Modification Of NaX Catalysts By Solid-liquid Continuous Mixing Method And Its Application In Side-chain Alkylation Of Toluene With Methanol

As an important organic chemical raw material,the traditional synthesis process of styrene relies mainly on two routes,Friedel-Crafts alkylation of ethylene with benzene and energy-intensive（550-700 ^oC）dehydrogenation of ethylbenzene.About 90%of styrene is produced by the ethylbenzene dehydrogenation in industrial application,which has disadvantages such as long process flow and high energy consumption.Compared with this method,preparation of styrene by side chain alkylation of toluene with methanol has a good commercial application prospect,whose advantages include milder reaction conditions,cheaper raw materials,lower energy consumption and higher atom utilization.Otherwise,the development of high-performance catalysts is the critical factor for the industrial application of this method.In the preliminary study of our research group,different modified materials,such as phenolic resins,aluminum-based catalysts,ordered mesoporous carbon,Cs X,NaX,Al PO₄-5,SAPO-5,were introduced in the side chain alkylation of toluene with methanol.The results showed that the moderate strong basic sites and the appropriate amount of strong basic sites can improve the catalytic performance of side chain alkylation of toluene with methanol.In order to increase the amount and distribution of medium strong basic sites and strong basic sites on the catalyst surface,NaX zeolite loaded with KOH,Na₂B₄O₇and KOH-Na₂B₄O₇dual components were prepared by solid-liquid continuous mixing method.The catalysts were characterized by XRD,SEM,FT-IR,Py-IR,BET,NH₃-TPD and CO₂-TPD etc.The effects of different loading amount of KOH,Na₂B₄O₇,and KOH-Na₂B₄O₇dual components on the strength,amount and ratio of acid-basic sites of the catalysts were investigated respectively.In addtion,the interaction law of the above factors with the catalyst structure and catalytic performance were also explored.The following conclusions could be drawn:1.NaX molecular sieve catalysts,uniformly loaded with KOH,were prepared by solid-liquid continuous mixing method for side chain alkylation of toluene with methanol.The results showed that:（1）Addition of a certain amount of KOH can increase the strong basic sites and decrease the medium strong acid sites of the catalysts,which will be beneficial to increasing the methanol conversion and the total selectivity of styrene and ethylbenzene.（2）When the loading amount of KOH is more than 3 wt.%,the selectivity of ethylbenzene is greatly improved.The selectivity of ethylbenzene reaches the highest of 94.73%when the KOH loading amount accounts for 7 wt.%.Meanwhile,the total yield of ethylbenzene and styrene accouts for 97.82%.（3）The Cat-n K series catalysts show the better stability,by which all the methanol conversions approach 100%.（4）It was preliminarily deduced that there are two competitive reactions for methanol over the strong basic sites and the medium strong acidic sites,respectively.If the proportion of strong basic sites is high,methanol will be inclined to dehydrogenize to form formaldehyde,which followed by reacting with the methyl of activated toluene to form styrene and ethylbenzene.In contrast,if the proportion of the medium strong acid sites is high,methanol will be hydrogenated to form methane or carry out the alkylation reaction with toluene to produce xylene.2.The NaX molecular sieve was loaded with different mass fractions of Na₂B₄O₇by the excessive impregnation method.After evaluation of the catalytic performance,it could be found that:（1）When a certain amount of Na₂B₄O₇is loaded on,the activities of the catalysts are improved.Cat-9B catalyst has the highest ethylbenzene selectivity of 67.59%,and Cat-13B catalyst has the highest styrene selectivity of 41.45%.At the same time,the total yield of ethylbenzene and styrene reached the highest of 72.61%.（2）The loading of Na₂B₄O₇can inhibit the producing of xylene.When the mass fraction is higher than 13 wt.%,no xylene is formed,but the selectivities of methane and formaldehyde begin to increase.3.The NaX molecular sieve uniformly loaded with KOH and Na₂B₄O₇were prepared by solid-liquid continuous mixing method for side chain alkylation of toluene with methanol.The results showed that:（1）The loading of KOH-Na₂B₄O₇dual components can improve the selectivity of styrene,and this kind of catalysts are better than single-component catalysts.When the loading amount of KOH and Na₂B₄O₇are 7 wt.%and 11 wt.%respectively,the selectivity of styrene is 65.87%,and the total yield of ethylbenzene and styrene reaches74.88%.（2）With the total loading amount of KOH and Na₂B₄O₇increasing,the alkalinities of the catalysts are correspondingly enhanced,which can inhibit the formation of xylene,while the selectivities of formaldehyde and methane increase.