities were the
products on molecular sieve prepared with SiO2/Al2O3 = 2.4 molar ratio in MTO
reaction. The conversions of DME towards light olefins were extremely low. Na-
SAPO reduced methane, and improved ethylene and propylene selectivity. Nonmodified
and Co-SAPO molecular sieves improved the selectivity towards light
olefins. Piperidine can be used as a template to synthesis SAPO-34 molecular sieve
within a short period of five days. SAPO-5 was synthesized instead of SAPO, and
DME was the main product instead of light olefins on the molecular sieves during
MTO reactions.