Tetrapropylammonium hydroxide

Polished titanium-aluminum-vanadium foil (Ti-6Al-4V, thickness 0.1 mm, purchased from Goodfellow Metals (GB)), was cut into 1 × 1 cm coupons. The edges of the coupons were rubbed down to eliminate contamination introduced by cutting the foil, and were then washed using an ultrasonic bath in sequence for 10 minutes (150 W) in acetone and isopropyl alcohol, after which they were rinsed in ethanol and were dried in air. The SFs were synthesized in situ from the reaction mixture of tetrapropylammonium hydroxide (TPAOH), tetraethylorthosilicalite (both Sigma-Aldrich) and deionized water, as described in the literature^{86 (link)}. The reaction mixture was aged for 2 hours. The synthesis proceeded for 3 hours in a Teflon-lined autoclave under autogenous pressure at 165 °C. The coupons were oriented upside down during synthesis. The as-synthesized SFs, assigned as SF-RT, were sonicated in deionized water (150 W, 10 minutes).
One part of the SFs was heat treated as follows: heating rate 1 °C min⁻¹ up to 500 °C, at which level the samples were maintained for 4 hours, after which they were cooled at a rate of 1 °C min⁻¹ in a stream of dry air (300 mL min⁻¹). The calcination temperature was chosen to minimize the presence of volatile condensed aromatic species, which had been observed in our preceding study³³ . The heat-treated samples were designated as SF-500.

The chemicals utilized in the preparation were citric acid ≥99.5%, L-cysteine hydrochloride monohydrate ≥98% and tetrapropylammonium hydroxide 1.0 M in H₂O purchased from Sigma Aldrich. Sugarcane was procured from the Sugar Illovo South Africa Company. The synthesis was done using deionized water from the Milli-Q water purification system (Millipore, Bedford, MA, USA).