Yamazaki Takahiro, Tanabe Tomohiro, Sugahara Takeshi
Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
ACS Omega. 2018 Oct 12;3(10):13154-13159. doi: 10.1021/acsomega.8b02102. eCollection 2018 Oct 31.
As a guest molecule of urea clathrate, a long-chain normal alkane and its derivative with low substituents in methanol solutions have been reported. To investigate the role of methanol in the urea clathrate formation, in the present study, we used propane (CH), propylene (CH), ethane (CH), and methane (CH) as guest molecules. Raman spectra and powder X-ray diffraction profiles revealed that, regardless of the existence of methanol, the CH, CH, and CH molecules are enclathrated into urea clathrates with a hexagonal structure, whereas there is no urea clathrate formation enclathrating CH. The pressurization of the urea clathrates including CH and CH reveals that no pressure-induced structural phase transition occurs at pressures up to 200 MPa. In spite of the guest molecule much shorter than the lattice constant of the -axis of the hexagonal channel structure, the urea clathrates have a fairly rigid structure against the compression. Methanol as an auxiliary solution is not always necessary for the urea clathrate formation. Methanol plays a role in decreasing the activation energy of the urea clathrate formation, although it makes urea clathrate thermodynamically unstable due to the high solubility of urea in methanol.
作为尿素包合物的客体分子，已报道了在甲醇溶液中具有低取代基的长链正构烷烃及其衍生物。为了研究甲醇在尿素包合物形成中的作用，在本研究中，我们使用丙烷（CH₃CH₂CH₃）、丙烯（CH₃CH=CH₂）、乙烷（CH₃CH₃）和甲烷（CH₄）作为客体分子。拉曼光谱和粉末X射线衍射图谱表明，无论甲醇是否存在，CH₃CH₂CH₃、CH₃CH=CH₂和CH₃CH₃分子都被包合到具有六方结构的尿素包合物中，而不存在包合CH₄的尿素包合物形成。对包含CH₃CH₂CH₃和CH₃CH=CH₂的尿素包合物进行加压表明，在高达200 MPa的压力下不会发生压力诱导的结构相变。尽管客体分子比六方通道结构的c轴晶格常数短得多，但尿素包合物对压缩具有相当刚性的结构。甲醇作为辅助溶液对于尿素包合物的形成并非总是必要的。甲醇在降低尿素包合物形成的活化能方面发挥作用，尽管由于尿素在甲醇中的高溶解度，它会使尿素包合物在热力学上不稳定。
