MTO Process Research Articles

Rational design of amino-functionalized pillar-layered Co6O6 cluster MOF for gas purification in the MTO process

Rational design of amino-functionalized pillar-layered Co6O6 cluster MOF for gas purification in the MTO process

A global methanol-to-hydrocarbons (MTH) model with H-ZSM-5 catalyst acidity descriptors

This work presents a dual cycle-based kinetic model that uses H-ZSM-5 catalyst acidity descriptors for the methanol-to-hydrocarbons (MTH, -olefins, MTO, and –aromatics, MTA) processes. This model was developed using data obtained in 12 periodic reactions with three H-ZSM-5 zeolites of different acidity (Si/Al = 15, 40 and 140). We decoupled the kinetics of the model from the catalyst identity by linking the calculation of the kinetic parameters to the zeolite acidity, allowing us to perform simulations of the reactor operating under various conditions and with different zeolite acidity values different from those used experimentally. The results obtained in the simulations let us identify the best operating conditions for the MTO and MTA processes, and pointed at the main difficulties found when implementing these two technologies industrially. In addition, the conditions and values obtained for the target products, either light olefins or aromatics, were comparable with those presented by several existing works in the literature for H-ZSM-5 zeolites of similar acidity. Moreover, the methodology detailed here using acidity descriptors can be extrapolated for its application to other catalytic processes.

Theoretical investigation of the paring mechanism of the MTO process in different zeolites

The paring mechanism, which belongs to the aromatic cycle of the methanol-to-olefins process and produces propene, is investigated for H-ZSM-5 (MFI structure), H-SSZ-24 (AFI structure) and H-SAPO-34 (CHA structure) with the heptamethylbenzylium cation as a co-catalyst using DFT and ab initio calculations. We focus on the mechanistic pathway of the paring mechanism that we have proposed recently for H-SSZ-13 [Catal. Sci. Technol. 12 (2022) 3516–3523.], in which ring contraction occurs from hexamethylmethylenecyclohexadiene and the resulting five-membered ring keeps an unsaturated methylene group throughout the process, leading to tetramethylfulvene as the intermediate after propene elimination. The highest free energy barriers (at 400 °C) for this mechanism are found to be 139 kJ/mol, 156 kJ/mol and 167 kJ/mol for H-SAPO-34, H-SSZ-24 and H-ZSM-5, respectively, compared to 127 kJ/mol for H-SSZ-13. All these barriers are low enough to be kinetically relevant and can thus explain the observed formation of propene as one of the main products of the MTO-process in these zeolites. The barriers for polymethylbenzene methylation to recover the active species are higher than the barriers for the paring mechanism in all zeolites (164 kJ/mol, 187 kJ/mol and 189 kJ/mol for H-SAPO-34, H-SSZ-24 and H-ZSM-5, compared to 163 kJ/mol for H-SSZ-13). The lowest barriers are found for H-SAPO-34, which is often the commercial catalyst in methanol-to-propene plants.

Controlling product selectivity and catalyst lifetime by altering acid strength, cavity size of SAPO, and diffusion rate of methanol in the MTO reaction: DFT and MD calculations.

The initiation mechanisms of the MTO process over silicoaluminophosphate (SAPO) catalysts with zeolite-like structures using first-principles calculations have been investigated. The supramolecular system of silicoaluminophosphates consisting of inorganic cages with Brønsted acid sites and trapped organic compounds was used as a catalyst in the MTO reaction. To study the structure-property relationship in more detail, the effect of acidity and cage size of different types of SAPOs (SAPO-18, SAPO-34, and SAPO-17 with CHA, AEI, and ERI structures, respectively) in the aromatic cycle of hydrocarbon pool mechanism was investigated. The differences in reaction barriers can be explained by the cage size, pore topology, and environment of framework protons of materials. Product selectivity was controlled by using cavity-type zeolite, the steric constraint of the cavity for the formation of critical intermediates, and acidic strength. The results show that ethylene selectivity increases as the cavity size decreases, and the elliptical pore size of the structures decreases, thereby decreasing the acidity of the zeolite structure, leading to an increase in propylene selectivity. SAPO-18 exhibits the longest reaction lifetime and has the highest amount of carbonaceous material after reaction completion. SAPO-17 with small pore and cavity size is selective to ethylene, although it shows a rapid catalyst deactivation rate.

