HMCM-22 Zeolite Research Articles

Resistance against Carbon Deposition via Controlling Spatial Distance of Catalytic Components in Methane Dehydroaromatization

Molybdenum-zeolite catalysts always suffer from severe carbon deposition and rapid deactivation in the methane dehydroaromatization (MDA) process. Herein, we present a strategy that controls spatial distance between Mo species and HMCM-22 zeolite over Mo/HMCM-22 catalysts, to inhibit the severe carbon deposition. Our characterization analyses demonstrate that the Mo/HMCM-22 catalysts possess the same active components, but the spatial distance plays a key role in determining product selectivity in the MDA process. The MDA performance reveals that Mo/HMCM-22-MM (mechanical milling) catalyst, with a medium spatial distance between Mo species and HMCM-22 zeolite, significantly inhibits carbon deposition and produces high selectivity to benzene. This work shows that spatial distance between molybdenum and zeolite is an important property for suppressing carbon deposition and improving benzene selectivity in MDA process.

Controlling the Evolution of Active Molybdenum Carbide by Moderating the Acidity of Mo/HMCM-22 Catalyst in Methane Dehydroaromatization

Effect of framework Bronsted Acidity by varying SiO2/Al2O3 ratio (SAR) of HMCM-22 zeolite is studied with respect to the formation of active molybdenum carbide and its anchoring over zeolite (HMCM-22) channels of Mo/HMCM-22 catalyst tested for methane dehydroaromatization (MDA) reaction. For this purpose, HMCM-22 is synthesized by conventional methods with varying SAR (30, 40 & 55) and is studied for MDA reaction with 5 wt% Mo loading. XRD, BET, NH3-TPD, H2-TPR, 27Al MAS NMR, Raman spectroscopy and XPS techniques are used to characterize 5Mo/HMCM-22 catalyst having different SAR. XPS analysis of carburized 5Mo/HMCM-22 (SAR-30, 40 & 55) catalyst confirms that higher content of molybdenum carbide (Mo2C) forms over HMCM-22 channels at SAR-30 as compared to SAR-40 and SAR-55 due to effective binding of initial MoOx species. Interaction of initial MoOx species with HMCM-22 zeolite framework is analyzed using 27Al MAS NMR and Raman spectroscopic studies which confirm that MoOx species bind differently at SAR-30, 40 and 55 which affects the catalytic performance. Lower reducibility of MoOx species at SAR-30 confirms that MoOx species strongly interact with HMCM-22 channels at SAR-30 as confirmed by H2-TPR study. Maximum transformation of MoOx species into active molybdenum carbide over HMCM-22 at lower SAR (30) during carburization results in higher activity of 5Mo/HMCM-22 (SAR-30) catalyst with lower coke content.

Non-oxidative conversion of methane into higher hydrocarbons over Mo/MCM-22 catalyst

Molybdenum impregnated zeolite catalyst has been well-known for methane conversion into higher hydrocarbons under non-oxidative condition. HZSM-5 & HMCM-22 zeolites are the effective supports for this purpose. However, the catalytic performance of HMCM-22 supported molybdenum catalyst is considered suitable than that for HZSM-5 catalyst with high aromatic selectivity due to unique pore structure and framework of MCM-22 zeolite support. Effect of Mo loading over MCM-22 zeolite has been studied for the activity test and observed that 5 wt% metal content over the support (MCM-22) is optimum for the proper tuning of acidic & metallic sites of the catalyst. Effect of silica/alumina ratio (SAR, molar) of MCM-22 zeolite has also been studied and observed that lower SAR (30) is suitable ( $$\hbox {C}_{6}\hbox {H}_{6}$$ selectivity, 37%) comparatively to higher SAR (55) ( $$\hbox {C}_{6}\hbox {H}_{6}$$ selectivity, 18%). Lower GHSV (720 mL/g.h) is effective for higher hydrocarbon production compared to higher GHSV (1200 mL/g.h) due to low residence time. Mo/MCM-22 catalysts with different Mo loading were characterized by BET surface area, XRD, Raman spectroscopy and $$\hbox {NH}_{3}$$ -TPD analysis. Unique pore systems [10 & 12 membered ring (MR)] and framework of MCM-22 zeolite support are the key factors for effective methane conversion to value added chemicals when loaded with molybdenum. Mo/MCM-22 bifunctional catalyst was studied in detail for direct methane conversion into higher hydrocarbons under non-oxidative condition. The effect of Mo loading and silica/alumina ratio of support was studied for the process. The results suggest that 5 wt% Mo loading and lower SAR (30) is effective for higher hydrocarbon formation.

