Improving the diffusional performance of traditional microporous zeolites has become of interest to the fields of catalysis, adsorption and separation. The development of hierarchical molecular sieves has occurred to solve the aforementioned issues. This review includes the theoretical analyses about how to improve the efficiency and diffusional performance of molecular sieves during catalytic reactions by recent approaches to synthesize hierarchical molecular sieves. Different hierarchical molecular sieves synthesized via constructive and destructive methods are reported, with emphasis on their strengths and weaknesses, regarding hydrothermal stability, acidity, operational constraints and scalability. This review also summarizes recent catalytic applications, in particular, the applications in catalytic cracking and hydrogenation. It is shown that the incorporation of mesopores to traditional microporous zeolites can improve the activity and selectivity of catalytic cracking and hydrogenation. However, development of hierarchical molecular sieves needs to consider cost, production of high purity products while reducing the production of unwanted by-products, and to provide readily scalable materials. These hierarchical molecular sieves should be able to withstand acidic conditions and have high hydrothermal stability. Development of such materials with the properties described above and structure-property-function relationships of hierarchical molecular sieves should be emphasized in the future.

Contents
1 Introduction
1.1 Background
1.2 Theoretical analyses
2 Methods of preparation of hierarchical molecular sieves
2.1 Destructive strategies
2.2 Constructive strategies
3 Catalytic applications of hierarchical molecular sieves
3.1 Fluidized catalytic cracking
3.2 Hydrogenation reactions
4 Conclusion and outlook