Chapter 12 NMR studies of FCC feeds, catalysts and coke

Abstract

Abstract NMR has become an indispensable tool for characterizing a variety of complex materials, includingheavy petroleum fractions, catalysts and coke. Determination of average molecular parameter by NMR is useful for the analysis of complex hydrocarbons like coal liquids, heavy oils, synthetic oils and high boiling petroleum fractions. This not only gives a brief idea of the molecules present but can also be used for prediction of crackability and coking tendency of the feedstock under a particular condition of operation. Although solid state NMR has not attained the resolution as of liquids, it has been proved as a better technique for molecular level characterization particularly in catalysis. This is because of the use of magic angel spinning, cross polarization (CP) and heteronuclear decoupling techniques in conjunction with routine and sophisticated solid experiments. In this chapter, NMR has been used to study the feeds, catalysts and coke of the FCC process of refining industries. Fluid catalytic cracking (FCC) feeds from two Indian refineries are structurally characterized by inverse gated decoupled 13C and distortionless enhancement by polarization transfer (DEPT) NMR methods. Detailed structural analyses are completely supported by a range of NMR information including chemical shifts of 1H and 13C, CHn type distributions and 1H-13C connectivities from 2D HETCOR NMR. The average structural parameters obtained from NMR analysis give a brief idea about the nature of feeds used in the FCC units. 29Si MAS, 27Al MAS and 27Al 3QMAS NMR methods are employed to study the structure of fresh and spent catalyst obtained after stripping. Other analytical techniques like HPLC, microcalorimetry, XRD, TGA, IR are used to compliment or augment the inferences obtained from NMR. The changes in structure of catalyst are described in terms of framework Si/Al ratio, the relative distribution of various acid sites (different framework Si atoms) and the changes in their relative populations and changes in unit cell sizes. The variations in structure of two catalysts are correlated in terms of their quadrupolar coupling constant values at the site of octahedral and tetrahedral Al nuclei obtained from the MQMAS studies. The soluble and insoluble coke concentrates are extracted from the spent FCC catalysts by chemical methods and are studied by different NMR techniques. Conventional 1H and 13C NMR is used to derive the average structure of soluble coke. Quantitative data about the insoluble coke aromaticity are obtained from solid state13C SHPE/MAS NMR. The information of protonated and non-protonated carbons are obtained from Dipolar Dephasing (DD) experiments in combination with SHPE/MAS. The compositional variation in the feed and changes in catalyst properties are used to explain the nature and structural differences of the coke.