A theoretical study of confinement effect of zeolite on the ethylene dimerization reaction

The influences of pore confinement and topology effect on ethylene dimerization over a series of zeolite, like HMOR, HZSM-5, HBEA and HMCM-22, have been systematically studied by density functional theory including dispersion interaction (DFT-D). Both of the stepwise and the concerted mechanisms are considered. Compared to the corresponding 8T models, the calculated activation energies are considerably reduced in the large models of zeolite due to the pore confinement effect. Moreover, the manners of ethylene dimerization are found to be related to the pore structure of zeolite catalyst. In stepwise mechanism, HMCM-22 shows better activity as suggested by the calculated activation energies. In concerted mechanism, the energy barriers in HBEA and HMCM-22 are much lower than that in HMOR and HZMS-5, indicting a better catalytic performance. By comparing the two mechanisms happened in one zeolite, we suggest that the concerted mechanism is preferable in HBEA, HMCM-22 and HMOR zeolite with large pores, while two mechanisms compete to each other in HZSM-5 with medium-size pores.

Active phase, catalytic activity, and induction period of Fe/zeolite material in nonoxidative aromatization of methane

Catalytic dehydroaromatization of methane has been investigated over 2–6wt.% Fe-modified HZSM-5 or HMCM-22 zeolite in the absence of an added oxidant. After pretreatment in a CH4/Ar/He mixture, Fe/HMCM-22, and more significantly, Fe/HZSM-5 exhibited high catalytic activity in nonoxidative conversion of methane to aromatic hydrocarbons. This conversion actually takes part in three consecutive steps, oxidation of methane by iron oxides, decomposition, and aromatization of methane. During this conversion, XPS and XRD results showed the gradual reduction of Fe2O3 to metallic Fe, followed by the carburization of the metallic species. The fact that no aromatic products were detected until iron carbide was formed demonstrated that carburized Fe was the active species for the aromatization of methane. Under the same reaction conditions, both metallic Fe and carburized Fe were highly effective for activation of methane, but they catalyzed dehydrogenation of methane to carbonaceous materials and to aromatics, respectively. Difference in the catalytic properties of those two Fe species suggested that carburized Fe could stabilize carbene, the primary intermediate of activated methane, possibly via formation of a metal–carbene complex, CFeCH2, so that carbene was able to dimerize into ethylene, which was then oligomerized to aromatics on the acid sites of the zeolite.

Bioethanol conversion into hydrocarbons on HZSM-5 and HMCM-22 zeolites: Use of in situ DRIFTS to elucidate the role of the acidity and of the pore structure over the coke formation and product distribution

In the last years there has been an increasing interest for the use of ethanol not only as biofuel but also as raw material for the production of petrochemicals. In the present work the conversion of ethanol into hydrocarbons was investigated over zeolites with different acidic properties and porous structure (HZSM-5 and HMCM-22). The samples used were active for ethanol conversion and both their acidic properties and porous structure played an important role on product selectivity. HZSM-5 zeolite was the most promising catalyst for propene formation. Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were also employed to investigate the carbon species deposited on the zeolites and the adsorbed species formed during the reaction, respectively. DRIFTS results show that coke formation was severely inhibited by the pore structure of HZSM-5 justifying the high stability of this zeolite.

