Heterogeneous Catalytic Synthesis of Methyl Lactate and Lactic Acid from Sugars and Their Derivatives

Abstract

Recent developments in sugar transformations to methyl lactate and lactic acid are critically summarized. The highest yield of methyl lactate from glucose obtained over Sn(salen)/octylmethyl imidazolium bromide catalyst was 68 % at 160 °C whereas the highest yield of lactic acid of 58 % was achieved over hierarchical Lewis acidic Sn-Beta catalysts at 200 °C under inert atmosphere. In addition to the desired products also humins are formed in water whereas in methanol alkyl glucosides- and -fructosides as well as acetals were generated, especially in the presence of Brønsted-acidic sites. The main challenges limiting the industrial feasibility of these reactions are incomplete liquid phase mass balance closure, complicated product analysis and a lack of kinetic data. In addition to reporting optimized reaction conditions and catalyst properties also catalyst reuse and regeneration as well as kinetic modelling and continuous operation are summarized.

Keywords: Lewis acids; heterogeneous catalysis; lactic acid; methyl lactate; sugars.

Figure 1

The main reaction pathways for synthesis of a) lactic acid adapted from Ref. [33] and methyl lactate from glucose, adapted from Ref. [30].

Figure 2

Yields of ML over Sn‐MWW‐nano, delaminated Sn‐MWW, nanosized Sn‐MFI, Sn‐MFI‐meso and Sn‐MOR bulk. ^[13] Conditions: 0.12 mol L⁻¹ glucose, glu/cat 1.4 wt/wt at 160 °C in 20 h.

Figure 3

Yields of ML over a) dealuminated Sn‐BEA (100) (▪), Sn‐Beta‐HT (100) (•) and desilicated and dealuminated Sn‐BEA(100) (□); b) dealuminated Sn‐USY (25) (▪) and dealuminated Sn‐USY (25) prepared via dissolution‐reassembly, acidic dealumination and Sn‐impregnation (□) adapted from Ref. [8]. Conditions: glucose 3 wt % in methanol, 1 wt % catalyst, 160 °C in 5 h.

Figure 4

Correlation between methyl lactate yield and ratio between meso‐ and micropore volume. Data is taken from Ref. [7] for mesoporous catalysts (▪) and from Refs. [7] (+), [4] (□) and [3] (▴) for microporous catalysts. Notation: numbers are given in Table 1. Reaction conditions: 0.014 mol L⁻¹ glucose in methanol, glu/cat 1.6 wt %/wt %, 160 °C, 10 h [3, 4] 0.14 mol L⁻¹ glucose in methanol, glu/cat 1.6 wt %/wt %, 160 °C, 6 h adapted from Ref. [7].

Figure 5

Effect of a) reaction temperature and b) pressure in glucose transformation over Sn‐Beta‐H4 at a) 5 bar N₂ and b) 160 °C. Conditions: 0.137 mol L⁻¹, glu/cat 1.6 wt %/wt %, 6 h adapted from Ref. [7].

Figure 6

Transformation of 1,3‐dihydroxyacetone over Sn/Al₂O₃ as catalyst at 80 °C under 250 kPa to different products in ethanol as a solvent. Notation: 1,3‐dihydroxyacetone (DHA), ethyl lactate (EL), glyceraldehyde diethyl acetal (GLADA), pyruvaldehyde (PA), pyruvaldehyde hemiacetal (PAHA), and pyruvaldehyde diethyl acetal (PADA). ^[16] Copyright received from Elsevier.

Figure 7

Simplified reaction scheme for transformation of 1,3‐dihydroxy‐acetone to ethyl lactate based on a kinetic model adapted from Ref. [16].

Figure 8

Amounts of sucrose (▪), glucose (▴) and fructose (□) as a function of time in their transformation to methyl lactate at 160 °C over HT Sn‐Beta catalyst adapted from Ref. [9].

Figure 9

Amounts of reactant and products as a function of time in a) fructose and b) glucose transformation to fructose at 160 °C over HT Sn‐Beta catalyst adapted from [9]. Notation: Fructose (□), methyl fructoside (▪), glucose (o), methyl glucoside (•), and methyl lactate (Δ).

Figure 10

Kinetics in fructose transformation to ML over Sn(Salen)IL catalyst at 160 °C under 20 bar nitrogen using 0.03 mol L⁻¹ fructose in methanol; adapted from Ref. [18]. Notation: (▴) methyl lactate, (□) methyl levulinate, (+) 5‐methoxy‐methylfurfural, (▪) pyruvaldehyde dimethylacetal and (o) methyl glycolate.

Figure 11

Fructose transformation in a fixed bed reactor as a function of time‐on‐stream The catalyst was regenerated ex situ at 550 °C for 6 h, adapted from Ref. [23].

Figure 12

Fructose transformation to glyceraldehyde and 1,3‐dihydroxyacetone via the retro‐aldol reaction on acid and base sites of ZrO₂ (adapted from Ref. [59]).