Suliman N. Basahel

Influence of the Reaction Temperature on the Nature of the Active and Deactivating Species During Methanol-to-Olefins Conversion over H-SAPO-34

The selectivity toward lower olefins during the methanol-to-olefins conversion over H-SAPO-34 at reaction temperatures between 573 and 773 K has been studied with a combination of operando UV–vis diffuse reflectance spectroscopy and online gas chromatography. It was found that the selectivity toward propylene increases in the temperature range of 573–623 K, while it decreases in the temperature range of 623–773 K. The high degree of incorporation of olefins, mainly propylene, into the hydrocarbon pool affects the product selectivity at lower reaction temperatures. The nature and dynamics of the active and deactivating hydrocarbon species with increasing reaction temperature were revealed by a non-negative matrix factorization of the time-resolved operando UV–vis diffuse reflectance spectra. The active hydrocarbon pool species consist of mainly highly methylated benzene carbocations at temperatures between 573 and 598 K, of both highly methylated benzene carbocations and methylated naphthalene carbocations at 623 K, and of only methylated naphthalene carbocations at temperatures between 673 and 773 K. The operando spectroscopy results suggest that the nature of the active species also influences the olefin selectivity. In fact, monoenylic and highly methylated benzene carbocations are more selective to the formation of propylene, whereas the formation of the group of low methylated benzene carbocations and methylated naphthalene carbocations at higher reaction temperatures (i.e., 673 and 773 K) favors the formation of ethylene. At reaction temperatures between 573 and 623 K, catalyst deactivation is caused by the gradual filling of the micropores with methylated naphthalene carbocations, while between 623 and 773 K the formation of neutral poly aromatics and phenanthrene/anthracene carbocations are mainly responsible for catalyst deactivation, their respective contribution increasing with increasing reaction temperature. Methanol pulse experiments at different temperatures demonstrate the dynamics between methylated benzene and methylated naphthalene carbocations. It was found that methylated naphthalene carbocations species are deactivating and block the micropores at low reaction temperatures, while acting as the active species at higher reaction temperatures, although they give rise to the formation of extended hydrocarbon deposits.

Single‐Particle Spectroscopy on Large SAPO‐34 Crystals at Work: Methanol‐to‐Olefin versus Ethanol‐to‐Olefin Processes

The formation of hydrocarbon pool (HCP) species during methanol-to-olefin (MTO) and ethanol-to-olefin (ETO) processes have been studied on individual micron-sized SAPO-34 crystals with a combination of in situ UV/Vis, confocal fluorescence, and synchrotron-based IR microspectroscopic techniques. With in situ UV/Vis microspectroscopy, the intensity changes of the λ=400 nm absorption band, ascribed to polyalkylated benzene (PAB) carbocations, have been monitored and fitted with a first-order kinetics at low reaction temperatures. The calculated activation energy (Ea ) for MTO, approximately 98 kJ mol(-1) , shows a strong correlation with the theoretical values for the methylation of aromatics. This provides evidence that methylation reactions are the rate-determining steps for the formation of PAB. In contrast for ETO, the Ea value is approximately 60 kJ mol(-1) , which is comparable to the Ea values for the condensation of light olefins into aromatics. Confocal fluorescence microscopy demonstrates that during MTO the formation of the initial HCP species are concentrated in the outer rim of the SAPO-34 crystal when the reaction temperature is at 600 K or lower, whereas larger HCP species are gradually formed inwards the crystal at higher temperatures. In the case of ETO, the observed egg-white distribution of HCP at 509 K suggests that the ETO process is kinetically controlled, whereas the square-shaped HCP distribution at 650 K is indicative of a diffusion-controlled process. Finally, synchrotron-based IR microspectroscopy revealed a higher degree of alkylation for aromatics for MTO as compared to ETO, whereas high reaction temperatures favor dealkylation processes for both the MTO and ETO processes.

Initial Carbon–Carbon Bond Formation during the Early Stages of the Methanol-to-Olefin Process Proven by Zeolite-Trapped Acetate and Methyl Acetate

Abstract Methanol‐to‐olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C−C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid‐state NMR spectroscopy with UV/Vis diffuse reflectance spectroscopy and mass spectrometry on an active H‐SAPO‐34 catalyst, we provide spectroscopic evidence for the formation of surface acetate and methyl acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C−C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.

Combined Operando UV/Vis/IR Spectroscopy Reveals the Role of Methoxy and Aromatic Species during the Methanol-to-Olefins Reaction over H-SAPO-34

The methanol‐to‐olefins (MTO) process over H‐SAPO‐34 is investigated by using an operando approach combining UV/Vis and IR spectroscopies with on‐line mass spectrometry. Methanol, methoxy, and protonated dimethyl ether are the major species during the induction period, whereas polyalkylated benzenes and polyaromatic species are encountered in the active stage of the MTO process. The accessibility of SAPO‐34 is linked with the amount of methoxy species, whereas the formation of polyaromatic species that block the pores is the main cause of deactivation. Furthermore, the reaction pathways responsible for the formation of olefins and polyaromatics co‐exist and compete during the whole MTO process, and both routes are directly related to the amount of surface polyalkylated benzene carbocations and methoxy species. Hence, a first‐order kinetic model is proposed and comparable activation energies for both processes are obtained.

