Acid-base Catalysts For Acrylic Acid Production By Gas-phase Dehydration Of Lactic Acid And The Impact Of Water On The Solid Acidity

Catalytic conversion of lactic acid?LA?,one of the top platform molecules derived from biomass resources,to value-added chemicals and materials has been an important aspect for sustainable chemical industry.Catalytic dehydration of LA to acrylic acid?AA?has been attracting more and more attention,as AA is a versatile monomer in the manufacture of a wide variety of polymers.In this dissertation,gas-phase dehydration of LA to produce AA?LTA?was comprehensively investigated on various solid acid-base catalysts.Moreover,we also developed powerful solid-state NMR techniques for making assessment of the surface acidity of some typical soild acids in the presence of water?steam?to meet the present demanding for knowledege about the acidic property of solid catalysts working in water or moisture for the valorization of biomass-derived platform molecules.Calcium-hydroxyapatites?HAPm-T?of varying molar Ca/P ratios?m = 1.58¹.69?and calcination temperatures?T = 360⁷00 oC?as well as alkali ion-exchanged ? zeolites?MxNa1-x?,M = Li⁺,Na⁺,K⁺,Rb⁺ or Cs⁺?with different exchange degrees?x = 0¹.00?were selected to investigate the correlations between catalyst surface acid-base property with thier catalytic performance for the LTA reaction.It is disclosed that the formation rate for AA showed a volcano-type dependence on the acidity/basicity ratio of the HAP m-T catalysts,while the acetaldehyde?AD,a main byproduct formed by LA decarboylation/decarbxylation?production rate increased linearly with increasing the acidity/basicity ratio.The more basic but less acidic K⁺-?x = 0.92⁰.98?,Rb⁺-?x = 0.85⁰.95?,or Cs⁺-?x = 0.71⁰.90?exchanged ? zeolites always showed a higher AA selectivity?60⁷0 mol%?than the acidic Li⁺-and Na⁺-exchanged ones?? 40 mol%?,which were more selective for AD production;while catalysts with an excessive or complete exchange degree of K⁺,Rb⁺,or Cs⁺ would be too basic to the production of AA but become favorable for LA condensation to 2,3-pentandione?2,3-PD?.Besides,AA selectivity decreased significantly when trifluoroacetic acid or NH3 molecules were introduced into the reaction to poison the basic or acidic sites,respectively.Thus,a kind of cooperative acid-base catalysis was invovled in the selective production of AA,while AD and 2,3-PD were produced by an acid-catalyzed decarbonylation/decarboxylation reaction and a base-catalyzed condensation reaction,respectively.The catalytic performance for the LTA reaction was compared among various K⁺-exchanged zeolites?KxNa1-xZ_y?to discuss the effects of zeolite type?Z = ZSM-22,ZSM-35,MCM-22,ZSM-11,ZSM-5,ZSM-5/ZSM-11 and ??and SiO₂/Al₂O₃ ratio?y?.It was shown that ZSM-5 and ? were found to be more efficient than other zeolites,and that not only higher AA selectivity but also better catalytic stability was obtained by lowering the y number of ? and ZSM-5.A K0.89Na0.11ZSM-5₂7 was identified as the best-performing catalyst to date for the LTA reaction,which offered high AA selectivity?75⁸4 mol%?for longer than 80 h without an obvious LA conversion decrease?? 5%?at 360 oC and a high LA space velocity?2.1⁴.2 h-1?with a LA partial pressure of 3.5⁷.4 kPa.Furthermore,an in situ calcination of the used catalyst at 450 °C in an air flow entitled complete catalyst regeneration.Solid-state 31 P and ¹H MAS NMR spectroscopies,using trimethylphosphine as a probe molecule,were developed to assess the acidic property of some typical solid acids?SiO₂,HZSM-5,?-Al₂O₃,ZrO2,SiO₂-Al₂O₃,hydrated niobium pentoxide and hydrated tantalum pentoxide?in the presence of water.It was demonstrated that water effects on the surface acidity were dependent on the nature of the solid acids.Water can not only lead to either part conversion of surface Lewis acid sites into Br?nsted ones for SiO₂-Al₂O₃ or complete conversion for hydrated niobium oxide and tantalum oxide,and even make Leiws acid sites become strengthened for SiO₂-Al₂O₃,but also poison the Lewis acid sites for ?-Al₂O₃ and ZrO2.On the other hand,it has little effect on the Br?nsted acid sites on HZSM-5.