A coprecipitation method was developed to synthesize fibrous γ-Al 2 O 3 with a large pore volume using a membrane dispersion microreactor with NaAlO 2 and Al 2 (SO 4 ) 3 as reactants. The pore size distribution was controlled because of the high mixing intensity and relatively homogeneous saturation in the membrane dispersion microreactor. The influences of the pH and concentration of the NaAlO 2 aqueous solution and of the two-phase flow rates were investigated. By controlling the pH to be around 9 and the concentration of NaAlO 2 aqueous solution to be 0.50−1.25 mol/L, we obtained γ-Al 2 O 3 nanofibers with pore volume of 0.95−1.52 mL/g. Under the maximal pore volume (1.52 mL/g), the specific surface area was 403.8 m 2 /g and the pore size distribution was 3−50 nm with a distribution variance of 0.2020. The length and diameter of the nanofibers were about 27.3 and 3.4 nm, respectively. This study provides a simple, economical method for preparing fibrous γ-Al 2 O 3 with both a large pore volume and a narrow pore size distribution, which could be used as an excellent catalyst support for the petroleum-refining industry.
A method based on
the combination of in situ small-angle X-ray
scattering (SAXS) and reactive molecular dynamics (MD) simulation
is proposed to investigate the nucleation and initial growth of nanosilica.
The formation and significant growth of silica nuclei within 1 s can
be observed using in situ SAXS in a custom-designed sample cell. Meanwhile,
reactive MD simulation confirms that the nucleation and initial growth
take place over a very short time period and quantitatively describes
the morphology of the formed particles. Both the experimental and
simulation methods reveal changes in the particle size and number.
A sudden increase in particle size is observed in the experiment,
which corresponds to a decrease in particle number, indicating the
transformation of the growth mode from ortho-silicic
acid attachment to particle coalescence. The simulation snapshots
also captures the process of particle coalescence, confirming the
existence of this growth mode. The effects of the synthesis conditions,
including temperature, sodium silicate concentration, and pH value,
are studied and discussed in detail. The results show that the combination
of in situ SAXS and reactive MD simulation is a useful method that
provides detailed information on the preparation of precipitated silica
and its regulatory mechanism.
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