Evaluation of energy distribution on a heterogeneous surface from heat of adsorption and its application to the ammonia/Na-Y zeolite system

The— function (energy level function) which represents surface heterogeneity was derived from theq— function (experimental heat function) in the case of reversible adsorption. The Langmuir equation was extended and applied to the procedure of calculations. An iterative calculation led to the most probable— function by setting theq— function and adsorption temperature. As an example for actual cases, the— function of Na-Y zeolite was calculated from theq— function obtained by the measurement of heats of adsorption of ammonia at 373 K. The— function thus derived seemed to be consistent with the positional distribution of sodium ions.     

Estimation of heat of adsorption from energy distribution on heterogeneous surfaces by a theoretical analysis of adsorption process

An attempt was made to calculate theq— function (experimental heat function) from the— function (energy level function) of a heterogeneous surface. In order to obtain the heat of adsorption, adsorption was assumed to take place on each energy level of the— function. In reversible adsorption, Langmuir's monolayer model was extended to determine the occupation of energy levels by molecules. On this basis, the heat of adsorption could be calculated. Irreversible adsorption was also examined on the basis of chemical kinetics. Molecules were supposed to be held on sites to which they first collided. These calculations were useful for estimating the extent of deviation of theq— function from the— function.     

Composite materials based on zeolite 4A for adsorption heat pumps

Some additives and binders were chosen for the preparation of 4A-zeolite-based composites with high equivalent thermal conductivity for heat pumps application. Additives (SiC, Si₃N₄, graphite) and binders (PTFE, Al(OH)₃) were tested for their effectiveness in terms of equivalent thermal conductivity and maximum water adsorption capacity of the composites. The influence of the equivalent thermal conductivity of the composite adsorbents on the specific power of the heat pump was also calculated. Results show a significant improvement in the equivalent thermal conductivity of the composite samples which are prepared using aluminum hydroxide as binder, over that of zeolite pellet beds. Such composite materials could be used to build adsorption heat pumps with higher specific power and, consequently, with lower investment cost.     

Description of NaX zeolite deformation during adsorption of xenon

The equation describing the experimental data of NaX zeolite deformation during adsorption of xenon at pressures of 0.001–7 MPa and temperatures of 252–333 K was obtained on the basis of the generalized Lennard-Jones pair interaction potential. The temperature dependences of the constants in the equation were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1887–1890, October, 1999.     

Equilibrium adsorption of an oxygen-nitrogen mixture on zeolite NaX

Dependences of the Helmholtz energy, entropy, and internal energy in single micropores on the total number of guest molecules and the oxygen to nitrogen ratio were found by analysis of experimental data on adsorption of the binary oxygen-nitrogen mixture on zeolite NaX using the statistical model. A substantially non-ideal character of the adsorption phase, which exhibits negative deviations from Raout's law, can result from heterogeneity of the adsorption field inside single cavities. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1077–1082, June, 1999.     

Adsorption properties of low-silica zeolite ZK-5

Water vapor, methanol, and argon adsorption isotherms, as well as the heat of adsorption of ammonia, methanol, and carbon dioxide have been determined on zeolite types A, ZK-4, and ZK-5. The results are explained by the distribution and the bond lengths of cations in the zeolite crystal lattice. It was shown that the exchange of 10–15% of the Na⁺ ions with Li⁺ in zeolite ZK-5 leads to a deformation of the 8-membered ring impeding the diffusion of adsorbed molecules. Cations contained in the 8-membered ZK-5 zeolite ring are bound more strongly to the framework, than in zeolite NaA, resulting in their smaller adsorption capacity.I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199164 St. Petersburg, Russia. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2494–2500, November, 1992.     

Steady state energy distribution function of adatoms in reactive adsorption

The interplay between the energy distribution function of adatoms and the rate of diatom formation in catalytic reaction is studied by means of mean field rate equations. The recombination of adatoms is described as a multi-channel process where adatom desorption arises from several energy levels. It is shown that the distribution function can be computed, analytically, as a function of the ratio between recombination probability and rate constant for energy disposal to the solid. This parameter is the key quantity of the kinetics since it governs both reaction rate and the shape of the energy distribution function. It is found that, in order to obtain steady state conditions, the control parameter is constrained within a well defined interval of values which result lower than unity. It is shown that a kinetic transition takes place for the highest value of the parameter, which entails hyperthermal reaction rate.     

Thermodynamics of CO<Subscript>2</Subscript> adsorption on zeolite NaX in wide intervals of pressures and temperatures

The dependences of the differential molar isosteric heat of adsorption and entropy of adsorption of CO₂ on zeolite NaX were determined in wide temperature (196–423 K) and pressure (0.1 Pa to 5.4 MPa) intervals. In the initial region of adsorption (a < 1 mmol g^–1), the differential molar heat of adsorption increases from 40 to 43 kJ mol^–1 and then decreases to 33 kJ mol^–1. The heat of adsorption remains virtually unchanged at 3 mmol g^–1< a < 6.5 mmol g^–1 and decreases sharply at high fillings of zeolite micropores (a > 7 mmol g^–1). The heat of adsorption was found to be temperature-dependent. The region with the constant heats shrinks with the temperature increase, and the heats begin to decrease at lower fillings of micro pores. The dependences of the change in the differential entropy of the adsorption system on the amount adsorbed were calculated at different temperatures. The specific features of the behavior of the thermodynamic functions of this adsorption system in the initial and medium region of fillings kre associated with interactions of adsorbate molecules with Na⁺ cations and walls of large cavities. For high fillings, an increase in repulsion forces between adsorbed molecules results in a sharp expansion of the adsorbent and a decrease in the heat of adsorption.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1570–1573, August, 2004.     

