Fabrication of the Hierarchical HZSM-5 Membrane with Tunable Mesoporosity for Catalytic Cracking of n-Dodecane

Hierarchical HZSM-5 membranes were prepared on the inner wall of stainless steel tubes, using amphiphilic organosilane (TPOAC) and mesitylene (TMB) as a meso-porogen and a swelling agent, respectively. The mesoporosity of the HZSM-5 membranes were tailored via formulating the TPOAC/Tetraethylorthosilicate (TPOAC/TEOS) ratio and TMB/TPOAC ratio, in synthesis gel, and the prepared membranes were systematically characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), N2 adsorption–desorption, N2 permeation, inductively coupled plasma (ICP), in situ fourier transform infrared (FT-IR), ammonia temperature-programmed desorption (NH3-TPD), etc. It was found that the increase of the TPOAC/TEOS ratio promoted a specific surface area and diffusivity of the HZSM-5 membranes, as well as decreased acidity; the increase of the TMB/TPOAC ratios led to an enlargement of the mesopore size and diffusivity of the membranes, but with constant acid properties. The catalytic performance of the prepared HZSM-5 membranes was tested using the catalytic cracking of supercritical n-dodecane (500 °C, 4 MPa) as a model reaction. The hierarchical membrane with the TPOAC/TEOS ratio of 0.1 and TMB/TPOAC ratio of 2, exhibited superior catalytic performances with the highest activity of up to 13% improvement and the lowest deactivation rate (nearly a half), compared with the microporous HZSM-5 membrane, due to the benefits of suitable acidity, together with enhanced diffusivity of n-dodecane and cracking products

Catalytic Cracking of Endothermic Hydrocarbon Fuels over Ordered Meso-HZSM‑5 Zeolites with Al-MCM-41 Shells

Ordered meso-HZSM-5 zeolites with Al-MCM-41 shells were synthesized by a surfacant-directed encapsulating process. It was found that ordered meso-HZSM-5, which was used as a core, could be synthesized using a designed amphiphilic organosilane as a surfactant and that the mesopore size of the MCM-41 shell can be adjusted from 2.5–3 to 4.5–10 nm by changing the structures of alkyltrimethylammonium bromide and the trimethylbenzene amount. Catalytic cracking performances of supercritical <i>n</i>-dodecane (500 °C and 4 MPa) show that ordered meso-HZSM-5 zeolites with Al-MCM-41 shells of 4.5–10 nm exhibit a 28% higher catalytic activity and a 25% lower deactivation rate compared to the conventional HZSM-5 zeolites. With an increasing mesopore size of the MCM-41 shell, the acid sites accessibility increased gradually and, thus, led to enhanced catalytic activity and decreased secondary reaction ability. In addition, ordered hierarchical zeolites show a well connection of ordered mesopores between the core and shell, which could also lead to the enhanced acid site accessibility