High resolution magic angle spinning (MAS) NMR is a powerful technique for studying structure and dynamics in a heterogeneous system containing a mixture of e.g., solid, semi-solid, liquid, and gaseous phases. Due to its intrinsic advantage of probing local structure at molecular level, MAS NMR is an attractive tool for in situ (operando) investigations of reaction mechanisms, including but not limited to the identification of active centers, intermediates, and the reaction dynamics associated with material synthesis or chemical reactions using solid catalysts. However, the commercially available reusable-MAS rotors are rarely capable of achieving 100% seal while fast spinning even at ambient conditions. In particular, reusable sample cells that can efficiently perform at combined high temperature (> 100 ?C) and high pressure (> 10 atm) are challenging due to technical complications associated with sealing heterogeneous solid/fluid/gaseous samples at high temperature and pressure while spinning at several kHz or more inside a strong magnetic field. Recently, we have developed a perfectly sealed MAS NMR rotor that is capable of sealing a heterogeneous sample containing solid, semi-solid, gases and liquids or a mixture of them under extreme experimental conditions of combined high pressure and high temperature.1 Herein, we would like to introduce this powerful operando NMR technique to the catalysis community with new examples of application in material synthesis and catalytic reactions, including but not limited to (a) Mechanisms of phenol-cyclohexanol alkylation in zeolite H-BEA studied by 13C and 1H NMR,2-3 (b) The crystallization of AlPO4-5 by 1H, 27Al, and 31P NMR,4 and (c) Genesis and stability of hydronium ions in zeolite channels by 1H and 1H-29Si CP NMR, and (d) Zeolite framework stability and degradation in hot water studied by high field and fast spinning 27Al MAS NMR.