Conversion of ethanol over zeolite H-ZSM-5

The conversion of ethanol over H-ZSM-5 was studied as a function of ethanol partial pressure, reaction temperature, weight hourly space velocity and Si/Al ratio. The results obtained were in qualitative agreement with most of those in the literature. Combination with all published results to give a significant regression model was not possible due to the large scatter of the data from various scientific groups. In mechanistic investigations, temperature programmed reaction measurements of ethanol, diethyl ether and ethene were performed. The formation of ethene from ethanol via direct elimination or from diethyl ether as intermediate could be confirmed. In the conversion of ethanol/water mixtures, the product distribution did not change significantly up to a water content of 60 wt%. Then, a pronounced increase of ethene formation and a considerable decrease of the yields of aromatics was observed. When several reaction mixtures from syngas conversion to ethanol were converted over H-ZSM-5, the coking rate depended on the product distribution in the feed. Product mixtures from processes with higher amounts of compounds having an unfavourable C/H ratio led to rapid deactivation of the zeolite.     

Identification and Influence of Acidity on Alkylation of Phenol with Propylene over ZSM-5

The alkylation of phenol with propylene has been studied over several H-ZSM-5s with different Si/Al ratios and Cs⁺-ion-exchanged H-ZSM-5s at temperature range 373–623°C. Both O- and C-alkylation, which were closely dependent on the reaction temperature and acidity of the catalysts, were observed. O-alkylated compound is found to be formed preferably at temperature lower than 250°C and over Cs⁺-ion-exchanged H-ZSM-5s. However, at higher temperature, only C-alkylation is observed. The acidic properties of the zeolites were characterized by solid-state ³¹P MAS-NMR of the probe molecule trimethylphosphine oxide and NH₃-TPD (temperature-programmed desorption) and it is suggested that in the case of C-alkylation, moderate acid sites are responsible for the formation of para-isopropylphenol, while ortho-isopropylphenol is favorable for weak acid sites.     

13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a¹³C nucleus in the CH₂ group (i-BuOH[1–¹³C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using¹³C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.     

Fischer-Tropsch synthesis with an iron catalyst: Incorporation of ethene into higher carbon number alkanes

Tracer studies with C labeled ethene show that for synthesis over a doubly promoted iron catalyst at 7 atm and ca. 60% CO conversion, higher carbon number products are formed from ethene initiation. About 10% of the added ethene (ethene/CO 0.02) is incorporated into C ₅ ⁺ products and that ca. 85% of the ethene that is incorporated does so by initiating chain growth.     

Radioisotope tracer study of co-reactions of methanol with ethanol using 11C-labelled methanol over alumina, H-ZSM-5 and Cu-ZSM-5

¹¹C-labelled methanol was introduced for study of co-reaction of methanol with ethanol over alumina, H-ZSM-5 and Cu-ZSM-5 between 200 and 320 °C. The radio-method was applied to distinguish the radio-labelled methanol derivatives and co-products from unlabelled ethanol derivatives during analysis by radio-GC. The co-reaction was compared to ¹¹C-single methanol transformations from the previous work. Besides ¹¹C-dimethyl ether, ¹¹C-methyl ethyl ether as a co-product was formed between 200–240 °C and completely transformed further between 240 and 320 °C over H-ZSM-5 and alumina and partly over Cu-ZSM-5. In view of our results the appearance of ¹¹C-methyl ethyl ether intermediate is related to higher selectivities to form ¹¹C-traced C₃ and C₄–C₅ paraffins compared to the single ¹¹C-methanol transformation over H-ZSM-5 catalyst. These results suggest a modified methanol transformation pathway in the presence of ethanol over H-ZSM-5.     

