Hydrocarbon cracking over a solid acid for endothermic fuel systems: A study of sub-micron particles and synthesized large zeolite crystals

Open Access
Author:
Zaman, Taslima Ahmed
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 30, 2017
Committee Members:
  • Robert M. Rioux, Dissertation Advisor
  • Robert M. Rioux, Committee Chair
  • Michael John Janik, Committee Member
  • Antonios Armaou, Committee Member
  • Randy L. Vander Wal, Outside Member
  • David E. W. Vaughan, Special Member
Keywords:
  • endothermic fuel system
  • hydrocarbon cracking
  • zeolite
  • ZSM-5
  • MFI
  • FAU
  • zeolite-Y
  • faujasite
  • solid acid
  • acid site density measurement
  • FAU-EMT intergrowth
  • zeolite large crystal synthesis
  • electron probe microanalysis
Abstract:
Storable liquid hydrocarbon fuels can undergo endothermic reactions and provide an additional heat sink to enable hypersonic flight without having to resort to cryogenic fuels such as liquid hydrogen. In this dissertation, the endothermic potential of small n-alkane cracking over HZSM-5 was measured utilizing differential scanning calorimetry (DSC) and gas chromatography-mass spectrometry (GC-MS). The study of both propane and n-butane cracking over HZSM-5 revealed that a decrease in space velocity increased the paraffin to olefin (P/O) ratio. Longer exposure times of hydrocarbon to cracking temperatures allow for secondary reactions to take place, which produce products that tend toward the exothermic equilibrium products and decrease the endotherm. Results also show that the heat of reaction measured by mass balance/product distribution (indirect) and calorimetry (direct) are in close
agreement as long as the carbon balance can be closed. Single large-body zeolite crystals of MFI have been synthesized and studied for a fundamental understanding of their structure-function relationship. The nature of acid sites present in both sub-micron sized and the synthesized single-large body ZSM-5 crystals were characterized using 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier transformed infra-red (FTIR) spectroscopies. For acid site density measurements, adsorption of simple amines including, isopropylamine (IPA) and pyridine were examined in both sub-micron sized and synthetic single-large body ZSM-5 crystals using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA). For each amine, an adsorption state corresponding to a coverage of about one molecule per aluminum atom was identified. The Si/Al ratio of each sample was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and was in close agreement to the acid site densities measured using TPD-TGA. The spatial distribution of aluminum content was mapped in the synthetic single large-body ZSM-5 crystals using electron probe microanalysis (EPMA) showing that Al is invariably concentrated in the rim portion of the crystals. These results implied that the acid sites were accessible in single large-body zeolites so long as the probe molecule was small enough to fit inside the zeolite cavities independent of particle size. Single large-body zeolite crystals of faujasite (FAU) have been synthesized and characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed the presence of both cubic (FAU) and hexagonal (EMT) structural phases. While it is known that FAU and EMT components segregate into contiguous blocks in materials like Y, CSZ-1, CSZ-3, ZSM-2, ZSM-3, ZSM-20, ECR-35, ECR-30 and EMC-2, such intergrowth has not been previously reported in large synthetic crystals.