(457b) Heat Effects Determine Catalyst Lifetime in Methanol-to-Hydrocarbons Reaction
The demand for ethylene and propylene is continuously growing. Ethylene was the leading petrochemical product, accounting for over 25% share of the global petrochemicals market in 2013. Nowadays the situation is changing in favor of propylene. Nevertheless, it is difficult to anticipate changes in global market, leading to the necessity of having a technology where both olefins could be produced simultaneously while their ratio could be tuned on market’s demand. Both chemicals are currently produced by fluid catalytic cracking (FCC), paraffin dehydrogenation and thermal cracking of ethane, propane or naphtha. In order to address their growing demand, the search for efficient alternative routes to the synthesis of these light olefins is now at the forefront of industrial and academic research. Among these routes, the methanol-to-hydrocarbons (MTH) conversion has gained a lot of attention during the last few decades. More specifically, the methanol-to-olefin (MTO) process provides an additional route for the production of ethylene and propylene. The indisputable advantages of MTO include high selectivity towards short olefins and possibility of controlling ethylene/propylene ratio via reaction conditions or catalyst modification, thus meeting current market requirements. In addition, methanol can be produced from synthesis gas, making any source of carbon-containing matter a potential feedstock for this reaction.
The MTO reaction pathway comprises the conversion of methanol over acidic zeolites, with SAPO-34 (CHA) and ZSM-5 (MFI) being successfully implemented in industry. Several processes are commercially available, and most of these rely on zeolite ZSM-5 as a catalyst. These include the Mobil Oil MTG process, the Topsøe integrated gasoline synthesis (TIGAS) process, and the Lurgi methanol to propylene (MTP) process. In the latter case, the selectivity is optimized towards propene rather than C5+ by tuning the reaction conditions (i.e., high temperature and low pressure) and by recycling long chain hydrocarbons. In the Norsk Hydro/UOP methanol-to-olefins (MTO) process, mostly ethylene and propylene are produced using SAPO-34 as catalyst.
In spite of the industrial success attained by processes based on SAPO-34 and ZSM-5, there is still ongoing research on improving selectivity to short olefins and, more importantly, catalyst lifetime, as both materials suffer from relatively rapid deactivation due to the coke formation. The latter issue being the most important problem to address in the MTO reaction, as both undesired coke and desired ethylene are formed from the same reaction intermediates. In many reports catalyst lifetime was improved by decreasing the amount of acid sites or their strength, the latter being accomplished either by isomorphous substitution of Al by other elements, by partial ion-exchange or by creation of extra porosity in hierarchical zeolites. Surprisingly, very little effort has been devoted to the reactor level in order to improve catalyst lifetime. This is indeed very remarkable when considering the extreme exothermicity of the MTO reaction (the methanol and DME protonation enthalpies are around −62 and −43 kJ mol−1, while protonation enthalpies of the oleﬁns vary from −11 kJ mol−1 to −53 kJ mol−1 for butene).
In a thorough literature analysis it is rather surprising to find results for the same catalytic systems (mainly commercial ZSM-5 Si/Al=40) tested under the same operation conditions that are significantly divergent, especially regarding lifetimes (up to 5 times). The only difference was in the testing reactor used, suggesting that catalyst deactivation strongly depends on heat and mass transfer phenomena on that level.
Taking into account all above-mentioned considerations, in this work an attempt has been made to relate the temperature profiles inside the catalytic bed with the rate of coke formation and to build a relevant deactivation model. For this reason, several parameters affecting catalytic performance were changed and their effect on catalyst lifetime and selectivity was monitored. First of all, catalyst deactivation was studied as a function of reaction temperature. In line with the literature available, catalyst lifetime depends on reaction temperature, namely by increasing the reaction temperature from 400oC to 500oC catalyst lifetime decreased from 30 to 10 hours. Moreover, carbon selectivity to C2-C4 olefins increased from 28 to 52 %. In addition to these experiments, dilution of the catalytic bed with silicon carbide (SiC) demonstrates the strong effects of heat transfer on catalyst deactivation. Interestingly, it was found out that by choosing appropriate amount of diluent (Catalyst: SiC=1:6 wt%) the catalyst lifetime could be extended by a factor of two independently on the chosen temperature. Extensive characterization of spent catalyst showed significant differences in nature and amount of coke species. Thus, faster catalyst deactivation without diluent was related to the formation of local hot spots that lead to the formation graphene like species which cannot be removed from the catalyst even after regeneration. In addition to the extensive experimental work, catalytic results have been modeled in order to gain insight into temperature and concentration profiles. Altogether, our results demonstrate the importance of the largely underestimated heat effects during catalyst performance testing for the MTO process.