Methanol Conversion to Hydrocarbons (MTH) Over H-ITQ-13 (ITH) Zeolite

Product flexibility is key to meeting fluctuating chemicals demands in the future. In this contribution, the methanol to hydrocarbons (MTH) reaction was investigated over two Ge-containing H-ITQ-13 samples, one with needle-like (H-ITQ-13(N), with (Si+Ge)/Al) = 42) and another with plate-like (H-ITQ-13(P), with (Si+Ge)/Al > 100) morphology. The samples were characterised using XRD, BET, SEM/EDS and FTIR spectroscopy, and their MTH performance was compared with the performance of H-ZSM-5 and H-ZSM-22. Similar specific surface areas (413 and 455 m² g^?1 for H-ITQ-13(N) and (P), respectively) and similar acid strength (Δν ~ ?327(?310) cm^?1) was observed for the two H-ITQ-13 samples. Testing of H-ITQ-13(N) at weight hourly space velocity (WHSV) = 2–8 h^?1 at 350–450 °C revealed that C₅₊ alkenes were the main products (35–45 % selectivity at 400 °C), followed by propene and butene. A low but significant selectivity for aromatic products was observed (6–8 % selectivity at 400 °C). Product selectivity was found to be independent of deactivation. The methanol conversion capacity of H-ITQ-13(N) was 120–150 g methanol g^?1 catalyst at 400 °C. Testing H-ITQ-13 at high (30 atm) and ambient pressure, respectively, at 350 °C showed that a high pressure led to enhanced C₅₊ selectivity, but close to a tenfold decrease in methanol conversion capacity. H-ITQ-13(P) was tested at 400 °C and 2 h^?1. It gave lower conversion than H-ITQ-13(N). Furthermore, when compared at the same conversion level, H-ITQ-13(P) gave higher C₅₊ alkene selectivity, lower aromatics selectivity, and a higher propene to ethene ratio than H-ITQ-13(N). The H-ITQ-13 samples yielded a product spectrum intermediate of H-ZSM-22 and H-ZSM-5. The effluent product cut-off of H-ITQ-13 was similar to that of H-ZSM-5 with tetramethylbenzene as the largest significant product, while H-ZSM-22 produced mainly linear and branched alkenes. The lifetime of H-ITQ-13(N) was clearly enhanced compared to H-ZSM-22, but inferior to H-ZSM-5.     

Preparation of In,H-ZSM-5 for DeNOx reactions by solid-state ion exchange

A simple method is proposed to prepare In,H-ZSM-5 catalyst for DeNOx reactions. This consists of mechanically mixing the fine powders of In₂O₃ and H-ZSM-5 followed by heating in oxygen free inert gas flow to 580 °C where indium undergoes thermal auto-reduction and moves into exchange positions as In⁺ without destroying the crystalline structure of the zeolite.It was evidenced by IR, temperature-programmed reduction (TPR) and reoxidation that, once In⁺ was introduced into the lattice either by reductive solid-state ion exchange (RSSIE) or by thermal auto-reductive SSIE, it can be oxidized by O₂ or in the DeNOx reaction to (InO)⁺. The formed (InO)⁺ can easily be reduced to In⁺ suggesting that In,H-ZSM-5 might be a good catalyst for reactions where a redox cycle in the catalyst is involved in the reaction mechanism.Selective catalytic reduction (SCR) by methane proved that only a small fraction of In exchanged, together with some acid sites of the zeolite formed the active center for the catalytic reaction. XRD, XPS and FT-IR using pyridine proved that the structure of the zeolite and these centers are stable under reaction conditions and In is mainly in the form of (InO)⁺ in the used catalyst.     

