HBEA Zeolite Research Articles

Alkylation of lignin-derived aromatic oxygenates with cyclic alcohols on acidic zeolites

Moderately strong BAS and spacious microporous environments (i.e., large-pore acidic zeolites HBEA and HY) are important criteria for efficient alkylation of phenols with cyclic alcohols and alkenes, while very strong BAS appear to be responsible for catalyst deactivation. BAS confined in HBEA and HY pores show substantially higher turnover frequencies compared to solids without molecularly sized pore constraints. HBEA favors the formation of mono-alkylates, while enhanced formation of di-alkylates is observed on HY. Dehydration of alcohols always dominates over the alkylation at the onset of the reaction. Carbenium ions, the direct electrophile for aromatic oxygenates, are produced mainly from the adsorption and protonation of olefins. The present study shows that for the alkylation of phenols and cyclic alcohols in apolar liquids, the alcohol concentration should be kept low to avoid the formation of unreactive surface dimers and mitigate their inhibitory effects on olefin adsorption and protonation.

Adsorption and catalytic decomposition of dimethyl sulfide on H-BEA zeolite

Catalytic direct decomposition of dimethyl sulfide (DMS) was performed using solid acid catalysts to develop an on-site hydrogen-free desulfurization system for utilization in small systems, such as fuel cells. DMS was decomposed to CH3SH and H2S at 500 °C on SiO2–Al2O3 and various zeolite catalysts. Among the catalysts, H-BEA zeolite with Si/Al = 18.5 (H-BEA-18.5) showed the highest performance for DMS decomposition at 500 °C. While the catalytic activity at 500 °C maintained a DMS conversion of greater than 30% for up to 114 h, a large amount of carbon deposition caused gradual deterioration. At a low temperature of 400 °C, DMS decomposition to CH3SH on H-BEA-18.5 continued for 100 h with a stable conversion of approximately 30%, although the adsorption of DMS on the catalyst surface was also confirmed. To achieve a high performance for the DMS decomposition, high temperatures were required to avoid the adsorption of sulfur species.

The Prins condensation between i-butene and formaldehyde over modified BEA and MFI zeolites in liquid phase

The liquid-phase Prins condensation between i-butene and formaldehyde was studied over parent and Mg-modified MFI and BEA zeolites with low Si/Al ratios. High catalytic activity and selectivity toward isoprene were obtained using the H-BEA zeolite. The H-MFI zeolite was confirmed as a highly selective catalyst for 3-methylbut-3-ene-1-ol production via Prins condensation. The influence of Mg-ion-exchange treatment on the catalytic behavior of the zeolites was studied; modification of BEA led to significant changes in the catalytic behavior, while Mg-MFI and H-MFI showed similar conversion of formaldehyde and similar product distribution.

Kinetics of aromatics hydrogenation on HBEA

Zeolite HBEA catalyzes hydrogenation of aromatic hydrocarbons—methyl-substituted benzenes (benzene and toluene), alkenyl-substituted benzenes (styrene), and polycyclics (naphthalene)—in presence of excess H2 at high-temperatures (573–748 K) with rates that depend linearly on aromatic and H2 pressures. The observed kinetic behavior can be rationalized based on a sequence of elementary steps where the first hydrogenation step of the adsorbed benzenic intermediate is rate-determining while subsequent hydrogenation and desorption steps are quasi-equilibrated and H+ is the most abundant surface species. Styrene hydrogenation exhibits the highest rates among the aromatics considered and results exclusively in ethylbenzene synthesis; in contrast, benzene/toluene and naphthalene hydrogenation results in formation of their triply-hydrogenated five-membered ring and doubly-hydrogenated ring open analogs, respectively. Based on independent studies involving co-reaction of cyclohexene and 1-methyl-1-cyclopentene with H2, we infer their facile interconversion and hydrogenation to methylcyclopentane implying that conversion of benzene to methylcyclopentane likely occurs via intervening formation of both five- and six-membered ring intermediates. Taken together, these studies demonstrate feasibility of aromatics hydrogenation and propensity of benzenic rings in these hydrocarbons to undergo ring reduction or ring opening during their activation with H2 on Brønsted acid zeolites.

