operando uv / vis spectroscopy measurements were performed to study the hydrocarbon species trapped inside hsapo34 cages , together with mass spectrometry to follow product formation during the mto reaction . timeresolved operando uv / vis spectra were recorded to monitor the reaction and regeneration under different reaction conditions and with different feedstocks . the operando uv / vis spectroscopy measurements were performed by using an avantes avalightdhsbal probe connected to an avantes avaspec 2048l spectrometer . online analysis of the reaction products during the mto reaction was obtained by a pfeiffer vacuum omnistar mass spectrometer with a mass range of 555 . the evolution of methanol , propylene , and dimethyl ether was tracked by monitoring the mass signals 31 , 41 , and 45 , respectively . operando uv / vis spectroscopy experiments were performed in a custombuilt reaction cell , which is outlined in figure  s5 in the supporting information . the reaction cell was built with a very low dead volume to directly link the spectroscopic data with the catalytic performance of the material . the average residence time (  ) for a molecule in the gas phase in just the pipelines of the setup was 102  s and in the custombuilt cell 125  s. the design of this cell is essential to this work as the it is the heart of the setup . a comparison between the custombuilt cell , and a commercial operando cell as well as more detailed information about the cell design and properties can be found in the supporting information (  s.4 ) . the hsapo34 catalyst powders were pressed into pellets of approximately 16  mm diameter and around 0.1  mm thickness , weighing between 57  mg , by using a laboratory pellet press and around 4  ton of pressure under vacuum . before every reaction , the catalyst pellet was activated in oxygen ( linde 5.0 ) . in this twostep activation , a 30  ml  min o2 flow was introduced . in the first step , the temperature was ramped to 423  k at a rate of 5  k  min , and held there for 5  min , aiming to eliminate water . then , the temperature was ramped to 723  k at a rate of 10  k  min to eliminate any other impurities in the zeolite material . after these two steps , the temperature was brought to the specific reaction temperature for that experiment and allowed to adjust to that temperature at a rate of 15  k  min . then , methanol was fed through a saturator using a 10  ml  min flow of n2 as a carrier gas , effectively containing approximately 13  % methanol . the catalyst used in the experiments described in this work is a commercial hsapo34 material . the crystal size of this material was 4.3  m ( with standard deviation =0.9 ) and the sio2:al2o3 ratio used in the synthesis of the sample was 0.4 , which can be translated theoretically into four acid sites per cage . lower acidity and longer lifetimes are reported for lower silicon contents.45a , 50 the tplot micropore volume of the sample under study is 0.25  cm  g , whereas the mesopore volume is 0.2  cm  g , calculated by using the barrett halenda ( bjh ) method of the material , argon physisorption isotherms , and the cumulative desorption pore volume in a range of 1300  nm . furthermore , the ( relative ) proton affinity measured by co adsorption is 1179  kj  mol , a characteristic value for an average of weak to medium strength acid sites.45b the supporting information shows an overview of these calculations for catalyst acidity characterization . nh3 temperatureprogrammed desorption ( tpd ) measurements displayed in figure  s7 ( in the supporting information ) revealed that , theoretically , the hsapo34 sample has approximately 0.9 acid sites per cage and suggests that about 25  % of the si forms a brnsted acid site . the co adsorption ftir spectra and nh3tpd profile can be found in the supporting information (  s.5 ) . it is important to mention that even highpurity methanol may contain enough organic impurities to create a hydrocarbon pool on a microporous solid acid catalyst . acetone and ethanol are known precursors for the formation of cyclic ( hydrocarbon pool ) species . to investigate the effect of the extrinsic impurities in the feedstock , two methanol feedstocks with a different degree of impurities an ultrapure feedstock was prepared by following the procedure of marcus et  al.53  acros analysis grade methanol was fractionally distilled and kept in a glovebox to ensure no water or other impurities were adsorbed from the atmosphere . this methanol was then passed over a 10  cm column filled with dry molecular sieves ( linde