Microwave Reaction Enhancement: The Rapid Synthesis of SAPO-11 Molecular Sieves

Abstract

The effect of microwave and conventional heating methods on the nucleation and growth of SAPO-11 molecular sieve was investigated. Microwave synthesis enhances both the nucleation and growth of SAPO-11. The rate of heating primarily influences the nucleation time for both microwave and conventional heating. A more uniform morphology and narrower size distribution are formed using microwave heating compared to conventional. This suggests that rapid and even nucleation is the enhancement mechanism. Crystallization by conventional heating can be accelerated by immersion in the pre-heated fluid due to the faster heating rate and improved thermal conduction. Microwave synthesis of SAPO-11 occurs in minutes and, thus, significant energy savings can be realized compared to the hours required for conventional synthesis.

Microwave Synthesis of SAPO‐11 and AlPO‐11: Aspects of Reactor Engineering

“…The higher activation energy calculated for microwave synthesized SAPO-11 (and AlPO-11) compared to conventional heating is due to the fact that microwave synthesis is activated by both thermal and non-thermal effects. [18] An increase in activation energy, for these reactions using microwave irradiation, is contrary to many reports that show a decrease in E a with microwave heating and hence accelerated rates due to a lower energy barrier. Furthermore, Gizdavic-Nikolaidis et al [42] reported an increase in the activation energy from conventional to microwave synthesis for NaA synthesis.…”

“…[18] A precursor solution was prepared by a mixture of two organic structure directing agents prepared by combining an aqueous solution consisting of 11.53 grams of 85 wt. % orthophosphoric acid and 22.0 grams of water with 6.9 grams of a hydrated aluminum oxide (a pseudo-boehmite, 74.…”

“…[17] reported the rapid synthesis of AlPO-11 using microwave hydrothermal processing at 150 8C within 2.5 h with no pretreatment. Recently, we reported [18] on the conventional and microwave synthesis of SAPO-11 using a duel-template system of n-Pr 2 NH (n-dipropylamine) and TBAOH (tetrabutylammonium hydroxide). Using a precursor composition of 1.0 Pr 2 NH: 0.5 A C H T U N G T R E N N U N G (TBA) 2 O: Al 2 O 3 : P 2 O 5 : 0.4 SiO 2 : 50H 2 O reacted at 175 8C, SAPO-11 was observed to nucleate after only 2 min and reach full crystallinity after 10 min, compared to 90 min and 720 min respectively for conventional heating.…”

“…The rate of heating primarily affects the nucleation time for the microwave synthesis. [18] A more uniform morphology and narrower size distribution were formed using microwave heating compared to conventional. This could indicate that either, rapid and even nucleation is occurring or that microwave heating yields a more uniform temperature distribution within the reacting system as suggested by de la Hoz et al [19] for organic synthesis.…”

“…All X-ray patterns were matched to standard diffraction pattern for aluminophosphate hydrate ICDD #43-0563. The crystallization curves were generated as described in Gharibeh et al [18] Crystallization rate was calculated as the slope of the linear region of the crystallization curve between 0.2 and 0.8 relative crystallinity. Nucleation time is defined as the reaction time required before 1 % relative crystallinity is observed by XRD.…”