Single-Particle Spectroscopy of Alcohol-to-Olefins over SAPO-34 at Different Reaction Stages : Crystal Accessibility and Hydrocarbons Reactivity

In situ synchrotron‐based IR and UV/Vis micro‐spectroscopy combined with isotopically labeled reactants have been used to identify the different hydrocarbon species formed as well as to assess the activity and accessibility of individual 50 μm‐sized SAPO‐34 crystals. For the methanol‐to‐olefins process, two reaction stages can be distinguished. The first involves the formation of methoxy species, protonated dimethyl ether, and polyalkylated benzene (PAB) carbocations, which do not affect the accessibility of the SAPO‐34 crystal. In addition, methoxy species are very dynamic during this stage. The second stage is related to the formation of polyaromatic (PA) species concentrated in the outer rim of the crystal, which are bulky and interact with the acid sites and thus alter the overall accessibility of the crystal. In contrast, the ethanol‐to‐olefins process only consists of one major stage, as the formation of PAB and PA species cannot be separated. Furthermore, the formation of these species is more internal, and coke formation is mainly concentrated in a layer located in the inner part of the SAPO‐34 crystal.

Oxidation of tyrosine by permanganate in presence of cetyltrimethylammonium bromide

In this paper we report the effect of cetyltrimethylammonium bromide, CTAB on the oxidative degradation of tyrosine by permanganate. The reaction rate bears a first-order dependence on the [MnO(4)(-)] under pseudo-first-order conditions (large excess of [tyrosine] for at least 10 times over [MnO(4)(-)]) in presence of 10.0x10(-4)moldm(-3) CTAB. The effect of total [CTAB] on the reaction rate was determined. When [CTAB] was less than its critical micelle concentration (CMC) the rate constants (k(psi)) values decreased from 18.5x10(-4) to 7.2x10(-4)s(-1). As the [CTAB] was greater than the CMC, the k(psi) values increase from 7.2x10(-4) to 15.8x10(-4)s(-1)at room temperature. The premicellar environment of CTAB strongly inhibits the reaction rate where as increase in rate constant ascribed to the incorporation of tyrosine and MnO(4)(-) in to the Stern layer of CTAB micelles. The reaction has acid-dependent and acid-independent paths and, in the former case, the zero-order kinetics with respect to [H(2)SO(4)] shifted to fractional-order at higher [H(2)SO(4)]. Experiments have been done to confirm the nature of Mn(IV) formed during the reduction of permanganate by tyrosine. The mechanism with the observed kinetics has been proposed and discussed. The presence of -OH group is responsible for the higher reactivity of tyrosine which easily transfers the proton to MnO(4)(-).

Thermal decomposition of ammonium trioxalatoferrate(III) trihydrate in air

Abstract The thermal decomposition of ammonium trioxalatoferrate(III) trihydrate in air has been studied using DTA-TG, electrical conductivity, SEM, XRD, FTIR and Mossbauer effect measurements. The first stage of decomposition of (NH 4 ) 3 [Fe(C 2 O 4 ) 3 ]·3H 2 O, starting at about 100°C, corresponds to evolution of the water of hydration and is followed by the second stage in which the sample ignites at around 260°C and burns rapidly to form finely divided iron(III) oxide. DC-electrical conductivity measurements showed two breaks corresponding to the two decomposition stages. Kinetic analysis of the two stages of the decomposition reactions was performed under isothermal conditions and the results were compared with those obtained under non-isothermal conditions using different integral methods of analysis. The fractional reaction-time data showed a sigmoid relationship and obeyed the Avrami-Erofeev equation characteristic of a solid-state nucleation-growth mechanism and consistent with the textural changes that accompany the decomposition, as revealed by SEM experiments. Mossbauer spectra of samples calcined at different temperatures are discussed and show that in the early stages of the decomposition at about 300°C, part of the Fe(III) oxide is formed in a superparamagnetic doublet state. As the temprature is increased, the crystallites grow and supermagnetism disappears.

Thermal decomposition of iron(III) oxalate-magnesium oxalate mixtures

Abstract The differential thermal analysis-thermogravimetry (DTA-TG) behaviour of chemically coprecipitated iron(III) oxalate-magnesium oxalate (1:1 mole ratio) was investigated. X-ray diffractometry (XRD) of samples calcined at different temperatures showed that magnesium ferrite is formed in samples heated at higher temperatures. Integral composite analyses of dynamic TG data of the decomposition reactions in the coprecipitated mixture were carried out using various solid state reaction model equations, and the results showed that the decomposition reactions are best described by the two- and three-phase boundary, R2 and R3 models. Kinetic analyses of dynamic data were also carried out in accordance with the integral methods of Ozawa and Coats-Redfern and the results are discussed in comparison with the integral composite analysis of data.

Kinetic analysis of thermal decomposition reactions: Part 8. Radiation effects on the thermal decomposition of ammonium oxalate monohydrate

Abstract The effects of gamma radiations on the kinetic parameters of the dynamic thermal decomposition-gasification reactions of ammonium oxalate monohydrate have been investigated using differential thermal analysis and thermogravimetry techniques. Kinetic analysis of the dynamic TG results are discussed in view of a composite integral method in comparison with the integral methods due to Coats and Redfern and to Ozawa. The activation parameters for the non-irradiated and irradiated samples are calculated and the results of the different methods of data analysis are compared and discussed.

Ammonium ion exchange equilibrium on potassium zinc hexacyanoferrate/II/ K2Zn3[Fe/CN/6]2

Potassium zinc hexacyanoferate/II/ K2Zn3[Fe/CN/6]2 was prepared and used for the investigation of the uptake of ammonium ion NH4+. The obtained equilibrium data indicate higher selectivity of ammonium ion relative to potassium ion and much lower relative to cesium ion.