Theoretical Insights into the Mechanism of Olefin Elimination in the Methanol-to-Olefin Process over HZSM-5, HMOR, HBEA, and HMCM-22 Zeolites

The mechanism of olefin elimination in the process of methanol-to-olefins (MTO) over a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by DFT-D calculations, which is a crucial step that controls the MTO product distribution. The results demonstrate that the manners of olefin elimination are related to the pore structure of zeolite catalyst and the interaction between proton transfer reagent (water or methanol) and zeolite acidic framework. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with large pores, as suggested by the energy barrier of rate-determining step and the potential energy surface (PES), but is unfavorable over HZSM-5 with medium-sized pores due to the steric hindrance of spiro intermediates. Over various zeolites, water and methanol perform differently in proton transfer to form the spiro intermediates; over HMOR and HBEA with strong acidity, water is superior to methanol in promoting propene elimination, whereas over HMCM-22 with relatively weaker acidity, methanol is more favorable as a proton transfer reagent.

Desulfurization of model FCC feedstocks by alkylation: Transformation of thiophenic compounds in presence of 2-methyl-1-pentene over acidic zeolites

The transformation of benzothiophene (BT) with 2-methyl-1-pentene was investigated over HBEA, HMCM-22 and HY zeolites having similar Brønsted acidities, at 85°C under atmospheric pressure in a fixed bed reactor. Over these zeolites, BT underwent only alkylation, whereas 2-methyl-1-pentene underwent alkylation, isomerization and oligomerization. HY was shown to be the most active and stable in BT conversion. HMCM-22 had a low initial activity and a poor stability, due to both diffusional limitations and fast deactivation of its acid sites, even those located in the external pockets. HBEA presented an intermediate behavior. The presence of toluene led to a decrease of BT conversion, which was attributed to competitive adsorptions of the aromatic and the olefin.Whatever the zeolite and the conversion, BT was predominantly transformed into monoalkylated products whereas 3-methylthiophene led essentially to dialkylated products. These results are in agreement with those obtained from theoretical calculations. HMCM-22 zeolite was shown to be the most selective toward alkylation versus olefin oligomerization.

Effect of the introduction of niobium carbide on non-oxidative dehydro-aromatization of methane over MoMCM-22

The effect of the introduction of niobium carbide was studied on the dehydro-aromatization of methane over a molybdenum containing HMCM-22 zeolite at 973 K. The introduction of niobium decreased Bronsted acidity, affecting catalyst behavior on the dehydro-aromatization of methane. Niobium carbide containing zeolite showed lower activity, lower coke deposition and predominant formation of naphthalene, in the expenses of benzene, the major product observed for Mo containing zeolite. Besides carbidic carbon, two other types of coke were observed: one associated to molybdenum and the other associated to Bronsted acid sites located both on the catalyst surface and inside zeolite pores.

Characterization of the pore architecture created by alkaline treatment of HMCM-22 using 129Xe NMR spectroscopy

The pore architecture of alkaline treated HMCM-22 zeolite was investigated by 129Xe NMR spectroscopy. The spectra show that additional porosities have been formed after the alkaline treatment. Up to a treatment with 0.20 M NaOH new micropores are formed which gradually grow in size to the mesoporous range at higher alkaline treatment conditions. Furthermore, interconnections between the two independent channel systems are formed, leading to an increase in the porosity and a further evolution of the porosity towards the mesoporous range. The results are in accordance with earlier characterization and catalytic results and conclusions drawn in our previous study.

Violating the general acidity–activity correlation: Computational evidence in a CpNa-modified HMCM-22 zeolite for ethene protonation

The catalytic activity of zeolite is well known to be associated with proton-donating ability at the Brønsted acid sites, i.e., high acidity versus high activity. In this paper, we firstly show that for modified zeolites, this simple acidity–activity correlation might be biased. We found that the proton hopping barrier (21.42 kcal/mol) within CpNa-modified HMCM-22 (Cp: cyclopentadienyl) is significantly higher than that of HMCM-22 (13.46 kcal/mol), indicating a clear lowering of acidity upon CpNa-modification. Surprisingly, the activation energy of ethene protonation is also significantly reduced from 30.43 kcal/mol in HMCM-22 to 24.74 kcal/mol in CpNa-modified HMCM-22, suggesting an enhanced catalytic activity upon CpNa-modification. The cause of violation from traditional “acidity–activity” correlation can be ascribed to the dual factor of CpNa, namely, Na + can “activate” the proton due to its influence on the charges of proton and oxygen atoms, whereas Cp − can “deactivate” the proton due to its effective interaction with proton.

