One Step Further in Understanding the Role of Hydrogen Bonds in Directing the Synthesis of CHA Analogue Molecular Sieves

In this work, the synthesis of CAL-4, a SAPO-44 structural analogue, is studied. This silicoaluminophosphate molecular sieve was prepared starting from the layered aluminophosphate n-butylammonium-AlPO-kanemite with the addition of silica and cyclohexylamine (CyHA). SAPO-44 was prepared using the conventional procedure and independent sources of silicon, aluminum, and phosphorus and the same structure-directing agent (CyHA). The materials prepared were compared in Fourier transform infrared (FTIR) light that showed the presence of CyHA and n-butylammonium (n-BA) in the chabasite cavities of CAL-4, while only CyHA was found in SAPO-44. No interaction between the codirecting agents CyHA and n-BA was detected by a Fourier transform infrared spectroscopy and single-crystal X-ray diffraction (XRD) combined study, contrary to what was found previously in another member of the same family of SAPO molecular sieves (Angew. Chem., Int. Ed. 2007, 46, pp 8895-8897). This study proposes that the H bond interaction of protonated CyHA with framework oxygen atoms through NH3+ groups prevented the formation of a supramolecular arrangement with n-BA and, thus, probably led to the formation of this particular chabasite analogue. This paper represents another step toward understanding the role of host-guest and/or guest-guest interactions in directing the synthesis of molecular sieves. The three cases studied here provide clear evidence of the key role of H bonds between structure-directing agent (SDA) amines and their framework in driving the synthesis toward a defined structure.