Formation Mechanism Of Thiophenes During Pyrolysis Of Oil Sands In Indonesia

As an unconventional oil and gas resource,oil sands are not the first choice to be exploited for economic reasons,but the research and reserve of oil sands is helpful to the national energy security.Pyrolysis and distillation can decompose oil sands into low molecular hydrocarbons for recovery,but sulfur atoms in oil sands migrate to tar products in the form of organic sulfur during pyrolysis,which affects oil quality and hinders industrial utilization of oil sands.The current research focuses on the removal of sulfur from the products,and we hope to explore how the hard-to-remove sulfur is formed.It is difficult to study the migration of sulfur in microcosmic level by traditional experimental techniques.Therefore,the mechanism of organic sulfur formation in the pyrolysis process of Indonesian oil sands was studied.In this paper,the occurrence of organic sulfur in Indonesian oil sands and tar from pyrolysis products was studied.Through Py-GC-MS,a large number of alkanes and olefin structures were found in oil sands,and strong vibration peaks of olefins were found in tar by FT-IR,and sulfur in tar products was found in the form of alkyl thiophene,benzothiophene and dibenzothiophene by GC-SCD.Based on the above experimental results,the molecular model for simulation was selected.The formation of thiophene compounds in oil sands during pyrolysis was studied at molecular level by using Materials Studio software.Based on the experimental results,21reaction paths were designed with alkene and H₂S as reactants,alkyl thiophene,benzothiophene and its derivatives,dibenzothiophene as products.Three-dimensional structures of reactants,intermediates and final products were constructed and optimized to obtain stable geometrical structures.The transition states were searched,optimized and confirmed based on the density functional.The energy barrier,bond length,Mayer bond order,thermodynamic parameters and kinetic parameters were calculated.The formation of alkyl thiophene is divided into three stages:the introduction of sulfur,the formation of five ring,the removal of hydrogen.Path3 connects the sulfur hydroxyl radical by breaking the pi bond of the double bond,and then the sulfur atoms are linked to the carbon in the carbon chain and dehydrogenated to form alkyl thiophene.The difficulty of one stage introduction varies with the number of carbon and the position of addition.The difficulty of two-stage cyclization increases with the increase of main chain length,but the existence of branching chain can reduce the barrier to some extent.The energy barrier of each dehydrogenation path is basically the same,and secondary dehydrogenation is easier to occur than primary dehydrogenation.Unsaturated mercaptan is formed by adding hydrogen atoms on the ortho-carbon of the double bond of Path3 with thiohydrogen radical.The ring structure is obtained by hydrogen transfer and then dehydrogenated to form alkyl thiophene.The first stage is more difficult than Path3,but the second stage is less affected by the structure,and the rule of the third stage is consistent with Path3.Benzothiophene and dibenzothiophene are formed by hydrogen removal on the basis of alkyl thiophene and after the side chains are connected.The energy barrier of dehydrogenation process is slightly higher than that of thiophene ring.The chemical processes in these three kinds of paths are thermodynamic spontaneous exothermic reactions.