Fabrication of porous supports

A sintered silica support was fabricated from crushed quartz fiber by using a silicon carbide binder (StarPCS SMP-10, Starfire Systems). Silica powder acquired from quartz wool (4-μm diameter) was well mixed with 600 μl of the 10 volume % (vol %) SMP-10 solution in toluene. The mixture was then pressed into a disk using a stainless steel press die with 22-mm diameter at a force of 4000 kg for 30 s. The pressed disks were dried overnight under ambient conditions and then at 70°C for a day. The pressed disks were then thermally treated at 400°C for 6 hours under N₂ atmosphere, to polymerize the inorganic binder, and then thermally treated at 1100°C for 3 hours under air. The ramp rates were 2°C/min for heating and 4°C/min for cooling.

The surfaces of porous silica supports were sequentially polished with 600-grit and 1200-grit SiC polishing papers. Then, the surfaces of supports were further flattened using 500-nm Stöber silica particles, as described previously (27). The top surface of a wet support was manually rubbed with the Stöber silica particles and then thermally treated at 1100°C for 3 hours under air with ramp rates of 2°C/min for heating and 4°C/min for cooling. The rubbing process was typically repeated six to eight times to yield a uniform and flat surface.

The sacrificial silica layers for the secondary growth of zeolite MFI (47–49) were prepared on top of porous silica supports by using 50-nm Stöber silica nanoparticles or mesoporous silica layers. Fifty-nanometer Stöber silica nanoparticles were deposited on porous silica supports with manual rubbing, followed by a heat treatment at 450°C for 6 hours. Alternatively, mesoporous silica layer (SBA-16) was used as a sacrificial silica layer. First, an epoxy layer was applied on the porous silica supports to block the pores between the 500-nm Stöber silica particles. Epoxy solution (5 weight %) prepared in acetone was spin coated on the porous silica supports with 1000 rpm for 1 min. The polymerization of the epoxy layer was facilitated by heat treatment at 70°C for a day. Then, mesoporous silica thin film was fabricated on top of the epoxy layer. Ethanolic Pluronic F127 silica sol, as reported elsewhere (50, 51), was spin coated on the epoxy-coated silica supports with 2000 rpm and 3000 rpm for 30 s each. The mesoporous silica–coated silica supports were dried under ambient condition for 2 days and treated with water vapor at 70°C for 2 days to strengthen the silica framework and prevent pore closing (51). Then, polymeric surfactants and epoxy layers were removed by calcination at 450°C for 6 hours with ramp rates of 1°C/min for heating and 2°C/min for cooling.