Biomass Route For Catalytic Conversion Of Lactic Acid To Acetaldehyde And Propionic Acid

Acetaldehyde and propionic acid as important commercial chemicals are widely used in the world, so that their demand is increasing year by year. Acetaldehyde and propionic acid are mainly produced through oil route in industry. With fast consumption of the fossil resources in industry, resulting in a drastically decrease, it is particularly important for us to look for alternative resources as raw materials to produce chemicals. In recent years, utilization in biomass has become one of advanced investigations in the world because it has regeneration and sustainability, receiving extensive attention.In this paper, under the guide of sustainable development, bio lactic acid as starting material were used to synthesize acetaldehyde and propionic acid through decarbonylation / hydrodeoxygenation process, focusing on the relationship between preparation methods and structure properties, the structure and activity. The main purpose is to design, select and develop the green, efficient, stable catalysts for catalytic conversion of lactic acid into acetaldehyde and propionic acid via decarbonylation or hydrodeoxygenation of lactic acid. The main research contents are as follows:The catalysts are prepared through precipitation or co-precipitation methods, and further treated with a series of steps including drying, calcinations, pressing and crushing. The structure, morphology and properties of catalysts were characterized by a variety of characterization methods such as XRD, FT-IR, BET, SEM, TEM, TG, CO2/NH3-TPD and H2-TPR; the activity and stability of catalysts were evaluated under the fixed bed reactor.In the research on decarbonylation and hydrodeoxygenation reactions of lactic acid, the preparation conditions of catalysts together with catalytic reaction conditions were investigated in detail. For the decarbonylation of lactic acid into acetaldehyde, metal sulfates together with heteropoly acids, Mg/Al composite oxides were mainly studied in this paper; while for the hydrodeoxygenation of lactic acid into propionic acid, Fe and its oxides were mainly studied. We optimized the process conditions of the lactic acid decarbonylation reaction or hydrodeoxygenation reaction, including preparation methods, activation temperatures, reaction temperatures, concentration of lactic acid, liquid hourly space velocity and reaction time. The optimal conditions were achieved. For metal sulfates-heteropoly acid system, under the optimal reaction temperature of 380 oC, aluminum sulfate was determined as an excellent catalyst for catalyzed decarbonylation of lactic acid to acetaldehyde, achieving lactic acid conversion of 100% as well as acetaldehyde selectivity of 92.1%. In high space hourly velocity of liquid（LHSV=31.2 h-1）, lactic acid can still completely converted. The time on stream for stability of aluminum sulfate can reach nearly 50 h. The spent catalyst can be completely regenerated and recovered to the initial activity by calcination in air atmosphere at 500 oC. For the Mg/Al composite oxide systems, we prepared magnesium aluminate spinel by co-precipitation; next to use it to catalyze lactic acid decarbonylation to obtain acetaldehyde. Mg-Al spinel was confirmed as active species by changing the Mg/Al precursor, Mg/Al molar ratio, calcined temperature. Under the optimum reaction conditions of the Mg/Al molar ratio at 1:2 and the activation temperature at 1000 oC, spinel catalyzed decarbonylation of lactic acid to acetaldehyde, achieving lactic acid conversion of 100% as well as acetaldehyde selectivity of 87.5%. On the basis of previous studies, we found that the formation of Mg/Al composite oxide is very sensitive to pH value in the course of adjusting the pH value. High pH value is favorable to the formation of MgAl2O4 structure; at low pH value of 7-8, we got another Mg/Al composite oxide of Mg_0.388Al_2.408O4. Under the optimum reaction conditions of the Mg/Al molar ratio at 1:2, the activation temperature at 1000 oC and high liquid hourly space velocity（LHSV = 13.0 h-1）, the Mg_0.388Al_2.408O4 can efficiently catalyze lactic acid to acetaldehyde via decarbonylation reaction, which can efficiently run 500 h on stream, acetaldehyde selectivity remaining almost unchanged（about 93%）. For the hydrodeoxygenation of lactic acid to propionic acid, we found that ferric oxide as catalyst precursor has excellent catalytic performance. With further study on structure-reaction time, we found that the real active species was Fe3O4. Under the optimum reaction conditions of reaction temperature at 390 oC, lactic acid concentration at 20wt%, feed flow rate of 1.0 g/h, carrier gas flow rate of 1.0 ml/min, and the yield of propionic acid reached 46.7%. At LHSV of 26.0 h-1, the catalyst can continuously run 100 h. Furthermore, the selectivity of the catalyst was almost unchanged, and lactic acid conversion rate decreased only a little.According to the characterization of the catalyst, we found that the surface acidity of catalyst is the key to influence the decarbonylation of lactic acid to acetaldehyde. The medium acid favored to the high selectivity of acetaldehyde, while strong acid will cause in several side reactions such as lactic acid polymerization and coking reaction; and weak acid is favorable to dehydration of lactic acid. Exploring the characterisitics of decarbonylation reaction has important significance to guide the design and development of high efficient catalysts.