Iron Tetrasulfonatophthalocyanine-Catalyzed Starch Oxidation Using H₂O₂: Interplay between Catalyst Activity, Selectivity, and Stability

Abstract

Oxidized starch can be efficiently prepared using H 2 O 2 as an oxidant and iron(III) tetrasulfophthalocyanine (FePcS) as a catalyst, with properties in the same range as those for commercial oxidized starches prepared using NaOCl. Herein, we performed an in-depth study on the oxidation of potato starch focusing on the mode of operation of this green catalytic system and its fate as the reaction progresses. At optimum batch reaction conditions (H 2 O 2 /FePcS molar ratio of 6000, 50 °C, and pH 10), a high product yield (91 wt %) was obtained with substantial degrees of substitution (DS COOH of 1.4 and DS CO of 4.1 per 100 AGU) and significantly reduced viscosity (197 mPa•s) by dosing H 2 O 2 . Model compound studies showed limited activity of the catalyst for C6 oxidation, indicating that carboxylic acid incorporation likely results from C−C bond cleavage events. The influence of the process conditions on the stability of the FePcS catalyst was studied using UV−vis and Raman spectroscopic techniques, revealing that both increased H 2 O 2 concentration and temperature promote the irreversible degradation of the FePcS catalyst at high pH. The rate and extent of FePcS degradation were found to strongly depend on the initial H 2 O 2 concentration where also the rapid decomposition of H 2 O 2 by FePcS occurs. These results explain why the slow addition of H 2 O 2 in combination with low FePcS catalyst concentration is beneficial for the efficient application in starch oxidation.

Benign catalytic oxidation of potato starch using a homogeneous binuclear manganese catalyst and hydrogen peroxide

“…The formed reducing end can be oxidised in its open form. 43,44 Thus, we hypothesise that under our reaction conditions, in analogy to FePcS oxidation, 26 partial hydrolysis forms reducing ends which can subsequently be oxidised to the carboxylic acids (Scheme S1†). We would not expect such a clear trend when the Mncat freely oxidises all alcohol groups in the anhydroglucose units.…”

“…Above, it was established that using Mncat with H 2 O 2 starch with relatively high DS COOH can be obtained. From this initial set of experiments and related reactions in literature, we realised that pH, 35 peroxide dosing, 26 and reaction time (also after complete H 2 O 2 consumption) have a large influence on the obtained starch properties. To exemplify, the oxidation using 1 wt% H 2 O 2 and 0.0021 mol% Mncat at 25 °C the degree of oxidation (0.018 ± 0.0002 mol mol −1 ) and recovery yield (98 ± 1) were reproducible, but the pasting behaviour between repeat experiments showed a variation in the viscosities ( i.e.…”

“…Moreover, the concentration of H2O2 is kept low and constant which was shown for the oxidation with FePcS to limit the decomposition of the catalyst. 26 Indeed an improvement in the oxidation efficiency was achieved reaching a higher degree of oxidation using the same amount of H2O2 (Fig 5a). Likewise, the pasting properties changed with a lowering in peak viscosity observed from 5344 to 674 mPa•s (Table S5 entries 2/14).…”

“…The performance of our system can be further illustrated by comparison with the best known catalytic oxidation system for starch using H2O2, which is FePcS that reached a DSCOOH of 0.014 mol/mol at a moderate recovery yield of 91% at a higher reaction temperature of 50 °C (Fig 2a). 26 Using our system, apart Please do not adjust margins Please do not adjust margins from the ability to reach a higher DSCOOH combined with a higher recovery yield, the oxidant is used far more efficiently requiring less than a fifth of the amount of H2O2 (1 and 5.6 wt%, respectively) and half of the catalyst loading (0.0021 and 0.0042 mol%, respectively) used by the FePcS system. Comparing to the oxidation using NaOCl, a higher DSCOOH (0.029) at a yield of 97 % was obtained when using the batch conditions and 1 wt% H2O2 (Fig 2a).…”

“…17,20,21 In recent years, an iron(III) tetrasulfonatophthalocyanine complex (FePcS) was studied for starch oxidation and allowed the more efficient use of H 2 O 2 at lower metal loadings (0.0042-0.017 mol%, entries 2-7). [22][23][24][25][26] However, the catalyst complex was found to decompose rapidly at optimal reaction conditions. 26 Furthermore, the loadings were still quite high (H 2 O 2 loading >5.6 wt%) and long reaction times were required (>420 min), while the starch products only reached moderate degree of oxidation (<0.016) and yields (<91%) which would make them economically unfeasible at an industrial scale.…”