Effect of dealumination by acid treatment of a HMCM-22 zeolite on the acidity and activity of the pore systems

Acid leaching of a HMCM-22 zeolite (Si/Al = 14.5) was carried out at 100 °C for different times (3, 30, 60 and 480 min) with a 4 M nitric acid solution. The resulting samples were characterised by elemental analysis, X-ray diffraction, pyridine adsorption followed by infrared spectroscopy (FTIR) and nitrogen adsorption and their catalytic properties were established with methylcyclohexane transformation at 350 °C. The role played in this reaction by the three micropore systems was established by coupling selective deactivation of supercages by coking and poisoning of the outer cup acid sites with 2,4-dimethylquinoline. The change in the acid and catalytic properties with the time of acid treatment shows that dealumination is dominated by the accessibility of the framework Al atoms. Thus, the outer cup acid sites are highly sensitive to the acid treatment. The acidity and activity of the supercages are also strongly affected whereas those of the narrow sinusoidal channels are hardly affected.

Alkaline Modification of MCM-22 to a 3D Interconnected Pore System and its Application in Toluene Disproportionation and Alkylation

Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This ‘desilication’ process led for mild alkaline concentrations (~0.10–0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3-acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Bronsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Bronsted acid site concentration. Decomposition and deconvolution of 29Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q3 SiOH groups were formed at the expense of the Q4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite.

1-Butene Cracking to Propene on High Silica HMCM-22: Relations Between Product Distribution and Feed Conversion Under Various Temperatures

HMCM-22 zeolite with a high Si/Al2 ratio (158) was applied to the 1-butene cracking reaction in a fixed-bed reactor. On basis of the product distribution, the main reaction pathways of C4 alkenes catalytic transforming on HMCM-22 were proposed. Also, the preferences of the reaction pathways influenced by the temperature and conversion level were discussed, which could provide good guidance on enhancing the propene production.

Highly effective conversion of syngas to dimethyl ether over the hybrid catalysts containing high-silica HMCM-22 zeolites

Pure HMCM-22 zeolites with different SiO2/Al2O3 ratios were synthesized and characterized by XRD, N2 adsoption, 27Al MAS NMR, pyridine-FT-IR and NH3–TPD. The HMCM-22 samples were mechanically mixed with a methanol synthesis catalyst and then applied in the one-step synthesis of dimethyl ether from syngas (STD process). The results indicated that with the increase of SiO2/Al2O3 ratio of zeolites HMCM-22, selectivity to dimethyl ether increased and the selectivities of by-products decreased due to the weakening of their acidities. It is concluded that the high-silica HMCM-22 zeolite is an effective dehydration component of the hybrid catalyst for the STD process.

Methylcyclohexane transformation over HMCM22 zeolite: Mechanism and location of the reactions

At 350 °C, over a HMWW zeolite ( Si / Al = 14.5 ) , methylcyclohexane (mch) transforms into four types of products: isomers (I), C 1–C 8 aliphatic hydrocarbons (C), benzenic hydrocarbons (A), and trapped carbonaceous compounds (coke). The role played by the various micropores was established through a two-step method: deactivation of supercages by coking and poisoning of the outer hemicage sites by 2,4-dimethylquinoline. More than 85% of mch transformation occurs in the supercages, ∼11% occurs in the outer hemicages, and only <2% occurs in the sinusoidal channels, which, considering the protonic site distribution, corresponds to turnover frequency values of 33, 82, and 2 h −1. The product distributions are very different: I, C 3–C 5 branched alkanes, and coke in similar amounts; essentially I and C 3–C 8 branched alkanes; and mainly C 1–C 8 and A products. The carbenium ion chain mechanism can account for the products formed in supercages and hemicages, whereas protolytic mechanisms play an important role in the narrow sinusoidal channels. Deactivation by coking is very fast in supercages and negligible in the other locations.

