TiO2 Catalyzed Dihydroxyacetone (DHA) Conversion in Water: Evidence That This Model Reaction Probes Basicity in Addition to Acidity

Abstract

In this paper, evidence is provided that the model reaction of aqueous dihydroxyacetone (DHA) conversion is as sensitive to the TiO₂ catalysts' basicity as to their acidity. Two parallel pathways transformed DHA: while the pathway catalyzed by Lewis acid sites gave pyruvaldehyde (PA) and lactic acid (LA), the base-catalyzed route afforded fructose. This is demonstrated on a series of six commercial TiO₂ samples and further confirmed by using two reference catalysts: niobic acid (NbOH), an acid catalyst, and a hydrotalcite (MgAlO), a basic catalyst. The original acid-base properties of the six commercial TiO₂ with variable structure and texture were investigated first by conventional methods in gas phase (FTIR or microcalorimetry of pyridine, NH₃ and CO₂ adsorption). A linear relationship between the initial rates of DHA condensation into hexoses and the total basic sites densities is highlighted accounting for the water tolerance of the TiO₂ basic sites whatever their strength. Rutile TiO₂ samples were the most basic ones. Besides, only the strongest TiO₂ Lewis acid sites were shown to be water tolerant and efficient for PA and LA formation.

Keywords: CO2 and NH3 microcalorimetry; FTIR of pyridine adsorption; acidity; basicity; dihydroxyacetone; titanium dioxide.

Scheme 1

Widely accepted route for DHA conversion into lactic acid via pyruvaldehyde formation.

Figure 1

Diffractogram of commercial TiO₂ samples.

Figure 2

N₂ adsorption/desorption isotherm of the commercial TiO₂ samples. Catalyst pretreatment: 300 °C for 2 h under vacuum (heating rate: 5 °C.mn⁻¹).

Figure 3

Infrared spectra of pyridine adsorption on P25, P90, UV100, HPX-200/v2, Rut160 and HPX-400C: (A) spectra of catalysts saturated with pyridine vapor at ambient temperature, then desorption of pyridine for 1 h at ambient temperature; (B) spectra after pyridine desorption at 150 °C for 1 h (L: characteristic vibration of Lewis acid sites, B: characteristic vibration of Brønsted acid sites, L + B: vibration common to Lewis and Brønsted acid sites, H: vibration of pyridine linked by hydrogen bonds). Conditions: Prior to pyridine adsorption, TiO₂ self-supported pellets were vacuum treated for 1 h at 150 °C (reference spectra), normalized spectra to 100 mg.

Figure 4

Ammonia adsorption on the TiO₂ samples: (a) isotherms and (b) calorimetric curves. Conditions: TiO₂ samples pre-treated at 150 °C under secondary vacuum for 5 h, NH₃ adsorption performed at 80 °C.

Figure 5

Carbon dioxide adsorption on TiO₂ (a) isotherms and (b) calorimetric curves. Conditions: TiO₂ samples pre-treated at 150 °C under secondary vacuum for 5 h, CO₂ adsorption at 30 °C.

Figure 6

DHA conversion with the time course of the reaction in the presence of the six TiO₂. Conditions: T = 90 °C, P_air = 1 atm, V_water = 200 mL, (DHA) = 0.1 mol.L⁻¹, (TiO₂) = 10 g.L⁻¹.

Figure 7

Products’ yield time course in the presence of the six TiO₂s. Conditions: T = 90 °C, P_air = 1 atm, V_water = 200 mL, (DHA) = 0.1 mol.L⁻¹, (TiO₂) =10 g.L⁻¹.

Figure 8

Increase of initial rate of hexoses formation as a function of TiO₂′s basic sites density.

Figure 9

Increase of initial rates of pyruvaldehyde and lactic acid formation with TiO₂′s total acid sites density.

Figure 10

Linear increase of the initial rates of pyruvaldehyde and lactic acid formation with TiO₂′s original strong acid sites density (sites with heat of ammonia adsorption > 130 kJ.mol⁻¹ as determined by calorimetry of ammonia adsorption).

Figure 11

Evolution of lactic acid (LA) yield with DHA conversion in the presence of the six TiO₂. Conditions: T = 90 °C, P_air = 1 atm, V_water = 200 mL, (DHA) = 0.1 mol.L⁻¹, (TiO₂) = 10 g.L⁻¹.

Figure 12

Evolution of the Products’ yields with the reaction time in the presence of (a) NbOH and (b) MgLaO. Conditions: T = 90 °C, P_air = 1 atm, V_water = 200 mL, (DHA) = 0.1 mol.L⁻¹, (catalyst) = 10 g.L⁻¹.

Figure 13

Evolutions of hexoses selectivity and Pyruvaldehyde/lactic acid (PA + LA) selectivities at 40% DHA conversion as a function of the TiO₂′s acid/base balance. Comparison with the reference solid acid, NbOH, and the reference solid base, MgLaO.

Scheme 2

Proposed mechanism which would prevail on bifunctional acid-base catalysts.