Study On The Characteristics Of PAl-NaX Catalysts For The Side-chain Alkylation Of Toluene With Methanol

As a convetional hydrophobic monomer,styrene plays an important role in the industrial synthesis of rubber,resin and plastic.Our cuntry is poor in oil and gas,it is meaning that synthesis of styrene from coal-based toluene with methanol.Compared with the traditional petroleum-based production method,preparation of styrene by the side chain alkylation of toluene with methanol has the advantages of low cost,less energy consumption and limited side reactions(no carbon dioxide)etc.However,the development of a stable catalyst with high activity is still a bottleneck problem that restricts the industrialization of this method.The previous research of our group discorers that the appropriate intensity of acid and base is the key to the side chain alkylation of toluene with methanol.In order to investigate the effects of catalyst acid-base sites and dehydrogenation components on catalytic activity and product distribution,a PAl-NaX zeolite with suitable acid-base intensity was designed and synthesized by in-situ hydrothermal method,based on which,we adjusted the acid-base intensity and composition of the catalysts,and studied the effects of acidity,basicity and dehydrogenation components of the catalyst on the performance in the side chain alkylation of toluene with methanol.Combined with X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),N2 physical adsorption-desorption and other characterizations and catalytic activity data,the following conclusions can be drawn:(1)The PAl-NaX catalysts were synthesized by hydrothermal method,and part of its NaX molecular sieve structure was transformed into phosphoaluminosilicate Na13Al24Si13P11O96·H2O structure,which can improve the ethylbenzene selectivity of the side chain alkylation of toluene with methanol from 0.00%to 21.28%.After PAl-NaX was modified with different mass fractions of NaOH,the surface basicity of the catalyst was increased,and the acidity was decreased.It was found that the high proportion of middle basic sites and the proportion of weak acid sites were beneficial to the side chain alkylation of toluene with methanol.When the NaOH loading was 9 wt%(Cat-PAI-NaX-9Na),the styrene selectivity is 45.84%,and the total yield of ethylbenzene and styrene reached 63.08%.The structure of phosphoaluminosilicate Na13Al24Si13P11O96·H2O appeares in Cat-PAl-NaX-9Na.However,the downside is the xylene selectivity is high in this series of catalysts.(2)In order to reduce the byproduct of xylene,the ion-exchanged between sodium ions and hydrogen was carried out firstly with different concentrations of NaOH in PAl-NaX,and then load 9 wt%NaOH on the obtained precursor to further adjust the base intensity.The results show that with the increase of Na loading,the basic intensity of the catalyst increases,and the acid intensity decreass,the selectivity of xylene is reduced obviously,which owes to the ion exchange.In summary,this series of catalysts inhibit xylene successfully,but the stability of the catalysts need to be improved.(3)In order to improve the stability of the catalyst,Si/Al ratio of the catalyst is enhanced by increasing the NaX feeding in synthesis of PAl-NaX,before modification with 9 wt%NaOH.The results showed that the stability of the catalyst is improved to some extent with NaX feeding increasing.The total yield of ethylbenzene and styrene is more than 40%during 6 days over this series of catalysts,Among which Cat-3-9Na shows the best performance.The styrene selectivity increases to 28.18%,the ethylbenzene selectivity is 45.49%,and the total yield of ethylbenzene and styrene of Cat-3-9Na reaches 73.57%.However,the CH4 selectivity is relatively high.(4)In order to reduce the byproduct CH4,the dehydrogenation component,5 wt%Cu,species was introduced before modification with 9 wt%NaOH.The results showed that the total yield of ethylbenzene and styrene is the highest of 91.11%over Cat-5Cu-15Na.And the styrene selectivity reaches 11.17%.It is confirmed that the introducing of Cu species improve the dehydrogenation capacity of the catalyst,and accordingly restrict the generation of CH4 and promote dehydrogenation of methanol to formaldehyde.