Cheng Chil-Hung, Shantz Daniel F
Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA.
J Phys Chem B. 2005 Oct 20;109(41):19116-25. doi: 10.1021/jp0524633.
In situ small-angle X-ray scattering (SAXS) is used to investigate the influence of alcohol identity and content on silicalite-1 growth from clear solutions at 368 K. Several tetraalkyl orthosilicates (Si(OR)4, R = Me, Pr, and Bu) are used to synthesize silicalite-1 from clear solution mixtures comparable to those previously investigated (i.e. 1:0.36:20 TEOS:TPAOH:H2O (TEOS = tetraethyl orthosilicate; TPAOH = tetrapropylammonium hydroxide), 368 K). All TPAOH-organosiloxane mixtures studied form silica nanoparticles after aging at room temperature for 24 h. Full-profile fitting analysis of the SAXS data indicates the particles are ellipsoidal and is inconsistent with the presence of "nanoslabs" or "nanoblocks". Synthesis using TEOS as the silica source have an induction period of approximately 7.5 h and a growth rate of 1.90 +/- 0.10 nm/h at 368 K. Changing the silica source to tetramethyl orthosilicate (TMOS) does not change the induction period; however the particle growth rate is decreased to 1.65 +/- 0.09 nm/h at 368 K. Variable-temperature SAXS measurements for syntheses with TEOS and TMOS show the activation energy for silicalite-1 growth is 60.0 +/- 2.9 and 73.9 +/- 2.8 kJ/mol, respectively, indicating the alcohol identity does influence the growth rate. By mixing tetrapropyl orthosilicate (TPOS) with TEOS (1.6:1.0 molar ratio) as the silica source, the precursor solution shows a shorter induction period (6.0 h) and a faster particle growth rate (2.16 +/- 0.06 nm/h). The alcohol identity effect is more pronounced when other organocations (e.g. alkyltripropylammonium cations) are used to make silicalite-1 at 368 K. Removing ethanol from the precursor solution decreases the induction period to approximately 4.5 h and increases the particle growth rate to 2.99 +/- 0.13 nm/h. Mixtures with 2 equiv of ethanol have an induction period and particle growth rate of 6.0 h and 2.04 +/- 0.03 nm/h, respectively. The results demonstrate the alcohol identity and content influence silicalite-1 growth kinetics. One possible explanation is varying the alcohol identity and content changes the strength of the hydrophobic hydration of the structure-directing agent and the water-alcohol interaction, resulting in less efficient interchange between clathrated water molecules and solvated silicate species.
原位小角X射线散射（SAXS）用于研究在368K下，醇的种类和含量对从澄清溶液中生长硅沸石-1的影响。使用几种原硅酸四烷基酯（Si(OR)4，R = 甲基、丙基和丁基）从与先前研究的类似的澄清溶液混合物中合成硅沸石-1（即1:0.36:20的原硅酸四乙酯:四丙基氢氧化铵:水（原硅酸四乙酯 = 原硅酸四乙酯；四丙基氢氧化铵 = 四丙基氢氧化铵），368K）。所有研究的四丙基氢氧化铵 - 有机硅氧烷混合物在室温下老化24小时后形成二氧化硅纳米颗粒。SAXS数据的全谱拟合分析表明颗粒为椭圆形，与“纳米片”或“纳米块”的存在不一致。以原硅酸四乙酯为硅源的合成在368K下有大约7.5小时的诱导期，生长速率为1.90±0.10nm/h。将硅源改为原硅酸四甲酯（TMOS）不会改变诱导期；然而在368K下颗粒生长速率降至1.65±0.09nm/h。用原硅酸四乙酯和原硅酸四甲酯进行合成的变温SAXS测量表明，硅沸石-1生长的活化能分别为60.0±2.9和73.9±2.8kJ/mol，表明醇的种类确实会影响生长速率。通过将原硅酸四丙酯（TPOS）与原硅酸四乙酯以1.6:1.0的摩尔比混合作为硅源，前驱体溶液显示出较短的诱导期（6.0小时）和更快的颗粒生长速率（2.16±0.06nm/h）。当在368K下使用其他有机阳离子（如烷基三丙基铵阳离子）制备硅沸石-1时，醇种类的影响更为明显。从前驱体溶液中除去乙醇会使诱导期缩短至约4.5小时，并使颗粒生长速率提高到2.99±0.13nm/h。含有2当量乙醇的混合物的诱导期和颗粒生长速率分别为6.0小时和2.04±0.03nm/h。结果表明醇的种类和含量会影响硅沸石-1的生长动力学。一种可能的解释是，改变醇的种类和含量会改变结构导向剂的疏水水合强度以及水 - 醇相互作用，导致笼形水分子和溶剂化硅酸盐物种之间的交换效率降低。
