The present invention provides a one-pot process of synthesis of phenyl naphthalene compounds that are employed as heat transfer agents. More particularly, the present invention provides a process of preparation of 1-phenylnaphthalene and 2-methyl-1-phenylnaphthalene using refinery spent catalyst. These molecules are known for application as synthetic heat transfer fluids that deliver outstanding performance and thermal stability at continuously high operating temperatures. The reaction is carried out in aqueous medium using a spent catalyst which is a palladium based charcoal catalyst as obtained from various refinery processes. Further, the present invention provides a heat resistant formulation using the synthesized polyhydrocarbons, wherein the formulation is optimized with a free radical scavenger.