Effect of silicon content on the catalytic behavior of chabazite type silicoaluminophosphate in the transformation of methanol to short chain olefins

Abstract

AlPO-34 and SAPO-34 molecular sieves with different silicon content have been synthesized using a hydrothermal system in presence of tetraethylammonium hydroxide (TEAOH) and evaluated as catalysts for the transformation of methanol to short chain olefins. The results showed that SAPO-34 materials prepared with medium-high silicon concentration in the framework present quite similar silicon environment distributions 2 and, when it is so, the key factor to obtain materials with improved catalytic performance is the crystallite size.

Complex relationship between SAPO framework topology, content and distribution of Si and catalytic behaviour in the MTO reaction

“…2). As reported before for other silicoaluminophosphates, 32 both parameters seem to be influenced by the Si content. However there is not a common pattern applicable to the different topologies reported in this work.…”

“…In our experience, most of the SAPO samples incorporate Si easily up to a certain concentration, and normally the molar Si/Al ratios in the solids are close to that from gels. 32,41 However, the topological differences and the structure directing agent used in the synthesis also play an important role, controlling both the silicon incorporation and the silicon distribution in the framework. 46 As shown in Table 1, the silicon content of all SAPO samples increases with the amount of silicon precursor in the synthesis gels, although each framework topology shows different silicon incorporation yield.…”

“…22,23,27,47 Besides, it is known for some SAPO materials that the silicon concentration in the synthesis gel influences the size of the obtained crystals. 32 The crystallite size and morphology of the SAPO samples obtained have been studied by scanning electron microscopy (Fig. 2).…”

“…Some authors have reported on the effect of the silicon content in the catalytic performance of SAPO catalysts. [31][32][33][34] The framework of microporous aluminophosphates is built up of AlO4 and PO4 tetrahedra connected through the oxygen atoms in an alternating arrangement. SAPO materials are formed by the isomorphous substitution of trivalent Al and pentavalent P atoms by tetravalent Si atoms.…”

Synthesis of nano-SSZ-13 and its application in the reaction of methanol to olefins

“…4,[13][14][15][16] In the case of SAPO-34, its performance has been improved by developing mesopores [17][18][19][20][21][22] and by synthesizing crystals of smaller size. [23][24][25][26][27][28][29][30][31] The use of dual templates, which implies small organic struture directing agent for the formation of micropores and large organic molecular surfactants for developing mesopores as a strategy to prepare meso-MFI, has been proposed 32,33 . The large molecular surfactant hexadecyl trimethyl ammonium bromide (CTAB) firstly used to prepare MCM-41 has been employed to the synthesis of hierarchical zeolites in the dual template method.…”