N-Heptane and methylcyclohexane transformation over a HMCM-22 zeolite. Origin of the features typical of large-or of medium-pore zeolites

The main features of methylcyclohexane (mch) and n-heptane (n-C7) transformation at 350°C were compared for fresh samples of HMWW, HMFI and HFAU zeolites. With both reactions, the behavior of HMWW was between those of the medium HMFI and large pore HFAU zeolites. Thus, the values of turnover frequency (TOF) and of the TOFmch/n-C7 ratio were similar with HMWW and HMFI and much higher with HFAU. In contrast, small differences were found between product distributions over HMWW and HFAU but large differences between HMWW and HMFI. These observations can be explained by alkane transformation in the large supercages with 10-MR apertures of HMWW: the narrow apertures limit diffusion of the mch within the supercages, but not the diffusion of the products which are therefore typical of the transformation within large cages hence without any steric limitation.

N-Heptane transformation over a HMCM-22 zeolite: Catalytic role of the pore systems

n-Heptane transformation was carried out at 350 °C over a HMCM-22 zeolite (Si/Al = 14.5) previously characterized by various techniques: X-ray diffraction, nitrogen adsorption, scanning electron microscopy, pyridine and 2,4-dimethylquinoline (2,4-DMQ) adsorption followed by FTIR. A pronounced deactivation was shown to occur in the first 10 min reaction, due to a very fast initial coke formation, followed by a quasi-plateau in activity. Cracking was the main reaction. The role played by each of the three pore systems was established by selectively deactivating the supercage sites by coking then by selectively poisoning the protonic sites of the external cups with a bulky base molecule (2,4-DMQ). The supercage sites (∼70% of the inner ones) were found to be responsible for 97% of n-heptane transformation, those of the sinusoidal channels (∼20%) for only 3%, which means that these latter sites were ∼16 times less active probably because of pronounced steric constraints. Unexpectedly, the protonic sites of the external cups, which were demonstrated as able to catalyse efficiently various reactions including methylcyclohexane cracking, were found to be completely inactive.

The use of CH4/H2 cycles on dehydroaromatization of methane over MoMCM-22

Abstract This work aims to study the non-oxidative conversion of methane in a HMCM-22 zeolite and to compare its activity and stability when using hydrogen regeneration cycles. Catalytic tests were carried with pure methane at atmospheric pressure and 973 K, with WHSV of 90 g CH 4 g Mo - 1 h - 1 . The use of hydrogen regeneration cycles promoted the increase on catalyst stability by decreasing coke deposits, especially the non-carbidic coke associated to Bronsted acid sites. The use of hydrogen regeneration cycles also affected the selectivity to products, with increase of naphthalene selectivity at the expense of benzene one.

Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts: Effect of the zeolite structure on the product selectivity and on the reaction mechanism

The transformation of n-decane, n-tetradecane and n-hexadecane was carried out in a fixed-bed reactor at 220 °C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA, HMCM-22 and HZSM-5 zeolite catalysts. The activities of the catalysts and especially the reaction scheme depended strongly on the zeolite structure. Whatever the reactant, monobranched isomers were the only primary reaction products on PtHBEA, while cracking was the main reaction observed on HMCM-22 and HZSM-5 with n-tetradecane and n-hexadecane. This was explained in terms of diffusion of the reaction intermediates inside the zeolites porosities: rapid diffusion inside the large channels of the HBEA zeolite, improved by the small size of the crystallites, blocking of the reaction intermediates inside the porosity of HMCM-22 and HZSM-5 yielding extensive cracking.