Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework.Tailoring the acidity,particularly the distribution and location of...Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework.Tailoring the acidity,particularly the distribution and location of the Bronsted acid sites in the zeolite is effective for making it a better catalyst for these reactions.We prepared a series of H-mordenite（H-MOR） samples by varying the composition of the sol-gel,using different structure directing agents and post-treatment.NH3-TPD and IR characterization of adsorbed pyridine were employed to determine the amount of Bronsted acid sites in the 8-membered ring and 12-membered ring channels.It was shown that controlled synthesis was a promising approach to improve the concentration of Bronsted acid sites in MOR,even with a low Al content.Using an appropriate composition of Si and Al in the sol-gel favored a higher proportion of Bronsted acid sites in the 8-membered ring channels.HMI as a structure-direct agent gave an obvious enrichment of Bronsted acid sites in the 8-membered ring.Carbonylation of dimethyl ether was used as a probe reaction to examine the modification of the acid properties,especially the Bronsted acid sites in the 8-membered ring channels.There was a linear relationship between methyl acetate formation and the number of Bronsted acid sites in the 8-membered ring channels,demonstrating the successful modification of acid properties.Our results provide information for the rational design and modification of zeolites with spatial constraints.展开更多
HMOR分子筛在二甲醚羰基化反应中具有类似酶催化的优异性能.关于骨架铝的分布和反应活性位的识别是研究该反应机理的关键科学问题.早期的工作是基于理论计算研究二甲醚羰基化活性位点,但缺乏直接的谱学证据.通过在不同温度下焙烧NH_(4)...HMOR分子筛在二甲醚羰基化反应中具有类似酶催化的优异性能.关于骨架铝的分布和反应活性位的识别是研究该反应机理的关键科学问题.早期的工作是基于理论计算研究二甲醚羰基化活性位点,但缺乏直接的谱学证据.通过在不同温度下焙烧NH_(4)MOR制备了一系列HMOR催化剂,通过多种谱学表征手段研究分子筛骨架铝的稳定性以及铝原子落位信息,进一步通过二甲醚羰基化反应活性关联MOR分子筛的酸性和铝分布关系获得反应机理的谱学证据.首先从XRD(X-Ray diffraction)和SEM(Scanning electron microscope)发现经过不同温度焙烧,MOR分子筛结晶度和宏观形貌没有发生明显变化,但是通过一维^(29)Si,^(27)Al和^(1)H魔角旋转固体核磁谱(MAS NMR)发现分子筛局部环境发生了脱铝现象,产生了明显的缺陷羟基以及B酸量的下降.焙烧温度对HMOR分子筛骨架Al稳定性的影响较大,随着温度升高,脱铝逐渐加剧.定量^(1)H MAS NMR结合红外(IR)光谱提供了HMOR分子筛不同孔道B酸含量的分布.进一步使用2D^(27)Al MQ MAS NMR的方法以及结合切片分峰拟合技术区分出分子筛骨架中的四种不同T位点,发现当温度低于600℃,不同T位脱铝速率相当;当焙烧温度为600℃时,T3位点的Al原子脱除速率加快.最后研究了二甲醚羰基化反应性能与酸分布和铝分布的关系,获得羰基化反应活性中心的确凿谱学证据,T3-O33位置的Al位是羰基化反应的活性中心.展开更多
Seed‐assisted low alkalinity gel system was developed to explore the organic‐free synthesis of MORzeolite.MOR nanoassemblies with Si/Al ratio(SAR)up to 9.4 and high solid yield(84–94%)weresuccessfully obtained unde...Seed‐assisted low alkalinity gel system was developed to explore the organic‐free synthesis of MORzeolite.MOR nanoassemblies with Si/Al ratio(SAR)up to 9.4 and high solid yield(84–94%)weresuccessfully obtained under controlled low alkalinity conditions.Characterization results demonstratethat the acid strength increases in parallel with the SAR,while the total acid amount and theproton distribution in the main channels and the side pockets are similar for the samples.The protondistribution in the H‐MOR is not straightforwardly related to the Na+distribution in theas‐synthesized MOR,implying the transfer of the protons among the oxygen sites of framework Tatom.Relative to low‐silica samples I‐5.3 and I‐7.4,sample I‐9.4 displays the best mass transferperformance and accessibility of the acid sites by pyridine due to its relatively low Al density andmild dealumination degree.Correspondingly,sample I‐9.4(pyridine‐modified catalyst)shows thebest activity with ca.100%selectivity of methyl acetate(MAc)in the DME carbonylation reaction.The high steady MAc yield(6.8 mmol/g/h)over sample I‐9.4 suggests the promising application ofMOR nanoassemblies synthesized by this economical organic‐free strategy.展开更多
Crystallographic sites of Brönsted acids(Si-OH-Al)in zeolites,which are closely associated with the Al sites,play a significant and unique role in the catalytic application,especially when they are distributed in...Crystallographic sites of Brönsted acids(Si-OH-Al)in zeolites,which are closely associated with the Al sites,play a significant and unique role in the catalytic application,especially when they are distributed in open channel systems or confined in cavities with small pore openings.In this article,we unraveled constrained Al crystallographic sites in FER-type zeolites containing the distinct local environments(10-ring channels and ferrierite cavities)by Rietveld refinement against the powder X-ray diffraction data.Final refinement demonstrates that regardless of the types of structure-directing agents and synthetic medium utilized,T1 and/or T3 are Al-rich positions,which are further confirmed by theoretical calculations.This new finding of constrained Al sites in the FER-type zeolite can well explain its limited catalytic activity in the DME carbonylation reaction.展开更多
