Mg(OH)2‐Facilitated Liquid‐Phase Conversion of Lactic Acid into 1,2‐Propanediol over Cu: An Experimental and Theoretical Study

Abstract Mg(OH)2 is found to exhibit superior performance in the liquid‐phase conversion of lactic acid (LA) into 1,2‐propanediol over Cu. A conversion of 90 % with a selectivity of 98 % is achieved at 513 K and 5 MPa H2. Mg(LA)2 could be identified as a crucial intermediate in this reaction, as it undergoes faster conversion than the combination of LA and Mg(OH2) and regeneration of Mg(OH)2 through the conversion of Mg(LA)2 as a substrate. DFT calculations reveal that the energetic span of the reaction decreases from 46.6 kcal mol−1 catalyzed with no cation to 43.6 kcal mol−1 with [Mg(OH)]+, confirming the facilitating effect of Mg(OH)2.


Catalyst Preparation
Cu/SiO 2 catalyst was prepared via an ammonia evaporation precipitation method. 4.05g Cu(NO 3 ) 2 ·3H 2 O was dissolved in deionized water (25 mL) and stirred for 0.5 h in a round bottom flask. 10.7 ml NH 3(aq) solution (25 wt. %) was diluted to 50 ml using deionized water. Afterwards, the diluted NH3 (aq.) was dropwise added into Cu solution. 1 h after the complete addition of NH 3(aq) , 40 wt. % colloidal silica was added to the slurry. The mixture was stirred at 750 rpm for overnight at room temperature in order to obtain a well-dispersed suspension. The ammonia was removed by heating in an oil bath at 363 K until the pH of the slurry was 7. The resulting powder was filtered of and washed with 2 L deionized water and subsequently dried in a drying oven at 353 K for overnight.
The prepared materials were calcined at 823 K for 4 h. The activation took place under H 2 at 723 K with 5 K/min for 4 h. The pristine catalysts were stored under Ar atmosphere.

Catalyst Characterization
N 2 -physisorption analysis. The textual properties of the samples were determined by N 2physisorption at 77 K using a QUADRASORB SI automated surface area and pore size analyzer.
Samples were pre-treated in vacuum at 473 K for overnight. The pore-size distribution was estimated by the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the adsorption isotherms. Specific surface areas were calculated using the Brunauer−Emmett−Teller (BET) method.
Transmission electron microscopy (TEM). Images were obtained using a Philips TECNAI G2 F20 system electron microscope at 100 kV equipped with a field emission gun. The sample powder was dispersed in isopropanol by ultra-sonication. Drops of the suspension were applied onto a copper grid-supported transparent carbon foil and dried in air. The particle size was determined by using the software ImageJ.

Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).
The catalyst was digested using a combination of the concentrated nitric acid and hydrochloric acid, followed by measurement of the solution by ICP-OES performed on an ICP Spectroflame D by Spectro.

Catalyst Activity Tests
Hydrogenolysis of LA was carried out in a 50 mL stainless steel autoclave equipped with a Teflon inlet. In a typical run, aqueous LA solution (0.5 g LA, 20 g H 2 O), Mg(OH) 2 (0.4 g) and 0.1 g 25%Cu/SiO 2 catalyst were charged into the autoclave. The reactor was purged five times and pressurized to 5 MPa with H 2 . The sealed autoclave was heated to 513 K and stirred at a speed of 750 rpm. Samples were taken periodically.

Recycling Tests
Recycling tests were carried out with 25%Cu/SiO 2 catalyst. After each test, the catalyst was washed by deionized water. 5% fresh catalyst was charged in next run in order to supplement catalyst loss during the recycling.

Analytical Method
The gas-phase was collected using a gasbag and analyzed by GC performed on a Agilent HP 6890 instrument with a Shin Carbon ST micropacked GC column (308-373 K, TCD, He). Liquid-phase products were analyzed by HPLC performed on a Shimadzu system (Rezex ROA Organic Acid H + (8%) column by Phenomenex, eluent: 0.05 M H 2 SO 4 , the mobile phase flow rate is 0.6 ml/min with a column temperature of 343 K). The LA conversion (Eq. 1) and selectivity (Eq. 2) were calculated on the basis of carbon.