A process for the production of, optionally functionalized, styrene monomers comprising the steps: (a) feeding benzene or functionalized benzene and ethylene, as well as a heterogeneous catalyst into a reactor forming a mixture of benzene or functionalized benzene and ethylene (the starting material), and (b) heating said mixture and applying pressure to obtain crude styrene; wherein the step (b) is carried out at a temperature ranging from and including 150°C to 300 °C; and at a pressure (of ethylene) of between and including 5 bar and 100 bar; wherein the heterogeneous catalyst is selected from graphene oxide, sulfonated graphene oxide, reduced graphene oxide (rGO), multiwalled carbon nanotubes, graphitic carbon nitride(g-C 3 N 4), sulfonated graphitic carbon nitride (S-g-C 3 N 4), palladium doped reduced graphene oxide (Pd/rGO), palladium doped multiwalled carbon nanotubes (Pd/CNT), palladium doped graphitic carbon nitride (Pd/g-C 3 N 4), boron-doped graphitic carbon nitride(B/g-C 3 N 4, palladium doped sulfonated graphitic carbon nitride (Pd/S-g-C 3 N 4), ceria doped sulfonated graphene oxide (Ce/SGO), ceria doped reduced graphene oxide (Ce/rGO), ceria doped multiwalled carbon nanotubes (Ce/CNT), ceria doped graphitic carbon nitride(Ce/g-C 3 N 4), ceria doped sulfonated graphitic carbon nitride (Ce-S-g-C 3 N 4), ruthenium doped reduced graphene oxide (Ru/rGO), ruthenium doped graphitic carbon nitride (Ru/g-C 3 N 4), ruthenium doped multiwalled carbon nanotubes (Ru/CNT), ruthenium doped sulfonated graphitic carbon nitride(Ru/S-g-C 3 N 4), platinum doped reduced graphene oxide (Pt/rGO), platinum doped graphitic carbon nitride (Pt/g-C 3 N 4), platinum doped multiwalled carbon nanotubes (Pt/CNT) and platinum doped sulfonated graphitic carbon nitride (Pt/S-g-C 3 N 4); and wherein the functionalized benzene is a benzene substituted on the phenyl-ring with 1 to 3 substituents selected from C 1-4 -alkyl. (a) feeding benzene or functionalized benzene and ethylene, as well as a heterogeneous catalyst into a reactor forming a mixture of benzene or functionalized benzene and ethylene (the starting material), and (b) heating said mixture and applying pressure to obtain crude styrene; wherein the step (b) is carried out at a temperature ranging from and including 150°C to 300 °C; and at a pressure (of ethylene) of between and including 5 bar and 100 bar; wherein the heterogeneous catalyst is selected from graphene oxide, sulfonated graphene oxide, reduced graphene oxide (rGO), multiwalled carbon nanotubes, graphitic carbon nitride(g-C 3 N 4), sulfonated graphitic carbon nitride (S-g-C 3 N 4), palladium doped reduced graphene oxide (Pd/rGO), palladium doped multiwalled carbon nanotubes (Pd/CNT), palladium doped graphitic carbon nitride (Pd/g-C 3 N 4), boron-doped graphitic carbon nitride(B/g-C 3 N 4, palladium doped sulfonated graphitic carbon nitride (Pd/S-g-C 3 N 4), ceria doped sulfonated graphene oxide (Ce/SGO), ceria doped reduced graphene oxide (Ce/rGO), ceria doped multiwalled carbon nanotubes (Ce/CNT), ceria doped graphitic carbon nitride(Ce/g-C 3 N 4), ceria doped sulfonated graphitic carbon nitride (Ce-S-g-C 3 N 4), ruthenium doped reduced graphene oxide (Ru/rGO), ruthenium doped graphitic carbon nitride (Ru/g-C 3 N 4), ruthenium doped multiwalled carbon nanotubes (Ru/CNT), ruthenium doped sulfonated graphitic carbon nitride(Ru/S-g-C 3 N 4), platinum doped reduced graphene oxide (Pt/rGO), platinum doped graphitic carbon nitride (Pt/g-C 3 N 4), platinum doped multiwalled carbon nanotubes (Pt/CNT) and platinum doped sulfonated graphitic carbon nitride (Pt/S-g-C 3 N 4); and wherein the functionalized benzene is a benzene substituted on the phenyl-ring with 1 to 3 substituents selected from C 1-4 -alkyl. wherein the step (b) is carried out at a temperature ranging from and including 150°C to 300 °C; and at a pressure (of ethylene) of between and including 5 bar and 100 bar; wherein the heterogeneous catalyst is selected from graphene oxide, sulfonated graphene oxide, reduced graphene oxide (rGO), multiwalled carbon nanotubes, graphitic carbon nitride(g-C 3 N 4), sulfonated graphitic carbon nitride (S-g-C 3 N 4), palladium doped reduced graphene oxide (Pd/rGO), palladium doped multiwalled carbon nanotubes (Pd/CNT), palladium doped graphitic carbon nitride (Pd/g-C 3 N 4), boron-doped graphitic carbon nitride(B/g-C 3 N 4, palladium doped sulfonated graphitic carbon nitride (Pd/S-g-C 3 N 4), ceria doped sulfonated graphene oxide (Ce/SGO), ceria doped reduced graphene oxide (Ce/rGO), ceria doped multiwalled carbon nanotubes (Ce/CNT), ceria doped graphitic carbon nitride(Ce/g-C 3 N 4), ceria doped sulfonated graphitic carbon nitride (Ce-S-g-C 3 N 4), ruthenium doped reduced graphene oxide (Ru/rGO), ruthenium doped graphitic carbon nitride (Ru/g-C 3 N 4), ruthenium doped multiwalled carbon nanotubes (Ru/CNT), ruthenium doped sulfonated graphitic carbon nitride(Ru/S-g-C 3 N 4), platinum doped reduced graphene oxide (Pt/rGO), platinum doped graphitic carbon nitride (Pt/g-C 3 N 4), platinum doped multiwalled carbon nanotubes (Pt/CNT) and platinum doped sulfonated graphitic carbon nitride (Pt/S-g-C 3 N 4); and wherein the functionalized benzene is a benzene substituted on the phenyl-ring with 1 to 3 substituents selected from C 1-4 -alkyl.