Studies On Surface Structure Regulation And Hydrogenation Performance Of Nano-Ru Catalysts

As an essential reaction in the fields of petrochemical industry,coal-to-chemicals industry,and organic synthesis,catalytic hydrogenation of aromatic compounds used to produce many important fine chemicals and intermediates.P-methylcyclohexylamine?PMC?was widely used in food,medicine,rubber,chemical industry and other fields,which was obtained by pnitrotoluene?PNT?hydrogenation generally.However,a large number of dicyclohexylamine by-products and tar will be produced in this process,resulting in huge waste of raw materials,difficult separation of products,inactivation of catalysts and unnecessary increase of PMC cost.The fundamental way to solve this problem is to develop a new type of efficient hydrogenation catalyst.At present,Ru catalyst is widely used in this field,impregnation and precipitation method are used to prepare Ru catalyst in normal conditions.However,there are many problems in its research and application:?1?low utilization rate and stability of Ru active site are poor;?2?The internal relation between catalyst acidity and catalytic hydrogenation performance is not clear;?3?Ru particles of the active component are easy to agglomerate,the means for controlling the dispersion structure of Ru particles is limited,and the efficiency of the Ru catalyst is still quite low;?4?The relationship between the active potential structure and catalytic action of Ru is not well understood.In view of the above problems,based on a lots of literatures,the paper studiedthe optimization of stability of Ru catalysts,acidity regulation of Ru catalysts,regulation of dispersive structure of Ru active sites,and preferential exposure of suitable Ru active sites.A series of Ru-based catalysts with high activity were obtained and the selectivity of PMC was improved effectively as well,accumulating a certain theory and experience for the development of PNT hydrogenation reaction and other supported catalysts for aromatic hydrogenation reaction.The specific research content and conclusions are as follows:?1?Study on performance of PNT hydrogenation over Ru-Y/NaY catalystsDue to the active component of Ru catalyst prepared by precipitation-precipitation method is easy to loss in the hydrogenation reaction.The NaY molecular sieve was selected as the carrier for its larger specific surface area,and the metal yttrium is used as the auxiliary agent,the stepwise dipping-precipitation method was applied to prepare the Ru-Y/NaY^x catalysts with high activity.XRD,TEM,H₂-TPR and ICP-AES were applied to analyze the structure and morphology of Ru catalysts.The results showed that the amount of Y has a great influence on the dispersion of Ru particles.Although suitable amount of yttrium is helpful to the dispersion of Ru active sites,excessive addition of Y will lead to agglomeration of the active ingredient and affect the utilization rate of Ru constituents.The hydrogenation performance of Ru-Y/NaY^xcatalysts were evaluated by PNT liquid-phase hydrogenation to PMC reaction.The results showed that the conversion of PNT over Ru-Y/NaY² catalyst was 99.7%and PMC selectivity was 82.5%.After using five times,the Ru loading of Ru-Y/NaY² is still close to that of the fresh catalyst,in addition,There was no significant decrease in activity of Ru-Y/NaY².It is proved that adding a certain amount of Y can improve the stability of Ru catalyst.?2?Study on acidity regulation and hydrogenation performance of Ru catalystsIn order to solve the problem of dehydration in PNT hydrogenation,a series of Ru catalysts modified by different alkaline earth metals?Ca,Mg,Sr?were prepared by stepwise dipping-precipitation method using Na Y zeolite as carrier and RuCl₃ as active component precursor.Using XRD,TEM,ICP-AES,and Py-IR analyzed the structure and acidity distribution of Ru catalysts.The results showed that appropriate amount of strontium is helpful to decrease the Ru particle size,and secondly,the loading of alkaline earth metals and Ru particles has a great influence on the acidity of the catalysts.According to the results of liquid-phase hydrogenation of PNT,it was found that when the molar ratio of Sr/Ru was 0.1,the content of Br?nsted and Lewis acid in Ru-Sr/NaY catalyst was the highest,when the B acid content was increased alone,the rate of dehydration reactions increased;If the content of B acid and L acid in catalyst were increased simultaneously,the dehydration reaction can be inhibited obviously.?3?Study on dispersed structure regulation of Ru active sites and hydrogenation performance of Ru-PEG_x/Na Y catalystBy adding PEG-400 during the catalyst preparation process,the affinity between PEG-400 and Ru³⁺can prevent the agglomeration of Ru³⁺effectively.A series of Ru-PEG_x/Na Y catalysts were prepared by stepwise dipping-precipitation method,the optimal preparation conditions for Ru catalysts was obtained by orthogonal experiments.Then,combining a variety of characterization analysis and PNT catalytic hydrogenation results,it was found that the addition of an appropriate amount of PEG-400 can regulate the dispersed structure of the Ru particles of the Ru catalysts effectively.When the PEG content was 7.5 mL,the Ru particles are highly dispersed on the surface of NaY,the average particle size was only about 2 nm,which is much lower than the average Ru particle size?>10 nm?obtained by conventional precipitation method.Under the optimal hydrogenation conditions,PNT hydrogenation reaction over Ru-PEG_7.5/NaY was tested,the conversion of PNT was 100%,the PMC selectivity was 95.2%.In addition,The effect of ammonia on PNT hydrogenation was investigated,the results showed that adding appropriate amount of ammonia in the solvent can inhibit the coupling reactions between PMC molecules effectively and the reaction rate of PNT hydrogenation was improved.?4?Study on proper exposure of Ru active sites and hydrogenation performance of Ru-La_x/AC catalystA series of Ru catalysts with different Ru/La molar ratio were prepared by stepwise impregnation-precipitation method using lanthanum as the promoter and activated carbon as the carrier.Using XRD,TEM,and low-temperature N₂ absorption/desorption characterized the structure of the Ru catalysts.It was found that the addition of lanthanum did not change the structure of the activated carbon.XPS was used to analyze the varies of the valence and binding energy of Ru and La elements in the Ru catalysts.It was found that La species were deposited on the surface of the Ru active site.The performance of Ru₁-La_x/AC catalysts was tested in PNT hydrogenation reaction,the results showed that the yield of 4,4-dimethyldicyclohexylamine was significantly decreased with the increase of the amount of La,which proved the High coordination of Ru active site in Ru catalyst was covered by La species,which increased the desorption capacity of PMC at the Ru active site and depressed the occurrence of PMC coupling reactions effectively.