Cooperative Effects of Active Sites in the MTO Process: A Computational Study of the Aromatic Cycle in H-SSZ-13

Brønsted acid sites in zeolites are typically described as single sites. Theoretical investigations of proximate acid sites have so far only found or considered indirect effects, where the additional acid sites influence the reactivity but do not directly participate in the reaction. Here, we investigate a case where a second acid site directly takes part in the reaction mechanism, and this leads to a significant lowering of the reaction barrier. This is shown for the side-chain mechanism of the aromatic cycle, which is believed to be the main source of ethylene in the methanol-to-olefins process. We investigate this mechanism based on the heptamethylbenzenium cation in H-SSZ-13 using quantum chemical calculations. We find that cooperative effects arising from the presence of a second acid site in the same cavity lower the barrier of the rate-determining step by about 40 kJ/mol, making this mechanism plausible. For the paring mechanism, the second site only has an indirect influence and leads to a destabilization of the transition state on the order of 15 kJ/mol compared to the single site case. The barriers for both the side-chain mechanism and the paring mechanism are found to be in the range of 150–170 kJ/mol. This is compatible with the experimentally observed formation of ethylene and propylene in comparable amounts. These findings suggest that higher propylene selectivity could be obtained for H-SSZ-13, if the majority of acid sites does not share a cavity with a second acid site. Based on simulations of random Al-siting, this is unlikely for typical Si/Al ratios < 100.

Single vs. dual-binder surface design of spray-dried Si–Al-sol-bound kaolin-matrixed SAPO-34 nanocatalyst for conversion of methanol to light-olefins in fluidized bed reactor

This study investigates shaping and characterization of fluidizable methanol to olefins catalysts using single-binder or dual-binder. Physicochemical and mechanical properties were investigated with different methods likewise FESEM, XRD, TGA, FTIR, TPD-NH3 and BET-BJH. Furthermore, MTO process tests were accomplished to investigate the catalytic activity in the fluidized bed and fixed bed reactor and revealed that the sample shaped using dual-binder has larger surface area than the shaped one using single-binder (alumina sol or silica sol). Catalysts with silica sol and alumina-silica sol binder show relatively lower attrition resistance than the catalysts with alumina sol binder. Furthermore, implementation of alumina-silica sol provides increased acidity of the MTO catalyst which led to the increased selectivity and life time in fluidized bed reactor compared to alumina sol and silica sol binder. In addition, the MTO catalyst shaped with alumina-silica sol exhibited slightly lower coke deposition compared to the one made with single binder.

Trace Compounds Confined in SAPO-34 and a Probable Evolution Route of Coke in the MTO Process.

Confined compounds in SAPO-34 cages are important to understand the activation and deactivation mechanisms of the methanol-to-olefin process. In this work, gas chromatography–mass spectrometry (GC-MS) chromatograms of CCl4-extracted samples of used SAPO-34 were denoised by subtracting signals of air compounds and stationary phase bleeding of the chromatographic column, which enhanced the identification of trace compounds. In addition to the generally noted methyl aromatics, this work also identified alkanes, cycloalkanes, alkyl (ethyl, propyl, and butyl) compounds, partially saturated compounds, and bridged compounds. These novel identified trace compounds favor the evolution route depiction of monocyclic, bicyclic, tricyclic, tetracyclic, and multicore hydrocarbons in the SAPO-34 cage. Confined compounds should grow via step-by-step alkylation, cyclization, and aromatization processes. C2+ side chains, especially C3+, favor the growth of rings. Alkyldihydroindenes should be key intermediates between monocyclic and bicyclic aromatics. Bridged soluble compounds provide evidence that insoluble coke is formed across cages in the SAPO-34 crystal.

Reaction pathway and kinetic modeling for transformation of light olefins over SAPO-34 in the absence of methanol

The current studies on SAPO-34 catalyzed MTO process are generally based on the hydrocarbon-pool mechanism, in which the individual olefin is generated in parallel by the competing reactions without interconversion of the resulted olefins. This situation is surely oversimplified and makes the reaction mechanism confusing. In this work, we investigated C3–C6 olefins transformation over SAPO-34 without methanol co-feeding to examine whether the oligomerization-cracking mechanism is applicable. Conversion pathways for individual olefin were discriminated with dimer-, trimer-, even tetramerization, by using Delplot analysis. The final reaction network was constructed with 11 reactions and a satisfactory correlation was achieved. Further validation tests were carried out by feeding C4 and C5 olefins mixture with excellent prediction performance obtained for the main product distribution, and comparison between SAPO-34 and ZSM-5 was done for a C4+ olefin feeding case, showing that SAPO-34 was much superior to ZSM-5 for higher olefin cracking.