A Novel Radioisotope Method for Studying Catalytic Transformations over Alumina, H-ZSM-5 and H-Beta Zeolite Catalysts: Investigation of Conversion of 11C-Labeled Methanol to 11C-Labeled Dimethyl Ether and Hydrocarbons

A novel radiochemical method for investigating the catalytic transformations of the ¹¹C-radioisotope labeled methanol over H-ZSM-5 and H-Beta zeolite catalysts has been introduced. The catalysis process was monitored by gamma detectors and the ¹¹C-labeled products were analyzed by radio-gas chromatography. The medium pore H-ZSM-5 and H-Beta zeolite catalysts were synthesized and characterized using X-ray powder diffraction, scanning electron microscope, nitrogen adsorption, X-ray fluorescency and FTIR spectroscopy. The investigations of ¹¹C-labeled product distributions and reaction mechanism of the conversion of [¹¹C]methanol over H-ZSM-5 and H-Beta zeolite catalysts have been elaborated in terms of structure and acidity of the catalysts. In microreactors the effect of natural carbon compounds from environment can be a disturbing effect for the detection of inactive carbon products. Applied radio detection method eliminates these disturbing effects and detects only ¹¹C-labeled compounds during the whole catalytic process. In the study of the transformations of carbon compounds, besides the well known ¹⁴C tracer technique and ¹³C MAS NMR spectroscopy investigation, the ¹¹C-method is a new, more sensitive and simple one to monitor the transformation of the starting ¹¹C-labeled compound by radio detectors (gamma detector) and for analyzing the ¹¹C-labeled products by radio-gas chromatography.     

In situ MAS NMR investigations of molecular sieves and zeolite-catalyzed reactions

Solid-state MAS NMR is a powerful technique to study heterogeneous catalysts and the way by which they operate. In situ MAS NMR has been demonstrated to be a powerful method to understand reaction mechanisms, to study the nature, dynamics and reactivity of surface intermediates and active sites, and to characterize structural modifications in the catalyst itself, in particular when using ¹³C strategically labelled substrates. In this paper, three examples selected from our own work are used to illustrate the potential of in situ MAS NMR. They are the formation of cumene and its isomerization to n-propylbenzene on zeolite H-ZSM-11, the activation of propane at low temperature and the alkylation of benzene with propane on zeolite H-ZSM-5, and the characterization of the aluminophosphate molecular sieve VPI-5 structure with temperature. Studies of the alkylation of benzene with propene confirmed that cumene was the primary reaction product. The undesired n-propylbenzene by-product results from the intermolecular reaction between cumene and benzene, enhanced by molecular shape-selective effects in medium pore size zeolites (e.g., H-ZSM-11). It explains why large pore zeolites, e.g., zeolite Beta, are used commercially today for this process. Propane can be activated at low temperature (ca. 573 K) on bifunctional medium pore size zeolites possessing intimately related acidic Brønsted sites and a dehydrogenation function provided by Ga or Zn species. In Ga/H-ZSM-5 catalysts, at 573 K, the activation of propane was shown to occur via a protonated pseudocyclopropane (PPCP) intermediate (or transition state). The latter evolves in a manner that can be formally described by the formation of CH ₃ ⁺ , C₂H ₂ ⁺ , and C₃H ₇ ⁺ carbenium ion intermediates. These species can react with olefins, alkanes, or other electron-rich molecules such as benzene. The primary reaction products of the reaction of propane with benzene are n-propylbenzene (in small amount), ethylbenzene and toluene. Their subsequent reactions lead eventually to toluene and xylenes as the final products. In the structural characterization of VPI-5, ²⁷Al, ³¹P, and ²⁷Al nutation MAS NMR spectra show that, at 294 K, fully hydrated VPI-5 contains three equally populated Al and P crystallographic sites and that one-third of Al is 6-coordinate. The VPI-5 structure then belongs to the P6₃ space group. Above 353 K, VPI-5, fully or partially hydrated, undergoes a structural transformation to a higher framework symmetry, i.e., the P6₃cm space group. The transformation occurs at nearly the same temperature in both cases, indicating that the breakdown of the hydrogen-bonded helical water structure inside the VPI-5 pores is not a factor in the process.     

