Catalyst For Conversion Of Methanol Research Articles

Highly active and selective ZIF‐derived cobalt catalyst for methanol conversion to dimethyl carbonate

The oxidative carbonylation of methanol to synthesize dimethyl carbonate (DMC) has been extensively studied over Cu‐based catalysts, but the activity and selectivity are not high. The Co catalysts exhibit high DMC selectivity, but the difficulty in recycling homogeneous Co catalyst and the low conversion of heterogeneous Co catalyst limit the application of Co catalysts. Here, the core–shell ZIFs materials were synthesized and carbonized to obtain solid core–shell cobalt catalysts, and then the catalytic performance for methanol conversion to DMC was investigated. The CoNC@NC catalyst, carbonized from Z67@Z8 with Z67 as the core and Z8 as the shell, shows that the carbonized NC shell effectively suppressed the aggregation of Co NPs and the Co NPs were only 15.4 nm, which was much smaller than those of NC@CoNC (34.5 nm) and CoNC (48.1 nm) catalysts. Compared with the CoNC catalyst, CoNC@NC significantly improved the pulse chemisorption of CH3OH and CO, leading to a significant increase in methanol conversion from 6.9% to 17.1%. Furthermore, the deactivation rate of the CoNC@NC catalyst (22.8%) was much lower than that of CoNC (59.4%) after five reaction cycles. The results of this work provide a new strategy for the design and preparation of solid cobalt catalysts for the oxidative carbonylation of methanol to DMC.

Design and fabrication of the alumina nanofibers decorated with isolated ZSM-5 using electrospinning method: A novel catalyst for methanol conversion to light olefins

ZSM-5 nanoparticles are widely used as the catalyst in the methanol conversion to olefins (MTO) process due to their high surface area and short diffusion path length. However, agglomerate formation is a challenge for this catalyst. To address this issue, in this study, electrospinning was used for the first time to synthesize fibrous catalysts comprised of ZSM-5 nanoparticles embedded in the alumina nanofibers. The catalysts were characterized using XRD, FTIR, SEM, TEM, EDX-map, BET, and NH3-TPD analyses. In the synthesized samples, ZSM-5 nanoparticles with an average particle size of 683 nm are uniformly and homogeneously dispersed over alumina nanofibers with a 300 nm diameter. These composites form core-shell structures and have a reactor porosity of 0.85, which is higher than that of parent ZSM-5. In the MTO process, electrospun catalysts demonstrate superior catalytic activity and a longer lifespan than powder catalysts. 80 wt% ZSM-5- 20 wt% Al2O3 with 47.76 % light olefin selectivity and complete methanol conversion after 321min on stream performed better than 50 wt% ZSM-5-50 wt% Al2O3. Moreover, a lower coke formation of about 3.28 % was achieved for the 80 wt% ZSM-5- 20 wt% Al2O3 sample. The reduced bulk density of the electrospun samples increases the probability of reactant and catalyst contact. Additionally, electrospinning is an effective method for reducing particle aggregation and improving catalytic process efficiency.

Formulated Mn-promoted SAPO-34/kaolin/alumina sol micro-size catalyst with a superior performance for methanol to light olefins conversion in a fluidized bed reactor

Formulation of zeolite molecular sieves powder is necessary to prepare an excellent micro-size catalyst for methanol conversion to light olefins (MTO) in a fluidized bed reactor. In this work, new fluidizable micro-meso pore catalysts were successfully fabricated by spray drying a slurry containing Mn(x)SAPO-34 powder, kaolin filler, alumina sol binder, and HCl additive. A mixed template of TEAOH/morpholine with different amounts of Mn ion was used in primary gel during hydrothermally synthesis of the crystals. The catalytic samples were characterized utilizing different methods. A moderate acidity reached by catalyst formulation using kaolin and alumina sol as well as through Mn incorporation into the chabazite cages was helpful for MTO performance of the final shape-selective catalyst in terms of selectivity to light olefins and lifetime. The enhancement effect of Mn promoter on ethylene production was much more than propylene because of entering Mn to the micro-pores and imposing restriction on heavier hydrocarbons diffusion. The uniformity and crystalinity were also appeared to be enhanced once Mn(0.05) was introduced into the gel composition. The suggested spray dried catalyst was SD-Mn(0.05)SAPO-34 (70–120 µm) in which provided a superior selectivity up to 88.4% to light olefins and catalytic lifetime of 330 min in a fluidized bed reactor at 450℃, WHSV = 4 h−1, and XMeOH = 0.75 in the feed.

