Chelbina Yu.V., Ilyin A.A., Tarabanko V.E., Kaygorodov K. L.

Institute of Chemistry and Chemical Technology SB of RAS,

Akademgorodok 50/24, Krasnoyarsk, 660036, Russia, veta@icct.ru

INTERACTION OF VANILLIN WITH CONCENTRATED SOLUTIONS OF SODIUM HYDROSULFITE

Oxidation of lignosulfonates is applied to produce vanillin up till now [1-6]. Primary purification of vanillin is carried out by hydrosulfitation and re-extraction from the organic phase into an aqueous solution of sodium hydrosulfite [7]:

Ar-CHO_org + NaHSO_{3 aq} = Ar-CH(OH)SO₃Na_{aq .} (1)

Quantitative data on the stability of the bisulfite-vanillin derivative is extremely limited (equilibrium constant of (1) K_obs = 350 ± 20, [NaHSO₃] < 1 M [8]).

The purpose of this paper is to find methods to improve the efficiency of stripping vanillin from the octanol phase with aqueous NaHSO₃ solutions.

Experimental

Mixture of vanillin in octanol and aqueous NaHSO₃ (volume ratio V_org: V_aq = 10: 1) in a tube were shaken for 5 min. The organic phase (octanol) was separated from the precipitate by filtration through a glass filter, and vanillin concentration in octanol was determined before and after extraction spectrophotometically.

Vanillin in the precipitate formed in the stripping process was determined by GLC after acid decomposition of vanillin bisulfate adduct and extraction. Vanillin content in the aqueous phase after separation of octanol and precipitate of vanillin-hydrosulfite-derivative was also analyzed by GLC.

The vanillin distribution coefficient was calculated as the ratio of equilibrium concentrations of vanillin in the aqueous and organic phases,

D = [Van] _aq / [Van] _org , (2)

were [Van] _aq and [Van] _org - equilibrium concentrations of vanillin in the aqueous and organic phases, respectively. In the case of three-phase system (when precipitate of vanillin-hydrosulfite-adduct was formed), effective distribution coefficient of vanillin from the organic phase was calculated, using the sum of the masses of vanillin in the aqueous and solid phases, referred to the volume of the aqueous phase V_aq instead of the equilibrium concentration in the aqueous phase:

[Van] _aq = (m (Van_aq) + m (Van_solid)) / V_aq, (3)

D = (m (Van_aq) + m (Van_solid)) / (V_aq [Van] _org), (4)

The ratio (4) characterizes the efficiency of vanillin stripping from the organic phase by sodium hydrosulfite.

Results and Discussion

The ionic strength (Na₂SO₄ solutions) strongly decreases the distribution coefficient in two-phase system (Fig. 1). As a result of this effect this coefficient decreases with [NaHSO₃] increasing above 4 M at low vanillin concentration (Fig. 2).

Fig. 1. Dependence of the distribution coefficient versus sodium sulfate concentration in the system 2.04 M solution of sodium hydrosulfite – octanol. 0.066 M vanillin initial concentration in the organic phase, Vorg / Vaq = 10:1.	Fig. 2. Dependence of the distribution coefficient of vanillin versus sodium hydrosulfite concentration in the octanol-water system. 0.02 M vanillin initial concentration in the organic phase, Vorg / Vaq = 10:1.

The studied system becomes more complicated and efficient at high vanillin and hydrosulfite concentrations as a result of precipitating the vanillin hydrosulfite adduct (Fig. 3). In the field of saturated concentration of sodium hydrosulfite (6.1 M), the dependence of the apparent distribution coefficient versus vanillin concentration takes the extreme nature with D_max values up to 250 (Fig. 3). These values are 10 times more than the values of distribution coefficient obtained previously at concentrations of sodium hydrosulfite below 1.2 M [8]. The most part of vanillin (appr. 90 %) was found in the precipitate at NaHSO₃ concentration 6.1 M, and this effect explains very high observed distribution coefficient.

Fig. 3. Dependence of apparent distribution coefficient of vanillin versus its initial concentration in the octanol-water system. Process conditions: initial sodium hydrosulfite concentration of 6.1 M (upper curve and its initial part to the right), 5.1 M (middle curve), and 3.6 M (lower curve), Vorg / Vaq = 10:1.

Thus, the vanillin hydrosulfite adduct was found to precipitate from saturated sodium hydrosulfite solution, and it does not require additional salting out agent, as is done in [9]. This effect reduces the consumption of sulfuric acid and alkali for decomposition of the vanillin hydrosulfite adduct down to the stoichiometric in opposite to the known methods [7] required 3 – 4 times more reagents.

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