H2O: Si molar ratio

In monolith febrication it is required to take into account the influence of the precursor gradient chemical composition on the sol-gel process. Parameters that affect the process are metal alkoxide precursor types, pH effect - type of catalyst used, H2O Si molar ratio (/ ), steric effect of precursor ligand groups, and functionality of organically modified silanes (ORMOSIL). 

SnCU-SHaO 221 H2O. The materials are designated as NxSnOa-MSM and HxSn02-MSM, where x is the Sn/Si molar ratio in the gel, N and H represent for the materials synthesized without or with hydrothermal treatment for 24 h. 

H2SO4 combined with water was mixed with 1-propanol. TEOS was then added dropwise under continuous stirring. Concentration of TEOS was 2 mol/1 and the amount of aqueous H2SO4 was adjusted in order to obtain a molar ratio a = H2S04/Si of 0.06 and a hydrolysis ratio h = H2O/TEOS = 4. Finally, the molar ratio Z = Zr/Si was fixed at 0.05 and 0.1. Sample with Zr/Si equal to 0 was used as a reference. 

The EMT- and FAU- type samples were prepared according to Delprato at al. [16] in the presence of the crown ethers 18-crown-6 and 15-crown-5, respectively. A gel of molar composition 1 Si02 0.1 AI2O3 0.21 Na20 0.07 R 14 H2O (R 18-crown-6 or 15-crown-5) was aged at room temperature for 24 h, then heated at 110°C for 15 days. The solids were filtered, washed, dried and calcined in air at 450°C for 6 h. The Si/Al ratio was 3.8. 

Mesoporous ZSM-5 named Z41(X), with X = Si/Al ratio used in the synthesis gel, were prepared by impregnation of the dried carbon cast with aqueous tetrapropylammonium hydroxide (TPAOH 1 mol/L) until incipient wetness. Subsequently a calculated amount of aluminum sulfate and TEOS were added to achieve a molar composition of 1 Si02 x AI2O3 0.36 TPAOH 8 H2O with x=0.005 for Z41(100) and =0.01 for Z41(50). The crystallization was carried out in a saturated water atmosphere at 180 °C for 72 h and the carbon cast and the occluded template cations were burned off at 550 °C for 6 h. 

Owing to the very fast hydrolysis of aluminum isopropoxide, TEOS should be dissolved separately and prehydrolyzed in ethanol at 75 °C. The time of prehydrolysis was varied from 0 to 3 h. Aluminum isopropoxide [Al(OCHCH3CH3)3] and H2O were added to the solution of TEOS. A homogeneous, optically clear gel formed in a few minutes at 60 °C. (The molar ratios applied were Al/Si= 1, ethanol/Si = 25, and H20/Si = 6.)... 

Fabrication procedures of silica monoliths by a one-step catalyst procedure are presented in Ref [20]. A basic procedure to prepare a silica xerogel monolith is by following the stoichiometry ratio (H2O Si) R = 4, while using a similar molar amount of alcohol as a common solvent for achieving homogeneous solution. In fabrication of sol-gel monoliths, the catalyst used has crucial influence. The common route for preparing silica monoliths is to use a recipe based on a one-step catalyst reaction. 

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