Carbon template synthesis of CeO2 catalyst for direct conversion of methanol and carbon dioxide to dimethyl carbonate

Dimethyl carbonate (DMC) is an interesting, value added chemical and a high energy additive to fuels. In this study, several CeO2 catalysts were successfully synthesized through carbon template method using two renewable sources of activated carbon. The structure of catalysts was fully resolved by XRD, SEM, N2-adsorption, NH3-TPD, CO2-TPD and XPS. CeO2 catalysts prepared using coconut shell activated carbon (CAC) template exhibit better surface morphology and improved catalytic performance compared to walnut shell activated carbon (WAC)-supported catalysts. We demonstrated a correlation between the efficiency of DMC synthesis and the abundance of moderately acidic and moderately basic active sites on the catalyst. It was found that the presence of oxygen vacancies (Vo) facilitates carbonyl bond cleavage of CO2. The best-performing catalyst was 4% CeO2/CAC owing to the richest Vo and the optimum acid/base properties. Moreover, a plausible catalytic mechanism of DMC formation was proposed, and includes the interaction between methoxy carbonate (CH3OCO2−) formed with the aid of Vo and moderately basic sites and methyl cation (CH3+) formed on the moderately acidic sites.

MOF-808: a novel solid-acid catalyst for methanol conversion to environmentally clean fuel dimethyl ether

In this study, metal–organic frameworks (MOFs) based on zirconium which are called MOF-808 were synthesized by solvothermal method. A series of Zr-MOF were synthesized at different temperatures and reaction times to find the best synthesis samples to produce dimethyl ether from dehydration of methanol. As synthesized MOFs were characterized with x-ray diffraction, standard electron microscopy, and BET, they showed high methanol conversion and selectivity for methanol oxidation. The relationship between crystallinity, reactivity and relation catalysts life was discussed. For the first time, this MOF was used as a solid catalyst in the conversion of methanol to dimethyl ether due to its thermal stability and moderate acidity. The results show that all samples had a crystalline structure and were properly synthesized. The performance of the synthesized catalysts, the selectivity, and the effect of reactor temperature in the conversion of methanol to dimethyl ether in a tubular reactor with weight hourly space velocity = 5 h−1 were investigated. The highest surface area is nearly 1360 m2/g, size dimension of samples is in the range of 200–800 nm, and the highest Langmuir surface area is 1530 m2/g.

Influence of Si/Al ratio of MOR type zeolites for bifunctional catalysts specific to the one-pass synthesis of lower olefins via CO2 hydrogenation

Highly efficient CO2 utilization methodologies should be developed to reduce greenhouse gas emissions. Combining methanol synthesis (CO2-to-methanol) catalysts and methanol conversion (methanol-to-olefins) catalysts realizes one-pass hydrocarbon production via CO2 hydrogenation. We focused on ZnO/ZrO2 as metal-oxide-based catalysts for methanol synthesis and MOR zeolites as solid acid catalysts for methanol conversion. These catalysts were physically mixed to produce bifunctional catalysts which work at relatively low pressure (10 bars). Compositions of zeolite catalysts, especially Si/Al ratios, are important since composition influences on properties of zeolites, such as acid density and strength. MOR zeolites with varied Si/Al ratios were tested and found MOR zeolites with Si/Al ratio of 104 showed the best olefin selectivity. The relation between coke formation behavior and acid sites was discussed for explaining catalytic activities. The novel bifunctional catalysts and knowledge of selectivity provide a new methodology for CO2 utilization.

