Carbonylation Of Dimethyl Ether Over Mordenite:Active Site Regulation And Mechanism Investigation

As a green alternative energy,ethanol is widely used as a fuel additive because of its environmental and long-term economic advantages,which conforms to the develop-mental direction of clean fule and energy diversification in China.Carbonylation of dimethyl ether?DME?to methyl acetate?MA?followed by hydrogenation is a promis-ing synthesis route for converting syngas into ethanol,owing to its superior atom econ-omy,mild reaction condition and potential implementation in industry.H-MOR,con-stituted by 8-MR and 12-MR channels,is proved an excellent candidate to catalyze this reaction.Due to the confinement effect of 8-MR side pockets,Br?nsted acid sites lo-cated at 8-MR in H-MOR selectively catalyzes the formation of MA,which were con-sidered as active sites.However,the rapid deactivation is caused by the coke formation that takes place on Bronsted acid sites in 12-MR.Focusing on MOR catalyst,the pre-sent thesis investigates the DME carbonylation reaction systematically in the aspects of regulating active sites and discussing catalytic mechanism over dual active site.Improving the ratio or the number of Br?nsted acid sites in 8-MR through catalyst design and engineering is a strategy to promote the catalytic activity.A series of Ce-containing MOR zeolites were synthesized by a facile one-step hydrothermal method.Comparing with the ball-milling counterpart,isomorphous substitution of framework T atom by Ce³⁺has been evidenced by multiple characterization techniques,such as XRD,UV-Vis,Raman and ²⁹Si MAS NMR.Quantitative analysis of acid site suggests that the distribution of Br?nsted acid sites in different channels?8-MR and 12-MR?of H-MOR are modulated by Ce incorporation.Incorporating a proper content of Ce,for instance,with an actual Ce content of 4.6%in the MOR zeolite,is able to enrich the Br?nsted acid sites in the 8-MR,whereas the number of Br?nsted acid sites in 12-MR are kept unchanged.As a result,the space time yield of MA is improved significantly,which is 32.6%higher than that on MOR without addition of Ce.Combining DFT cal-culations,acidic strength analysis and kinetic study,the regulation mechanism of Ce incorporation was explored:Ce preferentially located at T1 sites in 12-MR,and as a result,Al sitting on T3 site is more thermodynamically stable.This means that the Al atoms will be enriched in 8-MR that leads to more active sites.It results in an increase of activity.Pre-adsorption of pyridine is recognized as an efficient technique to improve sta-bility by suppressing coke formation,owing to selective shielding of Br?nsted acid sites in 8-MR.Therefore,the adsorption strength of pyridine is a key fator to determine the lifetime of MOR in industry.To clarify the influence of Br?nsted acid density and strength on the stability of H-MOR saturated with pyridine in the DME carbonylation process.The Br?nsted acidity were successfully modulated through facile thermal treat-ment.Quantitative results of the acid properties indicate that the concentration of Br?nsted acid site is the critical factor for catalytic stability and Br?nsted acidity is weakened as the amount of Br?nsted acid site increases.Furthermore,DFT calculation reveals that the absorbed stability of pyridine is decreased as decreasing the Br?nsted acid strength,which futher leads to the regeneration of active sites available for coke formation by pyridine desorption.The coke intermediate formed in the 12-MR will re-sult in the desorption of neighboring adsorbed pyridine due to the steric effect,which contribute to an accelerated deactivation.To clarify the catalytic mechanism for MA formation on Cu-Br?nsted acid site dual active site,a Cu/H-MOR catalyst with dual active sites?Cu cluster and Br?nsted acid sites?was constructed.Taking a comparative in situ FTIR study based on the Cu/H-MOR,Cu⁺/H-MOR and H-MOR together with DFT calculation and kinetic anal-ysis,the promoting role of Cu⁺for the reaction is excluded as the strong adsorption of CO on Cu⁺hinders the adsorbed CO from participating in the following reaction.Be-sides,a synergetic mechanism involving both the Cu and Br?nsted acid sites for DME carbonylation on Cu/H-MOR is proposed:DME dissociates on the Cu and Br?nsted acid site pair to form Cu–CH₃ and adsorbed methanol coordinated to the framework O.Subsequently,CO inserts into Cu–CH₃ and forms acetyl,followed by reaction with adsorbed CH3OH on the Br?nsted acid site,resulting in adsorbed MA.