基金supported by the National Natural Science Foundation of China(21325626,21406120)the Postdoctoral Science Foundation of China(2014M560181,2015T80214)~~
文摘Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework.Tailoring the acidity,particularly the distribution and location of the Bronsted acid sites in the zeolite is effective for making it a better catalyst for these reactions.We prepared a series of H-mordenite（H-MOR） samples by varying the composition of the sol-gel,using different structure directing agents and post-treatment.NH3-TPD and IR characterization of adsorbed pyridine were employed to determine the amount of Bronsted acid sites in the 8-membered ring and 12-membered ring channels.It was shown that controlled synthesis was a promising approach to improve the concentration of Bronsted acid sites in MOR,even with a low Al content.Using an appropriate composition of Si and Al in the sol-gel favored a higher proportion of Bronsted acid sites in the 8-membered ring channels.HMI as a structure-direct agent gave an obvious enrichment of Bronsted acid sites in the 8-membered ring.Carbonylation of dimethyl ether was used as a probe reaction to examine the modification of the acid properties,especially the Bronsted acid sites in the 8-membered ring channels.There was a linear relationship between methyl acetate formation and the number of Bronsted acid sites in the 8-membered ring channels,demonstrating the successful modification of acid properties.Our results provide information for the rational design and modification of zeolites with spatial constraints.
文摘HMOR分子筛在二甲醚羰基化反应中具有类似酶催化的优异性能.关于骨架铝的分布和反应活性位的识别是研究该反应机理的关键科学问题.早期的工作是基于理论计算研究二甲醚羰基化活性位点,但缺乏直接的谱学证据.通过在不同温度下焙烧NH_(4)MOR制备了一系列HMOR催化剂,通过多种谱学表征手段研究分子筛骨架铝的稳定性以及铝原子落位信息,进一步通过二甲醚羰基化反应活性关联MOR分子筛的酸性和铝分布关系获得反应机理的谱学证据.首先从XRD(X-Ray diffraction)和SEM(Scanning electron microscope)发现经过不同温度焙烧,MOR分子筛结晶度和宏观形貌没有发生明显变化,但是通过一维^(29)Si,^(27)Al和^(1)H魔角旋转固体核磁谱(MAS NMR)发现分子筛局部环境发生了脱铝现象,产生了明显的缺陷羟基以及B酸量的下降.焙烧温度对HMOR分子筛骨架Al稳定性的影响较大,随着温度升高,脱铝逐渐加剧.定量^(1)H MAS NMR结合红外(IR)光谱提供了HMOR分子筛不同孔道B酸含量的分布.进一步使用2D^(27)Al MQ MAS NMR的方法以及结合切片分峰拟合技术区分出分子筛骨架中的四种不同T位点,发现当温度低于600℃,不同T位脱铝速率相当;当焙烧温度为600℃时,T3位点的Al原子脱除速率加快.最后研究了二甲醚羰基化反应性能与酸分布和铝分布的关系,获得羰基化反应活性中心的确凿谱学证据,T3-O33位置的Al位是羰基化反应的活性中心.
文摘Seed‐assisted low alkalinity gel system was developed to explore the organic‐free synthesis of MORzeolite.MOR nanoassemblies with Si/Al ratio(SAR)up to 9.4 and high solid yield(84–94%)weresuccessfully obtained under controlled low alkalinity conditions.Characterization results demonstratethat the acid strength increases in parallel with the SAR,while the total acid amount and theproton distribution in the main channels and the side pockets are similar for the samples.The protondistribution in the H‐MOR is not straightforwardly related to the Na+distribution in theas‐synthesized MOR,implying the transfer of the protons among the oxygen sites of framework Tatom.Relative to low‐silica samples I‐5.3 and I‐7.4,sample I‐9.4 displays the best mass transferperformance and accessibility of the acid sites by pyridine due to its relatively low Al density andmild dealumination degree.Correspondingly,sample I‐9.4(pyridine‐modified catalyst)shows thebest activity with ca.100%selectivity of methyl acetate(MAc)in the DME carbonylation reaction.The high steady MAc yield(6.8 mmol/g/h)over sample I‐9.4 suggests the promising application ofMOR nanoassemblies synthesized by this economical organic‐free strategy.
文摘Crystallographic sites of Brönsted acids(Si-OH-Al)in zeolites,which are closely associated with the Al sites,play a significant and unique role in the catalytic application,especially when they are distributed in open channel systems or confined in cavities with small pore openings.In this article,we unraveled constrained Al crystallographic sites in FER-type zeolites containing the distinct local environments(10-ring channels and ferrierite cavities)by Rietveld refinement against the powder X-ray diffraction data.Final refinement demonstrates that regardless of the types of structure-directing agents and synthetic medium utilized,T1 and/or T3 are Al-rich positions,which are further confirmed by theoretical calculations.This new finding of constrained Al sites in the FER-type zeolite can well explain its limited catalytic activity in the DME carbonylation reaction.