In situ DRIFT study on NH3 selective catalytic reduction of NOx over HBEA zeolite doped with CeO2

The reaction mechanism of selective catalytic reduction of NO with NH3 on Ce-doped HBEA catalysts was investigated using in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results illustrated that the amide species adsorbed on Lewis acid sites (AlO)+ and NH4+ species adsorbed on Bro¨nsted acid sites (Al-OH-Si) were main intermediates on HBEA. Only Eley-Rideal (E-R) mechanism was presented over HBEA catalyst. By contrast, both E-R and Langmuir-Hinshelwood (L-H) mechanism were presented over Ce7.0HBEA and the E-R mechanism was the dominated reaction mechanism. NH4+ adspecies and nitrate adspecies were primary intermediates on Ce7.0HBEA. Meanwhile, the presence of Ce3+ could lead to the increase of surface oxygen concentration, which could actively take part in the activation process of ammonia adspecies and NOx adspecies and then facilitated the NH3-SCR reaction. Hence, the NOx conversion of Ce7.0HBEA was much higher than that of HBEA. In addition, the surface oxygen vacancy derived from ceria on Ce7.0HBEA could inhibit the formation of N2O. As a result, the N2 selectivity of Ce7.0HBEA is higher than that of HBEA. The above results are useful for guiding the way to develop Ce-containing SCR catalysts.

Synthesis of n-Butyl Levulinate Using Mesoporous Zeolite H-BEA Catalysts with Different Catalytic Characteristics

The present work focuses on the utilization of waste biomass for the improvement of key catalytic properties of conventional zeolite H-BEA. In the present endeavor, zeolite H-BEA has been modified using cetyltrimethyl ammonium bromide (CTAB) and rice husk (a waste biomass resource), via desilication post synthetic route, which is not reported so far. The synthesized mesoporous zeolite H-BEA catalysts have been characterized by various characterization techniques such as, SEM, 27Al and 29Si MAS-NMR, wide and low angle XRD, ICP-OES, FT-IR, TGA, NH3-TPD and BET surface area. The resultant mesoporous zeolite materials (MCCK and MCRK) exhibited bimodal porosity as well as improved physicochemical properties, and the utility of these modified zeolites as heterogeneous catalysts has been demonstrated in the production of n-butyl levulinate via levulinic acid (LA) esterification. The catalytic material, which has been modified using CTAB and rice husk, is found to exhibit better catalytic activity towards the synthesis of n-butyl levulinate (95.6%) as compared to other zeolite counterparts under the optimised reaction conditions, which is attributed to its enhanced surface area and lower Si/Al ratio as compared to other catalysts under study.

TTAB mediated synthesis of Meso-H-BEA and its application in the production of n-butyl levulinate

Abstract The present work is focusing on the improvement of catalytic characteristics of zeolite. In the present endeavor, zeolite H-BEA has been modified using sol-gel route by making use of tetradecyltrimethyl ammonium bromide (TTAB) surfactant as a structure directing agent. The synthesized mesozeolite was characterized by SEM, HRTEM, 29Si and 27A1 MAS-NMR, small and wide-angle XRD, FT-IR, TGA, NH3-TPD, ICP-OES and BET surface area analysis. The synthesized material was found to exhibit mesoporosity along with microporosity of parent H-BEA. The utility of modified H-BEA as heterogeneous catalyst was demonstrated in the production of value added chemical, n-butyl levulinate from biomass derived feedstock, levulinic acid (LA) via esterification reaction. n-butyl levulinate is one of the significant industrial chemical and has wide use as plasticizing agent, solvent and odorous substance. Various process variables such as molar ratio (LA to n–butanol), catalyst concentration (%) and reaction time (h) were optimized by response surface methodology (RSM), using the Box–Behnken Design (BBD). Analysis of variance (ANOVA) has been performed to investigate the suitability and significance of the quadratic model.

