Organic–inorganic hybrid porous sulfonated zinc phosphonate material: efficient catalyst for biodiesel synthesis at room temperature

Abstract

A new porous zinc phosphonate material (HZnP-1) has been synthesized via the reaction between p-xylenediphosphonic acid and anhydrous ZnCl₂ under hydrothermal and mildly acidic conditions (pH ∼ 5) in the absence of any structure directing agent. The phenyl group of this material has been sulfonated with concentrated sulfuric acid to obtain sulfonic acid functionalized material HZnPS-1. Powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE SEM), N₂ sorption, solid state ¹³C CP MAS and ³¹P MAS NMR, and FT IR spectroscopic tools are employed to characterize these materials. The crystal structures of both the materials are indexed corresponding to the new orthorhombic phases with unit cell parameters a = 11.00, b = 8.74, c = 14.62 Å, α = β = γ = 90° for HZnP-1, and a = 10.65, b = 13.52, c = 15.30 Å and α = β = γ = 90° for HZnPS-1. HZnPS-1 showed outstanding catalytic activity and high recycling efficiency for the synthesis of different biodiesel compounds via esterification of long chain fatty acids by using methanol as both reactant and solvent at room temperature. The green and eco-friendly catalytic system described herein can overcome the problem faced by the existing catalytic systems known in biodiesel synthesis, such as drastic conditions (high reaction temperature) and requirement of hazardous organic solvents.