Small pore SAPO-14-based zeolites with improved propylene selectivity in the methanol to olefins process

SAPO-14 zeolite with high Si content and SAPO-34/SAPO-14 composite zeolites have been prepared to improve the one-pass propylene selectivity in the MTO process.

A new mechanistic proposal for the aromatic cycle of the MTO process based on a computational investigation for H-SSZ-13

The paring mechanism of the aromatic cycle of the hydrocarbon pool is reinvestigated based on the heptamethylbenzenium cation adsorbed within H-SSZ-13 using quantum chemical calculations.

Unraveling the Relationship between Zeolite Structure and MTO Product Distribution by Theoretical Study of the Reaction Mechanism

Determination of the relative contributions of aromatic-based and alkene-based cycles in the MTO process is essential for understanding the reaction mechanism and estimating the product selectivity over various zeolites. However, it is a great challenge due to the high complexity of the reaction network. Thus, the olefin formation mechanisms are systematically investigated here by the density functional theory with van der Waals dispersive corrections over structurally different zeolites. It is shown that the aromatic-based cycle dominates the MTO process over H-SAPO-34 with a side-chain route as the major reaction pathway, while the alkene-based cycle plays a major role on H-BEA and H-ZSM-5. Over H-ZSM-22, the alkene-based cycle is the main route with C5+ alkenes as primary products. The energy barriers of the aromatic-based cycle are similar over the above four types of zeolites. A linear relationship is found between the ethene/propene molar ratios and the free energy barriers of alkene-based cycles over these zeolites, indicating that the ethene and propene selectivities are mainly determined by the propagation of the alkene-based cycle. This suggests that the ultimate free energy barrier of the alkene-based cycle can be used as a measure to predict the C2=/C3= molar ratios obtained on topologically different zeolites.

Stabilizing the framework of SAPO-34 zeolite toward long-term methanol-to-olefins conversion

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D 27Al, 31P MAS NMR, and 2D 31P-27Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Investigation of the Effect of Crystallization Temperature and Time in Synthesis of SAPO-34 Catalyst for the Production of Light Olefins

In this paper, the effect of variation of crystallization temperatures and times on the synthesis of SAPO-34 catalysts for conversion of methanol to light olefins has been investigated. These are two of the very effective parameters in the synthesis of SAPO-34 which affect phase purity, crystallinity, acidic properties, and finally the performance of catalysts. For this purpose, 9 samples have been synthesized by varying of temperature and time of crystallization in the range of 170–210°C, 12–36 h, respectively and the molar ratio of 1Al2O3 : 1P2O5 : 0.4 SiO2: 0.5 TEAOH : 1.5 MOR : 70 H2O and characterized by XRD, SEM, FTIR, BET, and NH3–TPD techniques. Finally, performance of catalysts was investigated in the process of conversion of methanol to light olefins in a fixed-bed reactor 410°C. The results show that crystallinity, phase purity, size, and distribution of particles, surface area, and acidic properties have great effects on performance and selectivity of ethylene and propylene and these properties themselves severely depend on Crystallization temperature and time. The results of characterized of catalysts show that high characterized temperature, reduces the phase purity and increases size in particles. The catalysts that have been synthesized at 190°C and 24 h have the highest crystallinity, suitable size, and distribution of particles, high surface area, and suitable acidic sites, so it has the highest selectivity of light olefins at MTO process.

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic

Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and ab initio calculations

The side-chain mechanism of the methanol-to-olefins process over the H-SSZ-13 acidic zeolite was investigated using periodic density functional theory with corrections from highly accurate ab intio calculations on large cluster models.

Comprehensive energy analysis and integration of coal-based MTO process

With low oil prices, the existing coal-to-olefin enterprises are forced to improve profitability by reducing energy consumption. This paper studies the comprehensive energy analysis and integration of coal-based MTO process. Process simulation is established first and is validated by comparing with the industrial data. Energy analysis of the MTO process was then conducted, calculating relevant information of each heat exchange stream. The heat exchanger network synthesis was proposed by using gradual optimization integration strategy based on the T-H diagram. It was found that the hot energy of purified water and steam condensate could be fully utilized. The cold energy of ethylene tower could replace partial −24 °C refrigerant. It was also found that there is the possibility of multi-stage heat transfer in this process. The maximum energy recovery is achieved under the principle of energy cascade utilization and stepwise matching. The result showed that the new design decreases the utility duty and the required exchanger area by 4.76% and 8.63% compared with the industrial process. The capital cost, operation cost and total annual cost of total site are 8.17, 14.64 and 17.27 million $, which are 8.29%, 2.78% and 3.66% less than the industrial design.