Propane activation on Zn-modified zeolite. The effect of the nature of Zn-species on the mechanism of H/D hydrogen exchange of the alkane with Brønsted acid sites

Zn-modified zeolites exhibit high activity for the aromatization of C2+ alkanes. To understand the effect of Zn-species of different nature on propane molecules activation and transformation, the H/D hydrogen exchange of zeolite Brønsted acid sites (BAS) with deuterated propane-d8 has been investigated. Two samples of the Zn-modified zeolite containing exclusively either isolated Zn2+ cations (Zn2+/H-BEA) or small (ZnO)n clusters (ZnO/H-BEA) have been studied. Zn-species of either type work jointly with BAS and may provide the synergy effect for propane CH bond activation resulting in dramatic acceleration of the H/D exchange between propane and BAS. The accelerating effect of isolated Zn2+ cations is more pronounced compared to zinc oxide clusters. Moreover, the H/D exchange occurs regioselectively into the methyl groups of propane on Zn2+/H-BEA zeolite. For ZnO/H-BEA, the methylene group becomes also involved in the exchange after some induction period. Various mechanisms providing the regioselective H/D exchange on Zn2+/H-BEA as well as the involvement of the methylene group after the induction period on ZnO/H-BEA are discussed.

Effect of acid strength on the formation mechanism of tertiary butyl carbocation in initial C4 alkylation reaction over H-BEA zeolite: A density functional theory study

Abstract As the first and essential step of the C4 alkylation reaction, the formation of tertiary butyl carbocation over H-BEA zeolite was first systematically studied utilizing density functional theory (DFT) calculation. Herein, two reaction pathways (i.e., skeletal isomerization and hydride transfer) were systematically compared involving the Gibbs free energy at 350 K. It is found that the overall apparent reaction barrier of the hydride transfer pathway is 76 kJ/mol, much lower than that of skeletal isomerization pathway (92 kJ/mol), indicating that the hydride transfer pathway is predominant over the H-[Al]-BEA. Moreover, effect of acid strength on reaction mechanism was further investigated over H-[Al]-BEA and H-[B]-BEA catalysts with stronger and weaker acidity, respectively. Results show that the hydrogen transfer route is still the dominant pathway to generate tertiary butyl carbocation when acid strength changes, which correlates well with the experimental results. However, rate-determining steps of skeletal isomerization and hydride transfer are changed, and the higher Gibbs free energy barriers are obtained by decreasing the acid strength. The insights in this work will be of great significance to understand the initial C4 alkylation mechanism and to design highly efficient catalysts.

Tert-Butylation of Toluene Catalyzed by Phosphotungstic Acid Supported on HBEA Zeolite

The performance of the catalysts prepared by impregnation of phosphotungstic acid (HPW) in HBEA zeolite with different HPW load (HPW(x)/HBEA) was evaluated in alkylation of toluene with tert-butanol with formation of 4-tert-butyltoluene (4-TBT). The obtained results showed that the HPW was highly dispersed on the HBEA support. The specific surface area of HPW(x)/HBEA zeolites decreased along with a decrease in pore volume. The Bronsted acidity of HPW(x)/HBEA showed a significant increase initially, reaching a maximum at x = 30% and then decreased with further increasing x. The Bronsted acidity of HPW(30%)/HBEA was 143.0 μmol/g, which is an increase of 69.8% when compared with that found for HBEA (84.2 μmol/g). The HPW(x)/HBEA zeolite with HPW loading of 30% exhibits the best catalytic activity at 180°C after 4 h. Toluene conversion over HPW(30%)/HBEA reached 73.1% with para-selectivity of 80.9%, which much higher than those found for HBEA (activity of 54.0% with para-selectivity of 69.5%). The improved toluene conversion over HPW(x)/HBEA can be attributed to their higher Bronsted acidity after loading HPW, while the improved selectivity can be attributed to the narrowed pores of HPW(x)/HBEA catalysts.

Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol.

The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15–407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–180 °C has been explored. We describe and discuss data concerning the strength and concentration of acid sites of H-BEA obtained by techniques of stepwise (quasi-equilibrium) thermal desorption of ammonia, X-ray diffraction, low-temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed pyridine and solid-state 27Al MAS NMR. The average values of the adsorption energy of NH3 on H-BEA were experimentally determined as 63.7; 91.3 and 121.9 mmol g−1 (weak, medium, and strong, respectively). In agreement with this, a correlation between the rate of ethyl-tert-butyl ether synthesis and the concentration of weak acid sites (ENH3 = 61.6–68.9 kJ mol−1) has been observed. It was concluded that the active sites of H-BEA for this reaction are Brønsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.

