Synthesis Of Mesoporous ZSM-5 Zeolites And Its Application In Catalyzing Ethanol And Oleic Acid To Prepare Light Olefins

Light olefins are one of the most important basic raw materials in petrochemical industry.At present,the main ways to produce light olefins are steam cracking and catalytic cracking of naphtha.With the increasing requirement for light olefins and the decreasing supply for fossil fuels,it has been the trend to develop non-petroleum routes to prepare light olefins.Ethanol and oleic acid,which originate from biomass,will become potential raw materials for the production of light olefins instead of petroleum resources because of several advantages:widely available,inexpensive and renewable.Traditional microporous ZSM-5 easily causes diffusion restriction of macromolecule feedstocks due to its smaller opening pore size and long channel,consequently affecting the conversion and cracking efficiency of macromolecule feedstocks.Three mesoporous ZSM-5 zeolites(SiO2/Al2O3=200)with different pore sizes(MZSM-5-A,MZSM-5-B and MZSM-5-C)were prepared by hydrothermal synthesis,and their major pore diameter are about 4.8 nm,16 nm and 22 nm,respectively.The obtained samples were characterized by XRD,SEM,N2 adsorption-desorption,ICP-OES,NH3-TPD and Py-IR.The effect of acidity and pore structure of catalysts on the catalytic performance was investigated by catalytic cracking of ethanol and oleic acid to produce light olefins.The influences of reaction temperature,WSHV and the carrier gas type on light olefin yield and selectivity were also studied.The effect of synthesized samples was compared with the traditional microporous ZSM-5 zeolite(HZSM-5).The characterization of catalysts showed that the orders of external specific surface area and mesoporous volume of the four samples were MZSM-5-C>MZSM-5-B>MZSM-5-A>HZSM-5.The four samples had similar total acidity and contained mainly Lewis acidic sites.The synthesized three samples were mesoporous zeolites.Catalytic cracking experiments with nitrogen as carrier gas showed that MZSM-5-C was the most effective in catalyzing ethanol into light olefins(ETO)reaction,and obtained the highest light olefin yield(318.3 mL/g)and ethylene selectivity(42.7%)at 400?;MZSM-5-B was the most effective in catalytic cracking of oleic acid into light olefins(OATO)reaction,achieving complete conversion of oleic acid at 500? and obtaining the highest light olefin yield(230.7 mL/g)and light olefin selectivity(38.1%)at 550?.The mesoporous structure of the synthesized ZSM-5 is helpful to decrease the diffusion limit for feedstock moleculars and intermediates,and thus to inhibit the carbon deposit,consequently promoting conversion and selective production of light olefins.The optimal catalyst selected for cracking of ethanol and oleic acid was different,which may be related to the size and chemical structure of feedstock molecular as well as the acidity of catalysts.MZSM-5-B contained more Br(?)nsted acidic sites,which could be the active sites for cracking and deoxygenation reaction for macromolecules oleic acid,and thus was most effective for oleic acid.Catalytic cracking experiments with steam as carrier showed that steam as carrier gas significantly improved the conversion of raw materials and the light olefin selectivity.In ETO reaction,MZSM-5-C achieved the highest light olefin yield(463.2 mL/g)and ethylene selectivity(77.5%)when the experiment was performed at 400? with ethanol/H2O mass ratio at 1:3;In OATO reaction,MZSM-5-B obtained the highest light olefin selectivity(69.8%)when the experiment was carried out at 500? with the rate of water injection was 1 mL/min.The stability tests showed that the synthesized MZSM-5-B and MZSM-5-C had good thermal and hydrothermal stability as well as renewability.