type 4   ) to remove any additional water . the methanol was subsequently passed over columns with pretreated amberlyst  15 and amberlyst  37 , successively . this ultrapure methanol contained 49  ppm of impurities ( gc , fid detector ) primarily consisting of 2,2dimethoxyacetone . in addition , ultrapure carrier gasses ( linde , 5.0 ) contain 110  ppm impurities , which might be a potential source of contamination . for this reason , a homebuilt gas trap with molecular sieves ( sigma  aldrich , 5   ) was installed to filter the o2 and n2 flows . the third methanol feedstock was created by adding 5  wt  % of ethanol to ultrapurified methanol produced by the method described above , to observe the influence of this common impurity on the methanol feedstocks.36 
  the addition of ethanol to the feedstock may influence the reaction in a twofold manner ; the kinetic diameter of an ethanol molecule is closer to that of the micropores of hsapo34 , which makes diffusion slower , but recent work has also shown that the ethanoltoolefins ( eto ) process leads to the faster formation of polyalkylated benzene carbocations and coke deposits.49 , 50 , 54 
  the type of impurities , concentrations , and gas methanol feedstock abbreviations are summarized in table  1 . all main impurities in the methanol feedstock fit inside the hsapo34 micropores of around 0.38  nm in size assuming that the respective calculated diameters are the maximum size , and pore entry will proceed in a poreparallel rather than poreperpendicular manner . operando uv / vis spectroscopy measurements were performed to study the hydrocarbon species trapped inside hsapo34 cages , together with mass spectrometry to follow product formation during the mto reaction . timeresolved operando uv / vis spectra were recorded to monitor the reaction and regeneration under different reaction conditions and with different feedstocks . the operando uv / vis spectroscopy measurements were performed by using an avantes avalightdhsbal probe connected to an avantes avaspec 2048l spectrometer . online analysis of the reaction products during the mto reaction was obtained by a pfeiffer vacuum omnistar mass spectrometer with a mass range of 555 . the evolution of methanol , propylene , and dimethyl ether was tracked by monitoring the mass signals 31 , 41 , and 45 , respectively . operando uv / vis spectroscopy experiments were performed in a custombuilt reaction cell , which is outlined in figure  s5 in the supporting information . the reaction cell was built with a very low dead volume to directly link the spectroscopic data with the catalytic performance of the material . the average residence time (  ) for a molecule in the gas phase in just the pipelines of the setup was 102  s and in the custombuilt cell 125  s. the design of this cell is essential to this work as the it is the heart of the setup . a comparison between the custombuilt cell , and a commercial operando cell as well as more detailed information about the cell design and properties can be found in the supporting information (  s.4 ) . the hsapo34 catalyst powders were pressed into pellets of approximately 16  mm diameter and around 0.1  mm thickness , weighing between 57  mg , by using a laboratory pellet press and around 4  ton of pressure under vacuum . before every reaction , the catalyst pellet was activated in oxygen ( linde 5.0 ) . in this twostep activation , a 30  ml  min o2 flow was introduced . in the first step , the temperature was ramped to 423  k at a rate of 5  k  min , and held there for 5  min , aiming to eliminate water . then , the temperature was ramped to 723  k at a rate of 10  k  min to eliminate any other impurities in the zeolite material . after these two steps , the temperature was brought to the specific reaction temperature for that experiment and allowed to adjust to that temperature at a rate of 15  k  min . then , methanol was fed through a saturator using a 10  ml  min flow of n2 as a carrier gas , effectively containing approximately 13  % methanol . the catalyst used in the experiments described in this work is a commercial hsapo34 material . the crystal size of this material was 4.3  m ( with standard deviation =0.9 ) and the sio2:al2o3 ratio used in the synthesis of the sample was 0.4 , which can be translated theoretically into four acid sites per cage . lower acidity and longer lifetimes are reported for lower silicon contents.45a , 50 the tplot micropore volume of the sample under study is 0.25  cm  g , whereas the mesopore volume is 0.2  cm  g , calculated by using