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With the aim to optimize alkaline and acid treatment of zeolites to obtain hierarchical mesoporous zeolites,dissolution and absorption mechanisms relevant to mesopore formation were investigated at a molecular level by combining density functional and reactive molecular dynamics calculations.In the alkaline dissolution processes,dealumination is energetically more favorable than desilication,though both processes can occur.This theoretical results supports the recent experimental hypothesis and clarifies the misunderstandings of "alkaline dissolves Si" lasting for several decades.On the other hand,the dissolved Al species prefer to be absorbed back onto zeolite surfaces whereas the dissolved Si species tend to aggregate in alkaline solution.Our study of acid treatment confirms that "acid prefers to dissolve Al" as common belief.The dissolution process promotes but the absorption process hampers the mesopore formation,laying foundation for understanding the mesoporosity influenced by the variations of zeolite framework and solution in alkaline and acid treatment.