Combined EXAFS and XPDF analysis of calcium methoxide short-range structure

Authors: Thokozile Kathyola (University of Leeds) , Elizabet Willneff (University of Leeds) , Sven Schroeder (University Of Leeds; Diamond Light Source) , Giannantonio Cibin (Diamond Light Source) , Philip Chater (Diamond Light Source)
Co-authored by industrial partner: No

Type: Conference Paper
Conference: XAFS2021
Peer Reviewed: No

State: Published (Approved)
Published: July 2021
Diamond Proposal Number(s): 14673 , 17686 , 17391

Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray Pair Distribution Function (XPDF) analysis are versatile complementary techniques that provide information about local structure and bonding in crystalline and non-crystalline materials. Both techniques provide information about the coordination number of neighbouring atoms, bond distances and the degree of static/thermal disorder. EXAFS is a well-established element-specific method that provides short-range order information around the X-ray absorber of interest. XPDF is not element-specific, but provides short-range order information similar to EXAFS, alongside long- range structure. Calcium methoxide (Ca(OCH3)2) is one of the prominent heterogeneous catalysts that are being used in the formation of sustainable biolubricants and biodiesel from natural/synthetic oils via transesterification. Other compounds that have been considered as viable catalysts include calcium oxide (CaO) and calcium hydroxide (Ca(OH)2). Unlike these two bases, which have been comprehensively characterised using EXAFS, the short-range crystalline structure of Ca(OCH3)2 is yet to be fully determined. Not much is known about its structure beyond the indication that it has the same hexagonal space group (P-3m1) as Ca(OH)2. This was determined over 40 years ago, using powder X-ray diffraction and mid infrared spectroscopy, and has not been investigated since. Hence, we present a comparative study on the local structure of CaO, Ca(OH)2 and Ca(OCH3)2 using EXAFS and XPDF analysis. Our results demonstrate how a new/refined model crystal Ca(OCH3)2 structure was obtained using XPDF data, crystal symmetry and unit cell dimensions. This model was subsequently used in EXAFS simulations to determine atom coordination numbers, bond distances and static disorder in Ca(OCH3)2. To the best of our knowledge, this is the first time EXAFS and XPDF data has been presented on a calcium alkoxide. The structural information obtained from this study could benefit development of more sophisticated alkoxide-based heterogenous catalysts. This multi-technique ex situ analysis of the three basic calcium catalysts could also inform operando studies of alkaline-earth metal catalysis with these techniques.

Subject Areas: Chemistry

Instruments: B18-Core EXAFS , I15-1-X-ray Pair Distribution Function (XPDF)

Added On: 09/08/2021 11:48

Discipline Tags:

Physical Chemistry Catalysis Chemistry

Technical Tags:

Scattering Spectroscopy Pair Distribution Function (PDF) X-ray Absorption Spectroscopy (XAS) Extended X-ray Absorption Fine Structure (EXAFS)