Section 2 Preparation of semi-synthetic molecular sieve cracking catalysts

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Since the introduction of zeolite molecular sieves into cracking catalysts in the 1960s, it has not only led to a series of significant reforms in the catalytic cracking process but also opened up a new field in catalysis – zeolite catalysis – which is experiencing rapid development. The molecular sieve initially used to prepare cracking catalysts was X-type molecular sieve, but later Y-type molecular sieve was used instead. Y-type molecular sieves are generally of the REY type (or REHY type). NaY can first be subjected to ion exchange with a rare earth solution to become RE-Y, and then added to the matrix; this is known as pre-exchange ； NaY can also be directly added to the matrix, and then ion exchange with a rare earth solution is carried out together with the matrix to form RE-Y; this is known as post-exchange. Pre-exchange and post-exchange have been applied in industry. During preparation, the location at which the molecular sieve is added can also vary: it is sometimes added to the water glass solution, and sometimes to the silicoalumina wet gel. The parent material of molecular sieve cracking catalysts was initially entirely synthetic aluminum silicate; such products are known as fully synthetic molecular sieve cracking catalysts ； Next, some kaolin is added to the synthesized aluminum silicate, or silica sol, aluminum sol, and similar substances are used as binders to combine the kaolin with the molecular sieve; such a product is known as a semi-synthetic molecular sieve cracking catalyst. Due to its abundant resources and low cost, kaolin generally has a low impurity content and relatively stable quality. As a matrix for cracking catalysts, it offers advantages such as good regenerability, low coke yield, and low sensitivity to metal contamination, thereby enabling the full utilization of the selectivity properties of molecular sieves. Currently, semi-synthetic molecular sieve cracking catalysts are all made by using silica sol or aluminum sol as binders to combine kaolin with molecular sieves. The molecular sieve used can be exchanged either beforehand or afterwards, depending on the specific process. This type of semi-synthetic molecular sieve catalyst, prepared by using a binder for bonding, not only overcomes the drawback of poor wear resistance associated with traditional semi-synthetic catalysts, but also features a simple production process. The resulting catalysts have excellent wear resistance, as well as good properties in terms of regeneration capability, stability, and resistance to metal contamination. As a result, they are widely used; today, most cracking catalysts are semi-synthetic molecular sieve catalysts.