Design,Synthesis And Performance Evaluation Of Novel Small Pore Zeolite For DeNO_x In Diesel Exhaust

Diesel vehicles are widely used due to their strong power and good economy.However,NO_x emissions in their exhaust gas have become not be ignored.At present,ammonia selective catalytic reduction(NH₃-SCR)technology is considered to be one of the most effective technologies for removing NO_x from diesel exhaust,and the key is the catalysts.Among the catalysts of De NO_x,copper-based small pore zeolite catalysts have become the current hotspot because of their excellent NH₃-SCR activity,high N₂ selectivity and good hydrothermal stability.The bottleneck of the catalyst is that the higher silicon-aluminum ratio(Si/Al>8)zeolite catalysts have good hydrothermal stability,but the synthesis cost is high and the crystallization time is long.Low silicon-aluminum radio(Si/Al<6.5)zeolite catalysts have good low temperature activity and wide temperature activity due to the rich in acidity and ion exchange site,however,the hydrothermal stability still need to be improved.Based on the above difficulties,for the high silicon-aluminum ratio zeolite catalyst,SSZ-13(Si/Al>13)was rapidly synthesized from industrial solid waste coal gangue.On the basis of ensuring catalyst performance,the cost of synthesized zeolite was reduced and the crystallization time was shorted to half.For the low silicon-aluminum ratio zeolite catalyst,novel Al-rich twin-crystal zeolites,such as OFF-CHA and OFF-ERI,with excellent NH₃-SCR activity and hydrothermal stability were synthesized.The NH₃-SCR performance and mechanism of hydrothermal stability were inverstigated.The main work and conclusion are as follows:(1)SSZ-13 zeolite was synthesized by ultrasonic pretreatment from coal gangue.By optimizing the experimental conditions and studying the effect of ultrasound time on the synthesis of SSZ-13.It was found that the ultrasonic pretreatment time of 1 h and the gel Si/Al of 15 were the optimal conditions for the synthesis of SSZ-13 zeolite(Si/Al=13).Under these conditions,the nucleation time of synthesizing SSZ-13 zeolite from coal gangue was shortened to nearly half compared with that from chemical.The rapid synthesis mechanism was clarified,that is,ultrasonics pretreatment was beneficial to the formation of locally ordered structural units such as Si-O-Si and Si-O-Al,which promote the rapid nucleation of zeolite.The SCR performance test found that NO_x conversion of Cu-SSZ-13 synthesized with crystallization time of 36 h and coal gangue as raw materils was more than 95%with gaseous hourly space velocity of 50,000 h^-1 in the range of 180-400?.And the NO_x conversion still maintained more than 94%in the range of 200-400?.after hydrothermal treatment at 750?for 6 h.(2)Stirring treatment was used to optimize the synthesis process of SSZ-13zeolite with coal gangue as raw material,and the formation mechanism of locally ordered structural units such as Si-O-Al was explored.The study found that the controllable preparation of SSZ-13 zeolite with higher crystallinity,smaller grain size and better NH₃-SCR performance could be achieved through process optimization.The presence of tetrahedral aluminum was the main reason to promote the rapid synthesis of SSZ-13,which also promoted the conversion of octahedral aluminum to Si-O-Al structural units,thereby further accelerating the nucleation and crystallization rate of SSZ-13 zeolite.The SCR performance test found that NO_x conversion of Cu-SSZ-13synthesized with crystallization time of 36 h and coal gangue as raw materils was more than 90%with gaseous hourly space velocity of 100,000 h^-1 in the range of 180-400?.And the NO_x conversion still maintained more than 80%in the range of 180-400?.after hydrothermal treatment at 750?for 12 h.These properties are similar to Cu-SSZ-13 synthesized with crystallization time of 60 h and chemicals as raw materials.Comprehensive comparison,the Cu-SSZ-13 catalyst prepared from coal gangue and stirring treatment has better development prospects.(3)Novel Al-rich OFF-CHA twin-crystal zeolite was synthesized by one-pot hydrothermal method and and its NH₃-SCR performance and mechanism was investigated.Results showed that at the gases hourly space velocity of 100,000 h^-1,the NO_x conversion of Cu_3.1-OFF-CHA was more than 90%at 160-550?.In addition,Cu_3.1-OFF-CHA also exhibited excellent water resistance and sulfur toleration.Moveover,Cu_3.1-OFF-CHA exhibitd more than 80%SCR activity in a temperature range from 180 to 450? after hydrothermal treatment at 750?for 16 h.The mechanism analysis found that the existence of the OFF-typezeolite is the reason for good hydrothermal stability of Cu-OFF-CHA.(4)OFF-ERI twin-crystal zeolite catalysts were synthesized by template-free hydrothermal method and its NH₃-SCR performance was investigated.Results showed that the NO_x conversion of Cu_4.0-OFF-ERI was more than 95%in the temperature range of 160-550? at the gases hourly space velocity of 100,000 h^-1.Moreover,Cu_4.0-OFF-ERI catalyst exhibited excellent water resistance,sulfur toleration,and sulfur and water resistance.By comparison,it was found that the NO_x conversion of Cu_4.0-OFF-ERI was significantly better than that of Cu_3.5-SSZ-13 before hydrothermal treatment.And the NO_x conversion of Cu_4.0-OFF-ERI is still maintain more than 80%in the temperature range of 180-450?after hydrothermal aging at 750? for 16 h.