the barrett halenda ( bjh ) method of the material , argon physisorption isotherms , and the cumulative desorption pore volume in a range of 1300  nm . furthermore , the ( relative ) proton affinity measured by co adsorption is 1179  kj  mol , a characteristic value for an average of weak to medium strength acid sites.45b the supporting information shows an overview of these calculations for catalyst acidity characterization . nh3 temperatureprogrammed desorption ( tpd ) measurements displayed in figure  s7 ( in the supporting information ) revealed that , theoretically , the hsapo34 sample has approximately 0.9 acid sites per cage and suggests that about 25  % of the si forms a brnsted acid site . the co adsorption ftir spectra and nh3tpd profile can be found in the supporting information (  s.5 ) . it is important to mention that even highpurity methanol may contain enough organic impurities to create a hydrocarbon pool on a microporous solid acid catalyst . for example ,  acros analysis grade acetone and ethanol are known precursors for the formation of cyclic ( hydrocarbon pool ) species . to investigate the effect of the extrinsic impurities in the feedstock , two methanol feedstocks with a different degree of impurities an ultrapure feedstock was prepared by following the procedure of marcus et  al.53  acros analysis grade methanol was fractionally distilled and kept in a glovebox to ensure no water or other impurities were adsorbed from the atmosphere . this methanol was then passed over a 10  cm column filled with dry molecular sieves ( linde type 4   ) to remove any additional water . the methanol was subsequently passed over columns with pretreated amberlyst  15 and amberlyst  37 , successively . this ultrapure methanol contained 49  ppm of impurities ( gc , fid detector ) primarily consisting of 2,2dimethoxyacetone . in addition , ultrapure carrier gasses ( linde , 5.0 ) contain 110  ppm impurities , which might be a potential source of contamination . for this reason , a homebuilt gas trap with molecular sieves ( sigma  aldrich , 5   ) was installed to filter the o2 and n2 flows . the third methanol feedstock was created by adding 5  wt  % of ethanol to ultrapurified methanol produced by the method described above , to observe the influence of this common impurity on the methanol feedstocks.36 
  the addition of ethanol to the feedstock may influence the reaction in a twofold manner ; the kinetic diameter of an ethanol molecule is closer to that of the micropores of hsapo34 , which makes diffusion slower , but recent work has also shown that the ethanoltoolefins ( eto ) process leads to the faster formation of polyalkylated benzene carbocations and coke deposits.49 , 50 , 54 
  the type of impurities , concentrations , and gas methanol feedstock abbreviations are summarized in table  1 . all main impurities in the methanol feedstock fit inside the hsapo34 micropores of around 0.38  nm in size assuming that the respective calculated diameters are the maximum size , and pore entry will proceed in a poreparallel rather than poreperpendicular manner . as a service to our authors and readers , this journal provides supporting information supplied by the authors . such materials are peer reviewed and may be reorganized for online delivery , but are not copyedited or typeset . technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors .
abstractoperando uv / vis spectroscopy with online mass spectrometry was used to study the effect of different types of impurities on the hydrocarbon pool species and the activity of hsapo34 as a methanoltoolefins ( mto ) catalyst . 
 successive reaction cycles with different purity feedstocks were studied , with an intermittent regeneration step . 
 the combined study of two distinct impurity types ( i.e. , feed and internal impurities ) leads to new insights into mto catalyst activation and deactivation mechanisms . in the presence of low amounts of feed impurities , 
 the induction and active periods of the process are prolonged . 
 feed impurities are thus beneficial in the formation of the initial hydrocarbon pool , but also aid in the unwanted formation of deactivating coke species by a separate , competing mechanism favoring coke species over olefins . 
 further , feedstock impurities strongly influence the location of coke deposits , and thus influence the deactivation mechanism , whereas a study of the organic impurities retained after calcination reveals that these species are less relevant for catalyst activity and function as 
 seeds  for coke formation only .