The SAPO-34/ZSM-5 composite catalyst (SZ-TS) was successfully fabricated using a two-step in situ crystallization method. X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, N 2 adsorption–desorption, and ammonia temperature-programmed desorption measurements were used to characterize the synthesized catalysts and evaluate their catalytic performances on methanol-to-olefin (MTO) reaction. The synthesis mechanism of two-step in situ crystallization was further investigated. Results showed that the SZ-TS synthesized by two-step in situ crystallization featured an intimately bonded composite structure owing to the synergistic effects of the interface. However, the physical mixture of SAPO-34 and ZSM-5 (SZ-PM) presented two discrete phase structures. By using the composite catalyst in MTO reaction, the SZ-TS exhibited the longest life of 1400 min, which was much higher than that of SZ-PM (720 min). Methanol conversion reached 97.3%, and the selectivity for light olefins totalled 93.0%. The excellent catalytic performance of SZ-TS was attributed to the hierarchical pore structure and suitably weak acid site distribution. Via pre-crystallization, alkalinity can be adjusted in the adjacent ranges of SAPO-34 and ZSM-5. By adding the pre-crystallized alkaline SAPO-34 slurry into the strong alkaline system of the parent ZSM-5 slurry, phase dissolution of SAPO-34 in the secondary crystallization solution was avoided. The external surface of SAPO-34 was etched in the alkaline environment of the secondary crystallization solution, resulting in hierarchical pore distribution, which benefited the MTO reaction. This in situ synthesis method features the advantages of simplicity, low time consumption, and low cost. Therefore, this approach possesses the potential to satisfy the demands of the MTO market.
The Cu-Ce modified SAPO-34/ZSM-5 (S-Z) composite molecular sieve catalysts were synthesized via a stepwise crystallization process using triethylamine (TEA) and tetraethylammonium hydroxide (TEAOH) as dual templates. This method significantly reduces the crystallization time from 24 hours to 12 hours. The incorporation of metal ions alters the structure and acid strength of the molecular sieves, thereby decreasing the rate of coke formation during the reaction, which in turn extends the catalyst's lifetime. Experimental results demonstrated that the catalyst's lifetime increased progressively with the addition of Ce. In the methanol-to-olefin reaction, the catalytic lifetime of the S-Z/Cu 0.05 -Ce 0.05 samples reached 1440 mins, approximately 480 mins longer than that of the S-Z samples, while maintaining a selectivity for low-carbon olefins at around 89.6%.
An alternative and promising route for the production of light olefins which realizes the objective of effective resource usage is the coupling conversion of n -hexane-methanol. The SAPO-34@ZSM-5 core-shell composite has been designed and fabricated by the steam-assisted crystallization (SAC) method for boosting the n -hexane-methanol synergies. SAC avoids the undesired phase transformation/dissolution of SAPO-34 caused by exposure to harsh basic conditions of the ZSM-5 precursor solution. In comparison with the nanosized ZSM-5 or physical mixture, the SAPO-34@ZSM-5 core-shell zeolite demonstrates a 32–38 wt% increase in the light olefin selectivity, a 28–67 wt% increase in the n -hexane conversion, and an 89–143% improvement in the catalyst lifetime under identical reaction conditions. With the aid of GC-MS analysis of carbonaceous deposits in spent catalysts, higher polymethylbenzenes concentration and lower polycyclic aromatic hydrocarbon (PHA) concentration are obtained from the extracted species of spent SAPO-34@ZSM-5 in comparison with that of other samples, which suggests the aromatic-based route to light olefins is enhanced and the carbonaceous deposition is slowed down. The core-shell configuration of the SAPO-34@ZSM-5 composite diminishes the diffusion rate of methanol towards the SAPO-34 core layer and increases the coupling of n -hexane derivatives with methanol in the hydrocarbon pool (HCP) route. The decreased number of acidic sites on the external surface of the SAPO-34 core layer may also be accounted for the inhibited carbon deposition.
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