Methods for methanol-to-gasoline conversion with forwarding methanol processing

The design of modified H-ZSM-5 catalyst for catalytic conversion of methanol to gasoline range hydrocarbons (MTG) was carried out using neuro-genetic approach. Four factors: SiO2/Al2O3 ratio in zeolite (denoted as Si/Al2), calcination temperature (Tc), reaction temperature (Tr) and nominal weight loadings (Lm) of three metals (Ag, Cu, Co) were modeled simultaneously by using combined experimental data and physicochemical descriptors of metals. The obtained optimum model was used as fitness function for hybrid genetic algorithm to find the optimum catalyst. The optimum catalyst was Cu/H-ZSM-5 with Si/Al2 = 40, Tc = 490 °C, Tr = 340 °C, and Lm = 9.24 wt% with methanol conversion of 96.23%, and gasoline yield of 13% that resulted in 4.88% increase in gasoline yield compared to H-ZSM-5 catalyst at the same conditions. The higher experimental yield of gasoline for predicted catalyst along with the fact that this approach can be applied easily for a large number of catalysts, confirms that this approach can be used for design of heterogeneous catalysts. UV–Vis DR spectra confirmed the possibility of ⋯O2−⋯Cu2+⋯O2−⋯Cu2+⋯O2−⋯ chain structures in the channels of zeolite for Cu/HZSM-5 catalyst as active components in MTG reaction.

Microwave Radiation Effects on the Acidic Properties of Fe/ZSM-5 Catalysts for Methanol Conversion

Aim: To use the microwave radiation as the post-treatment method to tune the acidic properties of Fe/ZSM-5 catalyst. Background: ZSM-5 zeolite is a widely used standard catalyst in the methanol conversion to olefins and high-octane gasoline range hydrocarbons. However, the coke deposition and the concentration of acid sites determine its overall catalytic activity. Thus, the concentration of acid sites more precisely, the number of Brønsted acid sites, is vital in determining the activity of ZSM-5 zeolite and product distribution in the methanol-to-hydrocarbons reaction. Objective: (1) To modify ZSM-5 using an iron solution to form Fe/ZSM-5 via the impregnation method. (2) To tune the acidic properties of Fe/ZSM-5 using microwave radiation. (3) To check and determine the concentration of acidic sites using n-propylamine temperature-programmed surface reaction. (4) To check the effect of microwave radiation and acidic properties in the methanol conversion (product distribution) via methanol temperature-programmed surface reaction. Methods: Two sets of zeolites were used, with iron being introduced by ion-exchange from ferric nitrate (Fe(NO3)3 9H2O, Sigma Aldrich, ≥ 98% metal) solutions. These two series were designated as Z (0.5FeZ10/0-700) and X (0.5FeX10/0-700) after microwave treatment. The Z and X series possess the Si/Al framework ratio of 30 and 80, respectively. The TPSR studies were then conducted for characterization and catalytic tests. Result: From the C3H9N-TPSR experiments, it was found that the concentration of Brønsted acid sites decreased with increasing microwave power level. Both X and Z series exhibited high selectivity to propene than ethene. Microwave treated catalysts (0.5FeZ10/280 and 0.5FeX10/462) with decreased concentration of Brønsted acid sites showed the highest propene/ethene ratios of 1.67 and 5.27, respectively. Conclusions: From the results obtained, it was found that the amount of methane evolved (as a measure of coke deposited) and the concentration of Brønsted acid sites decreased with increasing microwave power level (0-700 Watts). High selectivity to propene was found when using both X series (0.5FeX10/0-700) and Z series (0.5FeZ10/0-700) as catalysts. After decreasing the concentration of Brønsted acid using microwave treatment, the highest P/E ratios were observed for 0.5FeX10/280, 0.5FeX10/462, 0.5FeZ10/280 and 0.5FeZ10/462 catalysts. Other: It is reasonable to suggest that microwave radiation could be a feasible post-synthesis modification step to produce ZSM-5 based catalysts that exhibit reduced concentration of Brønsted acid sites, reduced methane formation, increased catalytic activity and selectivity.

