Effect of water on the organofunctionalization of SAPO-5 to form an efficient oleic acid esterification catalyst

Abstract

In this study, SAPO-5 having a high silicon content was synthesized and further modified by functionalization with sulfonic acid groups using 3-mercaptopropyltrimethoxysilane (MTPS) as a precursor. In particular, we focused on the effect of water on the organofunctionalization process using SAPO-5 either dehydrated under vacuum (S5–SO3H/Vac) or without any thermal pretreatment (S5–SO3H). Using X-ray diffractometry, inductively coupled plasma–optical emission spectroscopy, elemental analysis of carbon, nitrogen, hydrogen and sulfur, thermogravimetric analysis, Fourier transform infrared spectroscopy, and 29Si, 27Al, 31P, and 13C NMR spectroscopy, it was possible to identify important structural differences in the obtained hybrid materials. The results showed that S5–SO3H has a Si:S molar ratio five times larger than that of S5–SO3H/Vac. The presence of Tm sites (organic-modified silica sites, Si–C bonds), as revealed by 29Si MAS-NMR measurements, suggests that the organic pending groups are bonded covalently to the Si–OH groups in the SAPO-5 network during silanization. Further differences between the material obtained under dry conditions and that prepared from hydrated SAPO-5 were revealed by measuring the acidity via potentiometric titration: S5–SO3H was 40% more acidic than S5–SO3H/Vac. Both materials were also tested for their catalytic oleic acid esterification ability. Although the turnover frequency of S5–SO3H/Vac was four times higher than that of S5–SO3H, its conversion after five sequential cycles dropped from 97.4 to 28.3%. In contrast, S5–SO3H yielded a conversion of 84.4%, starting from 99.4%, after the same number of cycles; thus, microporous SAPO-5 is a promising catalyst support for reactions involving bulky molecules such as oleic acid.