KINETICS OF FORMALDEHYDE ADSORPTION FROM AQUEOUS SOLUTIONS WITH SYNTHETIC ZEOLITES IN PRESENCE OF PHOSPHORIC ACID

It is known that 4,4-dimethyl-1,3-dioxane is a key intermediate for the industrial synthesis of isoprene. It is obtained by condensation of the aqueous solution of formaldehyde with isobutylene in the presence of phosphoric acid. In the last decade, synthetic zeolites were used for the above process. However, in this work were not considered features of interaction of reagents and products of Prins reaction with porous materials. Therefore, the aim of this work was to study the adsorption of formaldehyde from aqueous solutions by synthetic zeolites in the presence of phosphoric acid. The kinetics of adsorption of formaldehyde from aqueous solutions by synthetic zeolites in the presence of phosphoric acid was carried out. For a series of zeolites of type A and X the values of the external and internal mass transfer diffusion coefficients were obtained. The effect of the value of the zeolite pore diameter in the contribution of the external or internal diffusion mass transfer in the process of adsorption of formaldehyde was founded. The time of the establishment of sorption equilibrium was obtained. As sorbents we have used synthetic zeolites with diameters of 3-9 angstrom. Adsorption of formaldehyde from aqueous solutions was investigated at (75 +/- 1) degrees C from a limited volume under constant agitation (laboratory mechanical stirrer, 17 rps). Samples of a porous sorbent (the weight was (2.65 +/- 0.10) g) were introduced into the solutions containing 50 mL of aqueous formaldehyde with the initial concentrations of 5.85-7.07 mol/l and 2.5 ml of 81% phosphoric acid. The contact time of the solution with samples of sorbents was ranged from 120 to 3600 s. The formaldehyde concentration in solution was determined by the sulfite method. Adsorption of formaldehyde (a) from aqueous solutions by synthetic zeolites material was evaluated according to the equation:. a [(c(0) - c(t)).V] / m, c(t) - adsorbate concentration at various time points, mol/l;c(0) - adsorbate concentration at the initial time, mol/l. Relative approach of the adsorption to equilibrium (gamma) was calculated according to the equation: gamma = a / amax - amax - adsorption at equilibrium, mg/g. Changing adsorbed formaldehyde (T) at different times was calculated according to the equation: T = - ln(1-gamma). External diffusion mass transfer coefficients (beta(n)) and internal diffusion from an aqueous formaldehyde solution were calculated using the equation: beta(n) = tga / T, tga - the tangent linear portion of the graph according to the inclination angle T = f (t). We have found that formaldehyde adsorption process from the aqueous solution in the presence of phosphoric acid for all used types of synthetic zeolites (KA, NaA, CaA, CaX, NaX) has been determined the values of the pore diameters of zeolites. It was shown that time of achievement of equilibrium sorption in the system formaldehyde-water-phosphoric acid in the case of all used synthetic zeolites is 600 s.