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.

Catalytic and photocatalytic properties of zinc-nickel ferrites

A synthetic procedure combining high energy ball-milling and calcination at 900 °C was applied to obtain mixed metal oxides of the type Zn1−xNixFe2O4 (x = 0.25; 0.5; 0.75). The formation of spinel-like structures and the phase composition were confirmed by XRD in combination with Mossbauer, IR and Raman spectroscopy. The crystallite size was analysed from X-ray diffraction data revealing nanometric crystallites with an average size in the interval 20–60 nm. The IR spectra showed two fundamental absorption bands in the range 650–400 cm−1, which are characteristics of the metal-oxygen vibrations in tetrahedral and octahedral configurations. The mixed metal oxides were tested as catalysts for the decomposition of methanol to syngas and the degradation of the water pollutant malachite green in model solutions under UV light irradiation. Both the catalytic and photocatalytic activities were found to increase with increasing content of Ni(II) in the mixed metal oxides. The reduction behaviour of the samples was characterized by temperature-programmed reduction—thermogravimetric analysis and related to the catalytic conversion of methanol in the presence of the oxides. The most active samples for malachite degradation had the lowest band gap energy of 1.43 and 1.45 eV. The measurements of the reaction liquor by atomic absorption spectroscopy verified the stability of the samples tested as photocatalysts during the photocatalytic test. Zn/Ni ferrites of the type Zn1−xNixFe2O4 (x = 0.25; 0.5; 0.75) active both as catalysts for methanol conversion (a) and malachite green decomposition in model solutions under UV light irradiation (b)

Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

H-ZSM-5 zeolite is a typical catalyst for methanol-to-olefins (MTO) conversion. Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework, the catalytic roles of the acid sites in different pore channels of the H-ZSM-5 zeolite are not well understood. In this study, the MTO reaction network, involving the aromatic cycle, alkene cycle, and aromatization process, and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel, sinusoidal channel, and intersection cavity of H-ZSM-5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations. The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels. In contrast, the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels, whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities. Consequently, the catalytic performance of H-ZSM-5 zeolite for MTO conversion, including activity and product selectivity, can be regulated properly through the purposive alteration of the acid site distribution, viz., the location of Al in the zeolite framework. This study helps to elucidate the relation between the catalytic performance of different acid sites in the H-ZSM-5 zeolite framework for MTO conversion, which should greatly benefit the design of efficient catalyst for methanol conversion.

Directly Printed Oxide/ZSM-5 Bifunctional Catalysts for Methanol Conversion to Dimethyl Ether with Exceptional Stability, Conversion, and Selectivity

Additive manufacturing, otherwise known as 3D printing, is an effective way of generating customizable heterogeneous catalysts. In this study, we utilized direct ink writing of insoluble oxides to ...

Preparation of hierarchical H-[B]-ZSM-5 zeolites by a desilication method as a highly selective catalyst for conversion of methanol to propylene

A hydrothermal route was exploited to prepare high-silica H-ZSM-5 as well as H-[B]-ZSM-5 zeolites, and the boron-modified type was then desilicated with alkaline treatment. The properties of catalysts were investigated by different techniques, and the catalytic performance was evaluated in the methanol to propylene reaction. The effects of three critical desilication parameters, including the alkali concentration, temperature, and treatment time were evaluated on propylene selectivity and mesopore volume. The results of characterizations evinced that the incorporation of B in the H-ZSM-5 zeolite reduced the total surface area as well as the pore volume of the H-[B]-ZSM-5 sample and increased the external surface area. Furthermore, a significant increase in the weak acid site density for the boron-containing specimen was observed. Also, the external surface area and mesopore volume of the desilicated specimens underwent an increment, while the micropore volume was almost constant. As opposed to the H-[B]-ZSM-5 catalyst, the optimum desilicated catalyst exhibited higher selectivity to propylene (48.8 vs. 40.7%) and total light olefins (76.9 vs. 71.2%), and a higher ratio of propylene to ethylene (5.66 vs. 4.15). The optimum catalyst had a higher lifetime in the MTP reaction (867 h) as opposed to the microporous H-[B]-ZSM-5 catalyst (498 h).

