last updated Tuesday, February 10, 2015

 
 

 
Porous materials like zeolites have been used for a long time in industrial application as catalysts or support of catalysts in the petroleum industry, in water purification, in gas separation, or as a drying agent (molecular sieves). They display porous structures that are based on a Si/Al-oxide  structures that contain additional cations i.e., Na+, K+, Mg2+, Ca2+, etc. that modify their properties like their Lewis acidity or the size of their pores/channels significantly. Due to the large channels in the structures, they usually display very low densities compared to other minerals (SiO2: 2.65 g/cm3, Al2O3: 4.00 g/cm3, CaO: 3.34 g/cm3). Some zeolites like clinoptilolite ((Na,K,Ca)2-3Al3(Al,Si)2Si13O36·12 H2O, r=2.15 g/cm3), stilbite (NaCa4(Si27Al9)O72·28 H2O, r=2.15 g/cm3) and natrolite (Na2Al2Si3O10·2 H2O, r=2.25 g/cm3) occur in nature. ZSM-5 (named after Zeolite Sconoy Mobile, NanAlnSi96–nO192·16 H2O (0<n<27), r=0.72 g/cm3,) is an artificial zeolite that is used for the isomerization of meta-xylene or ortho-xylene to para-xylene and as support for the copper-based oxidation of ethanol too

http://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Zeolite-ZSM-5-3D-vdW.png/220px-Zeolite-ZSM-5-3D-vdW.png

 
 
Recently, metal-organic frameworks (MOF) have garnered a lot of attention because of their unique properties. They consist of a metal ion and an organic ligand that links the metal ions together into larger arrays. Many dicarboxylic acids (i.e., oxalic acid, malonic acid, succinic acid, glutaric acid, terephthalic acid), tricarboxylic acid (i.e., citric acid, trimesic acid) or azoles (i.e., 1,2,3-triazole, pyrrodiazole) are used as linker.    MOF-5 (Zn4O(1,4-benzenedicarboxylate)3, r=0.13-0.20 g/cm3) consists of tetrahedral [Zn4O]6+ units  that are linked together with 1,4-benzene-dicarboxylate units. The opening in the structure is 9.3-13.8 Å depending on the orientation of the ring. MOF-5 can store a significant amount of hydrogen at low temperature (77 K: 7.1 wt % (40 bar), 10 wt % (100 bar)). While the hydrogen storage capacity in decent at 77 K (66 g/L), its ability to store hydrogen at room temperature is significantly lower (9.1 g/L), which limits its use as hydrogen storage medium.

 http://upload.wikimedia.org/wikipedia/commons/thumb/7/74/IRMOF-1_wiki.png/220px-IRMOF-1_wiki.png

MOF-177 (Zn4O(1,3,5-benzenetribenzoate)2) also consists of tetrahedral [Zn4O]6+ units are linked by large, triangular tricarboxylate ligands. Its hydrogen storage capacity is similar to the one of MOF-5 (77 K: 7.1 wt % (40 bar), 11.4 wt % (78 bar)). MOF-200 and MOF-210 display a little bit higher uptake of hydrogen at low temperature and are able to deliver 3 % at 100 bar at room temperature.

 http://www.chem-station.com/blog/images/MOF_177.gif

MIL-53 ([Al(OH)] (1,4-benzenedicarboxylate) (1,4-benzenedicarboxylic acid)0.7, r=0.4 g/cm3) consists linear chains of [AlO4(OH)2] octahedra that are linked together with 1,4-benzene-dicarboxylate units. Like many other MOFs, it is obtained by the reaction of the metal nitrate with the dicarboxylic acid under hydrothermal conditions. Recently, the iron, chromium and scandium analog of MIL-53 have been prepared as well. 

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/MIL-53ht.png/1024px-MIL-53ht.png

In this experiment, the students will synthesize a-magnesium formate, which also displays MOF–type properties as well, in solvothermal fashion. Since the synthesis of a-Mg3(O2CH)6 is carried out in N,N-dimethylformamide, the solvent will be included into the porous structure   (see picture on the left). Upon heating under reduced pressure, the guest can be removed without disrupting the framework as the crystal structure of the two compounds demonstrates. Other molecules (i.e., THF, Et2O, MeOH, EtOH, C6H6, C7H8, C6H12, etc.) may or may not be used to fill the voids of the guest-free structure depending on their size.

 

Experiment:

1. The drum vial is closed with a flat septum (Teflon side down, pushed inwards) and a compression cap.

2. The four compounds obbtained in this project will be characterized by infrared and 1H-NMR and 13C-NMR spectrum spectroscopy (in D2O)

3. The SPARTAN PART aims to determine the size of the molecules to be included.

Hints for the questions:

ad 1: What exactly happens during the heating part?

ad 4: Do you expect the structure to change after the inclusion of a solvent molecule?

ad 7: What determines the temperature needed to remove the solvent molecules?