Enhanced surface acidity of Si–Zr-MCM-41 mesoporous molecular sieve by sulfation and its catalytic activity in cyclic acetalyzation and functional group protection by pyranylation

Si-MCM-41 and Si–Zr-MCM-41(Si/Zr = 20) were synthesized hydrothermally using organic precursors and the materials were characterized by XRD, BET, NH3-TPD and XRF spectroscopy. The BET analysis showed bimodal distribution of pores in Si–Zr-MCM-41 structure. The surface area and pore diameter of the material diminished due to the incorporation of zirconium. An attempt was made to generate super acidity on Si–Zr-MCM-41(Si/Zr = 20) (referred as SiZrM20) for the first time by sulfation. The catalytic activity was examined in cyclic acetalyzation of substituted aromatic aldehydes and ketones with diol under solvent free autogeneous condition, and protection of alcoholic –OH group by pyranylation with dihydropyran in liquid phase at room temperature. Carbonyl compounds with electron withdrawing substituent formed respective cyclic acetal and ketal in good yields. Under mild conditions various alcohols formed the corresponding tetrahydropyranyl ethers (THP), in short reaction time. Hence the materials can be a convenient ecofriendly catalyst for cyclic acetalyzation and alcoholic –OH group protection by pyranylation.

Graphical abstractFor the first time sulfation was done on metal oxide (ZrO2) – mesoporous system synthesized using organic precursors. The NH3-TPD showed the formation of strong acid sites (high temperature region) HT on Zr-MCM-41 after sulfation. It catalyzed the protection of functional groups like alcoholic hydroxide by pyranylation and carbonyl group in aromatic aldehydes and ketone by acetalyzation in heterogeneous system. It revealed the material is a novel solid acid catalyst for acetalyzation and pyranylation and it is suitable catalyst for different reaction conditions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► For the first time SO42− impregnated on ZrO2–Si-MCM-41, generates strong acidity. ► BET revealed the presence of bimodal mesopore system in Zr–Si-MCM-41. ► The order of the catalytic activity was as follows SAZrM ≈ 8ASZrM > SiZrM20  > Si-MCM-41. ► Acetalyzation of acetophenone resist in forming cyclic ketal, took 5 h. ► 1:2 Ratio of EG:DHP needed to form THP ether of EG with in 90 min .