Highly shape‐selective Zn‐P/HZSM‐5 zeolite catalyst for methanol conversion to light aromatics

A highly shape‐selective and relatively long‐lifetime HZSM‐5‐based catalyst (Zn‐2P/HZSM‐5) was prepared by chemical modification with both ZnSiF6·6H2O and H3PO4 solution. The phosphoric acid modification could effectively modulate the Brønsted acid strength of the HZSM‐5 catalyst, which promotes the oligomerization, alkylation, cyclization, and hydrogen transfer reactions. The introduction of Zn‐Lewis acid sites significantly improved the dehydroaromatization of higher olefins. All of these were very beneficial for the generation of BTX (i.e. benzene, toluene, and xylene) hydrocarbons in aromatization of methanol. The coke amount and the average rate of coke formation decreased over the Zn‐2P/HZSM‐5 catalysts, which may largely be ascribed to its lower strong acid sites and lower outer surface acidity. The catalytic performance of methanol aromatization showed that the Zn‐2P/HZSM‐5 catalyst exhibited the highest BTX selectivity of about 46.76% and the longest catalytic lifetime of about 498 h at T = 400 °C, P = 0.1 MPa, and weight hourly space velocity = 0.7 h−1.

Metal-substituted sponge-like MFI zeolites as high-performance catalysts for selective conversion of methanol to propylene

Abstract In this study, sponge-like and M-substituted (M: Mn, W and Ce) sponge-like MFI zeolites were successfully synthesized by seed-assisted hydrothermal technique. The special bi-functional organic surfactant of C22-6-6Br2 with multi-ammonium groups was separately synthesized and used as structure directing agent (SDA). The prepared catalyst samples were examined in methanol to propylene (MTP) process and demonstrated superior catalytic performance compared to conventional H-ZSM-5 and M-substituted MFI zeolite nanosheets. The W-substituted sponge-like MFI zeolite effectively showed the most enhanced catalytic activity with complete methanol conversion, propylene selectivity of 56.3%, total selectivity of light olefins of 88.3% and catalytic life-span of 161 h. Moreover, to increase propylene selectivity, bi-metallic and tri-metallic sponge-like MFI zeolites were successfully synthesized via seed-assisted hydrothermal method by utilizing central composite design (CCD) strategy. For optimizing promoter compositions in lattice of bi-metallic and tri-metallic sponge-like MFI zeolites, the incorporated response surface methodology (RSM)-CCD technique and also an intelligent approach namely neuro-genetic method were applied. It was deduced that optimal bi-metallic sponge-like MFI zeolite suggested by neuro-genetic approach approved the highest propylene selectivity (66.7%). Synthesized catalysts were appropriately characterized by XRD, FE-SEM, EDX, TEM, FT-IR, N2 adsorption/desorption and NH3-TPD analyses which comparatively approved high specific surface area and dominant meso-porosity.

Neutron spectroscopy studies of methanol to hydrocarbons catalysis over ZSM-5

This article reviews recently published work showing how inelastic neutron scattering spectroscopy can be used to identify species found in zeolite ZSM-5 used as a catalyst for conversion of methanol to hydrocarbons. The advantages and disadvantages of the technique are illustrated, and current challenges identified.

Zirconium d-Transition Metal Phosphates As Catalysts for Selective Dehydration of Methanol to Dimethyl Ether

M0.5(1 +x)FexZr2 –x(PO4)3 (M = Ni, Cu, Mn; 0 ≤ x ≤ 0.5) phosphates have been studied as catalysts for methanol conversion in the temperature range 200–450°C. The results demonstrate that the most active and selective catalysts among these materials are Ni0.5Zr2(PO4)3 and Cu0.65Fe0.3Zr1.7(PO4)3. The dimethyl ether yield on the synthesized materials is comparable to that on available commercial catalysts.