Mass transfer and kinetic study on BEA zeolite-catalysed oil hydroesterification

In the present study, we have assessed the effect of mass transfer on the both steps of the zeolite catalysed oil hydroesterification to produce biodiesel (hydrolysis of the feed soybean oil producing free fatty acid and free fatty acid esterification). Both reactions were conducted over BEA zeolite in a batch reactor. Experimental results indicate the absence of internal and external mass transfer limitations on the overall rate of either reaction. A reversible, second order reaction was invoked in order to estimate the reaction rate constant, pre-exponential factor and activation energy of the esterification and hydrolysis reactions. Activation energy of 25.9 kJ/mol (soybean oil hydrolysis), 38.5 kJ/mol (ethyl acetate hydrolysis), 24.8 kJ/mol (oleic acid esterification) and 46.7 kJ/mol (acetic acid esterification) were calculated over H-BEA zeolite, illustrating the importance of reactant size and the fact that reactions take place on the pour mouth of the mesopouros zeolite in a case of large molecules reaction. Experimental results suggested that the relative hydrophobicity of the catalyst is a key parameter, which influences whether the reaction is under kinetic or diffusion control.

Preparation of 2-Methylnaphthalene from 1-Methylnaphthalene via Catalytic Isomerization and Crystallization

Large amounts of residual 1-methylnaphthalene are generated when 2-methylnaphthalene is extracted from alkyl naphthalene. In order to transform waste into assets, this study proposes a feasible process for preparing 2-methylnaphthalene from 1-methylnaphthalene through isomerization and crystallization. The 1-methylnaphthalene isomerization was carried out in a fixed-bed reactor over mixed acids-treated HBEA zeolite. The results showed that acidic properties of catalysts and reaction temperature were associated with the 2-methylnaphthalene selectivity, yield and catalytic stability. At a high reaction temperature of 623 K, the 2-methylnaphthalene yield was 65.84 %, and the deactivation rate was much lower. The separation of reaction products was then investigated by two consecutive crystallization processes. Under optimal conditions, the 2-methylnaphthalene purity attained 96.67 % in the product, while the yield was 87.48 % in the refining process.

Effect of acidity and porosity of hierarchical HBEA zeolite on catalytic stability of α-methylnaphthalene isomerization

Hierarchical HBEA catalysts were fabricated through alkaline treatment and evaluated in a fixed-bed reactor. The relative crystallinity, pore property, Si/Al molar ratio, coke formation and structure of catalysts were investigated by the methods including X-ray diffraction, N2 physisorption, X-ray fluorescence, thermogravimetric analysis and transmission electron microscope, respectively. The acidity of catalysts was studied by Fourier transform infrared spectrometry using pyridine as the probe molecule. These results demonstrated that pure NaOH treatment improved the mesoporosity of catalyst, while decreased the β-methylnaphthalene selectivity and catalyst stability due to the partially destroyed framework and an increase in the concentration of Lewis acid sites. The Lewis acid sites are responsible for the hydride transfer and subsequent reaction that form coke. In contrast, the presence of tetra-propyl-ammonium hydroxide in the mixed-alkali treatment resulted in regular mesopores development, preserved intrinsic zeolite properties and Lewis sites proportion, leading to the maintained β-MN selectivity and coke deposition rate. The increased mesoporosity and strong Bronsted acid sites amount per number of coke were responsible for the improved coke tolerance of mixed-alkali treated HBEA, and thus its deactivation rate was 41.7% lower than that of HBEA catalyst.

Mechanistic insight into the passive NOx adsorption in the highly dispersed Pd/HBEA zeolite

ABSTRACT Passive NOx adsorption using zeolite supported Pd catalysts has been considered one of most effective methods for low-temperature NO storage in the diesel engine emission control. In the present work, first-principles density functional theory calculations were used to study NO adsorption at the highly dispersed Pd sites, i.e., monomeric PdIIO and dimeric [PdII-O-PdII]2+ in the HBEA zeolite. It has been found that each PdIIO and [PdII-O-PdII]2+sites could bind with three and four NO molecules, respectively. Upon adsorption, NO could be oxidized to NO2, resulting in the reduction of PdII to Pd0 for PdIIO and PdII to PdI, for [PdII-O-PdII]2+, respectively. With increasing NO coverage at both sites, NO oxidation becomes more facile with low activation barriers of 13 and 22 kJ/mol. At the [PdII-O-PdII]2+ site, the binding of formed NO2 becomes weak at high NO coverage. Both PdIIO and [PdII-O-PdII]2+sites are protonated to the more stable [PdIIOH]+ and [PdIIOH]+/[PdIIOH]+ sites in the presence of H2O or neighboring BAS sites. Finally, we studied the CO effect on the NO adsorption in Pd/HBEA. With co-fed CO in the NOx mixture, both PdIIO and [PdII-O-PdII]2+ could be reduced to Pd° and [PdI-PdI]2+ sites. Compared to the NO adsorption, our calculations show that CO adsorption is slightly stronger at the [PdII-O-PdII]2+site while it is slightly weaker at the PdIIO site. CO oxidation, rather than NO oxidation readily occurs if NO and CO co-adsorbed. The reduced PdI site and the [PdIIOH]+/[PdIIOH]+ site enhances NO adsorption.