MTO performance over seed-assisted SAPO-34 zeolites synthesized by reducing template consumption

SAPO-34 samples were prepared via a seeding-induced method with a considerable reduction of template concentration. The resultant SAPO-34 zeolites were investigated by XRD, FESEM, EDS, N2 adsorption, and NH3-TPD analyses. The samples represent different catalytic activity in the MTO process due to their differences in the physic-chemical properties, including crystallite/particle sizes, surface area, acid strength and concentration. Pure SAPO-34 was obtained with a 60% reduction of organic template for the first time. However, due to the increase of particle sizes, it did not show the desired catalytic performance. An excellent catalytic performance was achieved for the seeding-induced catalyst prepared by a 40% reduction of template content according to its appropriate high acidity and relatively large surface area. This sample exhibited 91.8% selectivity toward light olefins and about 330 min of lifetime, which were 14.4% higher in term of selectivity and more than twice lifetime than the unseeded sample. This substantial achievement was obtained under the industrial feed conditions (72%wt methanol in water) and provided a cost-effective and environmentally friendly route for preparing promising MTO catalyst with notably decreased template dosage.

Comparative Synthesis and Characterization of Nanostructured SAPO-34 Using TEA and Morpholine: Effect of Mono vs. Dual Template on Catalytic Properties and Performance toward Methanol to Light Olefins

Production of light olefins from methanol was studied over SAPO-34 molecular sieves exploring the effect of mono and dual templates. Herein, the single templates of TEA, morpholine, and mixed templates of TEA/morpholine (equal molar ratio of TEA and morpholine) were used to synthesize SAPO-34 catalysts. The prepared samples were prepared via hydrothermal synthesis method and characterized with XRD, FESEM, PSD, EDX, BET, and FTIR techniques. It was found that the crystallinity decreased upon applying TEA as a template and it can also be noted that the intensity of the SAPO-34 phase peaks increased by increasing the morpholine in template mixture. Production of much smoother particles for the catalyst synthesized with a binary template mixture of TEA/morpholine can be dependent on the crystallinity increase. Si incorporation value was decreased for the catalyst with a major phase of SAPO-5 (topological structure of AFI). It is indicative that the TEA application would facilitate the formation of AFI structure, which is incapable of incorporating higher amounts of Si into the crystalline framework. The nature of the template determines the morphology of the final product due to the different rates of crystal growth obtained in accordance with XRD and FESEM results. Therefore, the catalyst synthesized with the TEA/morpholine mixture shows the best performance among synthesized samples in terms of lifetime in the MTO process, sustaining light olefins selectivity at higher values (about 90% after 630 min TOS).

CFD Modeling of Methanol to Light Olefins in a Sodalite Membrane Reactor using SAPO-34 Catalyst with In Situ Steam Removal.

In this work, the performance of a sodalite membrane reactor (MR) in the conversion of methanol to olefins (MTO process) was evaluated for ethylene and propylene production with in situ steam removal using 3-dimensional CFD (computational fluid dynamic) technique. Numerical simulation was performed using the commercial CFD package COMSOL Multiphysics 5.3. The finite element method was used to solve the governing equations in the 3- dimensional CFD model for the present work. In the sodalite MR model, a commercial SAPO-34 catalyst in the reaction zone was considered. The influence of key operation parameters, including pressure and temperature on methanol conversion, water recovery, and yields of ethylene, propylene, and water was studied to evaluate the performance of sodalite MR. The local information of component concentration for methanol, ethylene, propylene, and water was obtained by the proposed CFD model. Literature data were applied to validate model results, and a good agreement was attained between the experimental data and predicted results using CFD model. Permeation flux through the sodalite membrane was increased by an increase of reaction temperature, which led to the enhancement of water stream recovered in the permeate side. The CFD modeling results showed that the sodalite MR in the MTO process had higher performance in methanol conversion compared to the fixed-bed reactor (methanol conversion of 97% and 89% at 733 K for sodalite MR and fixed-bed reactor, respectively).

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors.

The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host-guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.