Toward a comparative description between transition metal and zeolite catalysts for methanol conversion.

Transition metals and zeolites are extremely different catalysts used for methanol conversion. Zeolites are able to catalyze methanol conversion to hydrocarbons like gasoline and olefins, while transition metals show the selectivity of syngas. It is quite important to establish a general description from a catalysis point of view for a variety of catalysts. In this work, we have employed density functional theory calculations to correlate adsorption energies for all intermediates over a set of transition metals and zeolites. We have successfully unveiled the difference in chemical reactivity and catalytic activity for zeolites and transition metals; a comparative description has been finally established between the acidity (and porous effects) of zeolites and electronic (and geometrical) effects over transition metals. The hydrogen adsorption strength was suggested to be a general descriptor for both transition metal and zeolite catalysts. In addition, it was found that some zeolites with the same ammonia adsorption strength, which was always used to describe the acidity in experimental studies, are likely to have different theoretical acidity (hydrogen bonding strength). This eventually opens one more dimension for rational selection and design of zeolites for catalysis application.

Polyoxometalate modified SAPO-34: A highly stable and selective catalyst for methanol conversion to light olefins

Abstract Highly stable and selective SAPO-34 catalysts were synthesized with direct synthesis incorporation of tungstophosphoric acid (HPW) and silver salt of HPW (AgPW) in SAPO-34 framework. The physicochemical properties of synthesized SAPO-34 samples were investigated using various analyses, including XRD, FTIR, UV–vis/DRS, NH3-TPD, ICP-AES, FESEM, BET and TGA. Furthermore, the catalytic performances of SAPO-34 samples in the reaction of methanol to olefins (MTO) were studied at 450 °C and 1atm. Incorporation of a moderate amount of HPW (HPW/Al2O3 = 0.010) as a Bronsted acid promoter in the SAPO-34 framework significantly reduced the SAPO-34 crystals size, enhanced the BET surface area and boosted the acidic properties. MTO catalytic tests revealed that the existence of a higher amount of Bronsted acid site as an active site could promote the light olefins selectivity and lifetime of HPW-modified SAPO-34 samples. The best HPW-SAPO-34 sample had a 375 min lifetime and 95.34% selectivity to light olefins, while the unmodified SAPO-34 represented 185 min and 83.28% of catalytic lifetime and selectivity to light olefins, respectively. AgPW-SAPO-34 with AgPW/Al2O3 = 0.010 illustrated superior catalytic performance and a lower deactivation rate due to the small well-defined crystals, the highest BET surface area and sufficient created Bronsted acid sites with a lower strength than the other synthesized samples. The AgPW-SAPO-34 sample showed a 403 min lifetime and 96.61% light olefins selectivity in the MTO process.

Small ZSM-5 crystals with low defect density as an effective catalyst for conversion of methanol to hydrocarbons

Abstract This work presents the synthesis of nearly defect-free ZSM-5 nanosized crystals, prepared in fluoride medium by seeding with silicalite-1. This material was carefully characterized and its catalytic performances in the methanol to hydrocarbons (MTH) reaction were assessed. Such fluoride-based material was compared to a reference ZSM-5, produced through a conventional alkaline synthesis but from the same seeding. Despite both the materials show closely identical morphology and they have a comparable acid site population, the catalyst prepared using the fluoride route showed significantly longer lifetime in MTH compared to the catalyst prepared using conventional synthesis at high pH. The slower deactivation for the samples prepared using the fluoride route was ascribed, thanks to a thorough in situ IR spectroscopy study, to its lower density of internal defects. According to the UV-Raman characterization of coke on the spent catalyst, the fluoride-based ZSM-5 catalyst produces less molecular coke species, most probably because of the absence of enlarged cavities/channels as due to the presence of internal defects. On the basis of these observations, the deactivation mechanism in the ZSM-5 synthesized by fluoride medium could be mostly related to the deposition of an external layer of bulk coke, whereas in the alkali-synthesized catalyst an additional effect from molecular coke accumulating within the porous network accelerates the deactivation process.