13C solid state NMR evidence for the existence of isobutyl carbenium ion in the reaction of isobutyl alcohol dehydration in H-ZSM-5 zeolite

Using two-dimensionalJ-resolved and CP/MAS¹³C NMR, the pathway for the transfer of the¹³C label from the CH₂ group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydration inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH₂ group of the isobutyl silyl ether reaction intermediate (IBSE), and then into the CH and CH₃ groups of the isobutyl fragment (-CH₂CH(CH₃)₂) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl carbenium ion is formed as a reaction intermediate or transition state during the transformation of isobutyl silyl ether into butene oligomers.     

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to²H NMR, the proton of the acidic Al-OH-Si group of the zeolite is transferred into the CH₂= group of the octene-1 molecule. (2) According to¹³C NMR the¹³C label inserted into the terminal CH₂= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.     

Catalytic Activation of Methane Using n-Pentane as Co-reactant over Zn/H-ZSM-11 Zeolite

The catalytic conversion of methane (C1) into higher hydrocarbons using n-pentane (n-C5) as co-reactant over Zn/H-ZSM-11 zeolite material was studied. The aromatics yield was very high, achieving values of over 40 mol% at 500 °C and w/f = 30 g h mol_-1 with a C1/(C1+C5) molar fraction (XC1) = 0.30. Contact time and time-on-stream effects on the product distribution were analyzed in detail in order to obtain information about the evolution of different species. The C1 conversion was as high as 30 mol% without CO _x reaction products.     

Stability of protonic zeolites in the catalytic oxidation of chlorinated VOCs (1,2-dichloroethane)

The deactivation of protonic zeolites in the catalytic oxidation of 1,2-dichloroethane (DCA) was evaluated. DCA oxidation reactions were carried out in a conventional fixed-bed reactor at atmospheric pressure under conditions of lean DCA concentration in air (1000 ppm). The outlet composition was analysed by a gas chromatograph, an IR spectroscopy-based analyser and another UV analyser. The effect of the zeolite crystalline structure was examined in order to track the catalytic stability of H-ZSM-5, H-MOR and H-BEA under typical reaction environment and conditions (1000 ppm DCA, 300 °C, 13,500 h⁻¹). With the aim of a better understanding of the deactivation pathway, the influence of the space velocity and temperature on the durability of protonic zeolites was analysed. Since some products formed during reaction could also cause deactivation, H₂O and HCl were introduced in the feed stream along with the DCA itself, so as to evaluate their effect. In general terms, coke formation was concluded to be the main reason for zeolite catalyst deactivation. Coke was formed from the intermediate vinyl chloride (VC), which resulted from a first dehydrochlorination step of DCA.     

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with¹³C in CH₃ or C-O groups (t-BuOH[2–¹³C₂] andt–BuOH[1-¹³C]), as well as selectively deuterated in methyl groups (t-BuOH[2-²H₉]), was studied on H-ZSM-5 zeolite simultaneously with¹³ C CP/MAS and²H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2–¹³C₁] andt-BuOH[1–¹³C] give rise to identical₁₃C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the side reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.     

Preparation and application of Ba/ZSM-5 zeolite for reaction of methyl vinyl ether and methanol

The ZSM-5 zeolite is widely used to catalyze the reactions of methanol to olefins. Herein, we have prepared the H-ZSM-5 doped with barium (Ba/ZSM-5) using incipient wetness impregnation method. The Ba modified catalysts were used to catalyze a new reaction of methanol with methyl vinyl ether to improve the selectivity of ethylene and propylene (C₂⁼ + C₃⁼). The reaction catalyzed by Ba doped H-ZSM-5 shows higher propylene selectivity over H-ZSM-5. The reaction mechanism is discussed.     

