Methanol-to-olefin Reaction Over SAPO-and A1PO-type Molecular Sieves

As an oil-free process to obtain olefins, the conversion of methanol to olefins（MTO） on microporous solid acids has attracted significant attentions. In the presentstudy, several types of silicoaluminophosphates （SAPO） and aluminophosphates（AlPO） with different framework structure were synthesized via hydrothermalmethods and applied as MTO catalysts in fixed-bed reactor. The mechanism of MTOreaction and the factors influencing MTO conversion were investigated by means of aseries of characterization techniques.The catalytic performances of SAPO-type samples with different amounts ofBr nsted acid sites and different framework structure were studied under the sameconditions. The effects of Br nsted acid sites and framework structure on MTOconversion were studied by¹H MAS NMR, in-situ UV/Vis, GC-MS and TGAanalysis. The results indicated for the first time that SAPO-41with one-dimensional10-ring pores and a significant concentration of Br nsted acid sites could be goodcatalyst for MTO, or more generalized MTH, reaction.The accessible Br nsted acid sites and benzene-based carbenium ions on workingSAPO-34catalysts after different MTO reaction time were measured via a novelmethod consisting of ammonia adsorption and quantitative¹H high speed MAS NMRspectroscopy for the first time. The results indicated that the accessible Br nsted acidsites were gradually consumed during the MTO process, but the benzene-basedcarbenium ions were firstly formed and then gradually consumed. The totalconsumption/coverage of benzene-based carbenium ions, instead of the totalconsumption/coverage of Br nsted acid sites, is the direct reason for catalystdeactivation.The effects of organic impurities originated from the incomplete combustion oforganic template on MTO reaction were studied over SAPO-46withthree-dimensional12-ring pore structures. SAPO-46samples calcined at lowertemperatures exhibited good initial MTO activity. GC-MS results indicated the tolueneoriginated from the incomplete combustion of organic template may be the primaryhydrocarbon pool or the precursor for the formation of primary hydrocarbon pool thatinduces the initial MTO reaction. Under MTO reaction conditions, residual toluene inSAPO-46can be transformed to polymethylbenzenes and alkylnaphthalenes, i.e. thewell-known active hydrocarbon pool species for MTO process. In order to study the effects of trace amounts of Br nsted acid sites on MTOconversion. Several AlPO-type molecular sieves with different framework structureand trace amounts of Br nsted acid sites were synthesized and studied as possibleMTO catalysts. The changes of Br nsted acid sites and organic species during MTOconversion were studied by in situ FTIR, is situ UV/Vis,¹H MAS NMR,¹³C MASNMR and TGA analysis. Based on the results, it is revealed that trace amounts ofBr nsted acid sites are enough to realize MTO reaction. Moreover, good MTOperformances can be obtained on catalysts with trace amounts of Br nsted acid sitesand suitable framework structure.SAPO-34obtained after MTO conversion times of0to30min were investigatedby¹H PFG NMR spectroscopy for studying the self-diffusivities of ethane and ethene.The MTO reaction over SAPO-34were investigated by in situ UV/vis,¹³C MASNMR, and¹H MAS NMR spectroscopy giving insights into the organic deposits andacid sites in different periods of the catalyst lifetime. The results indicated that theorganic deposits can lead a growing hindrance of molecular diffusion in the pores ofSAPO-34. However, the deactivation of SAPO-34was not caused by the block ofpore, but should be due to the cover of benzene-based carbenium ions by coke.