Phosphomolybdic acid hydrate

All reagents were of analytical grade unless otherwise stated. Calibration curves for ICP analysis were obtained using the ICP standard solution of B, Ca, Cu, Fe, K, Mg, Mo, Mn, Na, P, and Zn purchased from Merck (Darmstadt, Germany). The HCl, HNO₃, and H₂O₂ were purchased from Sigma-Aldrich (Germany). All solutions were prepared using high purity water from a Milli-Q Element system (Millipore, Molsheim, France). HPLC grade methanol and acetonitrile were obtained from Merck (Italy). The amino acid standards, 2-aminoadipic acid (99%), diethyl ethoxymethylenemalonate (DEEM, 99%), L-fucose (>99%), L-arabinose (99%), L-glucose (99%), D-xylose 8>99%), D-mannose (>99%), D-mannitol (>98%), ferrous sulfate eptahydrate (>99%), potassium dichromate (0.1 N), sodium hydroxide (>97%), citric acid (>99.5%), ammonium formate (LC-MS grade), trifluoroacetic acid (>99%), potassium chloride (>99%), ascorbic acid (99%), sodium acetate (>99%), sulfuric acid (95.98%), hydrogen peroxide solution (30% w/w), phenol (99%), hydrochloric acid solution (37% w/w) chloride, phosphotungstic acid hydrate (99.995%), phosphomolybdic acid hydrate (99.99%), and Folin–Ciocalteu reagent (2N) were purchased from Merck (Italy). The deuterated solvents CD₃OD and D₂O were purchased from Sigma-Aldrich.

Composite 1 was synthesized using commercial silica gel powder with a pore size of 150 Å (Sigma-Aldrich) as a hard template. Typically, 2.555 g (1.4 mmol) of phosphomolybdic acid hydrate ([H₃PMo₁₂O₄₀]·xH₂O (=PMo12) Alfa Aesar) was dissolved in 5 g of water, and then 1.25 g (3.63 mmol) of sucrose (Merck Millipore (Calbiochem)) was added and dissolved. Finally, 1.0 g of silica powder was dispersed in the above solution and stirred overnight at room temperature. The mixture was heated in air to 100 °C for 6 h and subsequently to 160 °C for another 6 h. This impregnation process was repeated with a second solution containing 0.8 g (2.32 mmol) of sucrose and 1.643 g (0.9 mmol) of PMo12 in 5 g of water. The air-dried materials were carbonized at 900 °C for 5 h under Ar at a heating rate of 5 °C min⁻¹. To remove the silica template, the as-prepared composites were stirred in 50 mL of 10% aqueous hydrofluoric acid (HF_aq, ≥ 48%, Sigma-Aldrich) for 48 h, and then washed with water and ethanol three times respectively, and finally dried at 100 °C overnight. During this step, PMo12 was reduced and converted to MoO₂, while the subsequent HF treatment did not significantly affect the chemical makeup of the molybdenum-based species in 1. Note that hydrofluoric acid is toxic and necessary handling precautions need to be taken.

Briefly, 20 mL melamine (Alfa Aesar) solution (2 mg mL⁻¹ in aqueous phase) was first mixed with 20 mL GO solution (4 mg mL⁻¹ in aqueous phase, prepared by a modified Hummers method) by stirring at 700 rpm for 15 min. Then, phosphomolybdic acid hydrate (27.4 mg, 0.015 mmol [H₃PMo₁₂O₄₀]⋅x H₂O (=PMo₁₂), Alfa Aesar) and Cobalt(II) acetate tetrahydrate (Co(OAc)₂⋅4 H₂O, Merck, containing 44.8, 22.4 and 14.9 mg (0.18, 0.09 and 0.06 mmol) for composite 2, 3 and 4) were dissolved in 40 mL H₂O and added to the above mixture under stirring at 1000 rpm for 6 h. Afterwards, the obtained mixtures were hydrothermally reacted at 180 °C for 12 h, and then cooled down to room temperature. The obtained solid composites were filtered off, washed with water for three times, and dried at 80 °C for overnight. Finally, the dried composites were calcined in the tube furnace programmed with two heating steps under Ar atmosphere, first at 400 °C for 2 h with a heating rate of 1 °C min⁻¹, and then at 800 °C for 2 h with a heating rate of 2 °C min⁻¹. This gave the final composites 2, 3, and 4 with the Mo/Co molar ratio of 1:1, 2:1 and 3:1, respectively. For comparison, we also prepared the catalyst 1 without adding Co(OAc)₂⋅4 H₂O, and the other conditions were kept the same.