Evaluation of H-type zeolites in the destructive oxidation of chlorinated volatile organic compounds

The deep oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over H-type zeolites (H-Y and H-ZSM-5) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppmv) in dry air, between 200 and 550°C in a conventional fixed-bed reactor. H-ZSM-5 zeolite resulted more active than H-Y zeolite in the decomposition of both chlorinated volatile organic compounds. It was established that Brønsted acidity plays an important role in controlling the catalytic behaviour of the H-type zeolites. DCE was completely decomposed at 400°C, whereas TCE required higher temperatures (550°C). Vinyl chloride was identified as an intermediate in the DCE oxidation in the range of 250–400°C. When vinyl chloride is destroyed at higher temperatures, both zeolite catalysts show a high selectivity (>90%) towards HCl formation. Trace amounts of tetrachloroethylene were detected in the TCE oxidation, which peaked at 500°C. CO was promoted in quantity in the destruction of both DCE and TCE reflecting the difficulty of carbon monoxide oxidation over H-type zeolites.     

Performance of zeolites and product selectivity in the gas-phase oxidation of 1,2-dichloroethane

The deep oxidation of 1,2-dichloroethane (DCE) over H-type zeolites (H-Y, H-ZSM-5 and H-MOR) was evaluated. Experiments were performed on conditions of lean chlorocarbon concentration (around 1000 ppmv) under dry and humid conditions, between 200 and 550°C in a conventional fixed-bed reactor. The high density of Brønsted acid sites, proved by temperature-programmed desorption (TPD) of ammonia and diffuse reflectance FT-IR of adsorbed pyridine measurements, make H-ZSM-5 zeolite an effective catalyst for DCE decomposition. Vinyl chloride was identified as an intermediate in 250–400°C range. When vinyl chloride was destroyed at higher temperatures, all the zeolites showed a great selectivity (>90%) to HCl. CO was promoted in quantity reflecting the difficulty of its oxidation over these zeolite catalysts. The activity of the zeolites was reduced in the presence of water vapour (15,000 ppmv). It was noticed that the addition of water to the feed stream did not alter the order of activity observed in the dry experiments. Moreover, the presence of water in the DCE decomposition changed significantly the reaction product distribution. Vinyl chloride formation was found to be significantly lowered over the three zeolites, and selectivity to CO₂ formation was largely enhanced. The X-ray powder diffraction (XRD) analysis of the deactivated samples indicated partial destruction of the zeolite crystal structure during reaction.     

An In Situ ESR Study of Pd/H-ZSM-5 Interaction with Different Adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd⁺ sites; Pd⁺/H₂O; Pd⁺/C₆H₆) upon interaction of isolated Pd²⁺ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O₂, H₂O, H₂, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd²⁺ to Pd⁰ reduction by H₂. Strong bonding of NO _x ligands to Pd²⁺ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd²⁺/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd⁺/benzene complex, the paramagnetic Pd⁺/benzene structure indicating a surprisingly high resistance to further reduction to Pd⁰. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.     

Methane aromatization on Zn-modified zeolite in the presence of a co-reactant higher alkane: How does it occur?

By using ¹³C solid-state NMR and GC–MS, the analysis of the ¹³C-label transfer from methane-¹³C into the products of methane and propane co-aromatization on Zn/H-BEA zeolite at 823–873 K has been performed. A high degree involvement of ¹³C-carbon atoms of methane into aromatic products (benzene, toluene, xylenes) has been demonstrated. The main pathway of methane conversion into aromatics has been determined to consist in the methylation of aromatics, which is produced exclusively from propane, by methane. The methoxy species formed by the dissociative adsorption of methane on ZnO species of the zeolite is responsible for the methylation.     

Fischer-Tropsch synthesis. Evidence for two chain growth mechanisms

The results show that n-pentanol serves to initiate Fischer-Tropsch synthesis reactions. Product accumulation in the CSTR is not adequate to explain the deviation from a constant¹⁴C activity/mole with increasing carbon number for alkane products. A second Fischer-Tropsch synthesis mechanism that produces only alkanes is needed to explain the deviation of the C activity/mole with increasing carbon number for n-alkanes. Furthermore, the two chain growth pathways must be completely independent without the possibility of a carbon number species that is common to both mechanisms. It is suggested that the pathway that incorporates added¹⁴C labeled alcohol has an oxygen containing surface intermediate while the other reaction pathway involves an oxygen-free reaction intermediate.