Tert-Butylation of Toluene with Tert-butanol Over Transition Metal Oxide-Modified Hbea Zeolite

A series of transition metal oxide-modified HBEA (M x0 y/HBEA) zeolite catalysts were prepared by the wetness impregnation method, and investigated for the alkylation of toluene with tert-butanol to synthesise 4- tert-butyltoluene (4-TBT). Their physico-chemical properties were characterised by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller and NH3 temperature-programmed desorption methods. M x0 y/HBEA zeolite showed higher para-selectivity than parent HBEA due to its improved structural and acidic properties. Though the toluene conversion of M x0 y/HBEA catalyst decreased, the 4-TBT selectivity increased significantly at 190 °C after 4 h. The narrowed pores after loading the M x0 y prompted an increase in selectivity for 4-TBT by increasing the shape selectivity. In addition, the decrease in strong acid sites increased selectivity for 4-TBT by suppressing further isomerisation of the 4-TBT formed on acid sites. Among all the modified HBEA zeolites, Fe203/HBEA exhibited the best catalytic activity and para-selectivity. The factors affecting the reaction over Fe203/HBEA have also been investigated extensively.

Heterogeneously catalyzed synthesis of oxymethylene dimethyl ethers (OME) from dimethyl ether and trioxane

Oligomeric oxymethylene ethers (OMEs) are promising fuels for internal combustion engines with self-ignition. A tremendous reduction of soot, nitrogen oxide and hydrocarbon emissions can be realized by employing OMEs. This work describes the synthesis of OMEs from dimethyl ether (DME) and trioxane. Zeolite H-BEA 25 was found to be a suitable catalyst for this reaction and a study on reaction parameters (e.g. temperature, reactant feed ratio and reaction time) has been carried out. A maximum DME conversion of 13.9 wt% could be obtained and the maximum reactant-related product yield for OME3–5 was 8.2 wt%. An OME formation mechanism is proposed which explains the observed product spectrum.

Intraparticle Diffusional versus Site Effects on Reaction Pathways in Liquid-Phase Cross Aldol Reactions.

Chemo- and regioselectivity in a heterogeneously catalyzed cross aldol reaction were directed by tuning the nature of the sites, textural properties, and reaction conditions. Catalysts included sulfonic acid-functionalized resins or SBA-15 with varying particle size or pore diameter, H-BEA zeolites, and Sn-BEA zeotype; conditions were 25 °C to 170 °C in organic media. Benzaldehyde and 2-butanone yielded branched (reaction at -CH2 - of butanone) and linear (reaction at -CH3 ) addition and condensation products; and fission of the branched aldol led to β-methyl styrene and acetic acid. Strong acids promoted the dehydration step, and regioselectivity originated from preferred formation of the branched aldol. Both, resins and functionalized SBA-15 materials yielded predominantly the branched condensation product, unless particle morphology or temperature moved the reaction into the diffusion-limited regime, in which case more fission products were formed, corresponding to Wheeler Type II selectivity. For H-form zeolites, fission of the branched aldol competed with dehydration of the linear aldol, possibly because weaker acidity or steric restrictions prevented dehydration of the branched aldol.

27Al MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

27Al single pulse (SP) MAS NMR spectra of HBEA zeolites with high Si/Al ratios of 71 and 75 were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field 27Al MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on the results obtained from 27Al MAS and MQMAS NMR acquired at 19.97 T, four different quadrupole peaks are used to deconvolute the 27Al SP MAS spectra acquired at various fields by using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate that, for fully hydrated HBEA zeolites, the effect of second-order quadrupole interaction can be ignored, and quantitative spectral analysis can be performed by directly fitting the high fi...

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.