Magnesioaluminophosphate molecular sieves with ATS topology: Synthesis by dry-gel conversion method and catalytic properties in the isopropylation of biphenyl

MgAPO-36, a magnesioaluminophosphate molecular sieves with ATS topology, was successfully synthesized by means of dry-gel conversion (DGC) method. Heating protocol during the synthesis was a key factor for the crystallization of pure MgAPO-36. The best results were obtained by heating at 140 degrees C for 24 h after aging of the gel at 105 degrees C for 48 h. However, ATS phase without magnesium was not obtained under our synthesis conditions because of co-crystallization of AFI phase. MgAPO-36 with high surface area was obtained by the calcination of as-synthesized samples in nitrogen stream or in vacuo; however, the calcinations in air stream resulted in decrease of surface area due to collapse of microporous structure. Characterization was performed by XRD, NH3-TPD, TG-DTA, SEM, N-2 adsorption, and ICP analyses. Bronsted acidic character appeared in MgAPO-36 because of the replacement of a part of Al3+ with Mg2+ in the framework. The selectivity for 4,4'-diisopropylbiphenyl (4,4'-DIPB) was about 30-35% among DIPB isomers in the isopropylation of biphenyl (BP) over MgAPO-36: this value is lower than MOR and other zeolites with straight channels. Because the selectivity of 4,4'DIPB is higher than the composition in equilibrium, the catalysis is controlled by the pores of MgAPO-36. The relative low selectivity for MgAPO-36 is not due to their pore entrance, but to differences in their pore structure. ATS channel with annular side pocket is too loose for selective formation of the transition state to 4,4'-DIPB, and the formation of bulkier transition states for the other isomers, such as 3,3- and 3,4'-DIPB, are also allowed inside the channel. (c) 2005 Elsevier Inc. All rights reserved.