Y型分子筛结构破坏的动力学分析(英文)

Y型分子筛是催化裂化(FCC)的速率控制组分.FCC过程中,催化剂在反应器和再生器中往往面临高温水蒸气存在的苛刻环境.因此,分子筛的热稳定性和水热稳定性是催化剂最为关注的性能之一.由于FCC原料中通常含有V、Ni、Na、Fe等不同数量的金属污染物,会对催化剂造成污染及钝化.进料中存在的卟啉类有机复合物持续不断的沉积在催化剂表面,由于含钒的有机金属卟啉化合物在反应中转化形成V_2O_5,V_2O_5在水热条件下形成H_3VO_4组分,在高温水热气氛下加速分子筛骨架结构水解,破坏了Y型分子筛的晶体结构,从而降低了催化剂活性,影响产品选择性.稀土Y型分子筛在FCC中扮演重要的角色,稀土交换分子筛可以提高催化酸性、裂化活性和热与水热稳定性.此外,Na在高温水蒸气条件下也会对分子筛结构造成破坏.一方面,钠能够中和Y型分子筛B酸中心,降低催化裂化活性;另一方面,水热条件下钠的存在会加速破坏Y型分子筛的结构.有关Y型分子筛结构破坏的机理解释较多,然而该过程的动力学研究鲜有报道.反应动力学不能提供一个直接的反应机理,但是任何反应机理的提出必须符合反应动力学的数据.本文采用离子交换法分别制备了一系列不同Na含量USY,不同稀土含量USY,以及含钠和稀土的USY分子筛,通过固相动力学模型考察了上述Y型分子筛水热结构破坏活化能的变化及钒对其活化能的影响.结果表明,Y型分子筛的结构破坏存在三种路径,分别是脱铝、脱硅和La-O键的断裂.钒加速了分子筛骨架水解速率;钒钠具有协同作用,同时存在时对分子筛破坏作用更加显著;NaOH的形成是速率控制步骤;稀土稳定了分子筛的结构,降低了分子筛的水热脱铝速率;钒与定位于分子筛小笼里稀土作用,破坏分子筛的[RE-OH-RE]~(5+)的RE-O键夺取分子筛的骨架氧,导致骨架结构崩塌.由于稀土本身稳定了分子筛的结构,同时钒稀土作用时形成稳定的REVO_4固定了钒的流动性,因此钒对REY结构的影响是几种因素相互叠加和抵消的结果.     

用密度泛函理论研究NO在H-ZSM-5分子筛不同酸性位的吸附

利用密度泛函理论(DFT),基于7T簇模型,在B3LYP/6-31G(d,p)水平上研究了NO分子在H-ZSM-5分子筛孔道中α,β,γ酸性位的吸附.在计算过程中,首先对H-ZSM-5的α,β,γ酸性位进行优化计算,然后对NO分子η1-N和η1-O两种吸附模式的红外光谱和吸附能进行计算.计算结果表明,NO分子以η1-N模式吸附于H-ZSM-5分子筛酸性位上,不同酸性位对NO分子的吸附能力排序为:α酸性位>β酸性位>γ酸性位.此外,H-ZSM-5分子筛直型孔道更有利于NO分子的吸附和扩散,因而可更有效地促进NO分子催化分解反应的进行.     

Enhancement of the heat and mass transfer in compact zeolite layers

Heat and mass transfer during the adsorption of water on zeolite has been studied both theoretically and experimentally. A dynamic simulation model of a zeolite layer has been developed to estimate the predominant transport resistances and calculations were carried out to assist the simultaneous experimental investigations. On one hand, a metallic matrix was added to the compact zeolite layer to improve the heat transfer. On the other hand, pore-forming materials such as melamine or tartaric acid were used. These organic components are removed during drying of the zeolite so that the mass transfer inside the zeolite is significantly enhanced compared to a granulated zeolite bed. The experimental investigations show that the theoretically deduced possibilities of improving the adsorption process can be realized in the manufactured zeolites.The investigations described here are of interest for the development of adsorption heat pumps. Due to the thermodynamic characteristics the adsorption system zeolite-water is a promising working pair for this application. The investigations show that the main shortcoming of these machines, the thermal output, can be increased significantly.     

Cu-loaded dealuminated Y zeolites active in selective catalytic reduction of nitric oxide with ammonia

The selective catalytic reduction of NO with ammonia in the presence of oxygen has been carried out on Cu-loaded dealuminated Y zeolite catalysts. Copper was introduced by the usual ion-exchange procedure with an aqueous solution of cupric acetate. On deeply dealuminated USY zeolites, Cu²⁺ was supported in the amount larger than 2Cu/Al = 2, resulting in the formation of CuO fine particles in addition to the isolated and dimer Cu²⁺ species. The specific catalytic activity per surface copper on the CuO particles was very high compared with these Cu²⁺ species. NO adsorption measurement revealed the higher dispersion of CuO on the deeply dealuminated USY than on SiO₂, which made Cu/USY a better catalyst for the reduction of NO. The reaction intermediates were investigated through the IR spectra of adsorbed species.     

Effect of pressure and temperature on the adsorption equilibrium in the H2-Ne-zeolite NaA system

The effect of the temperature, pressure, and gas-phase composition on the parameters of the adsorption equilibrium in a zeolite NaA-neon-hydrogen system was studied. From the results obtained optimum conditions for the adsorption separation of a binary neonhydrogen gas mixture were found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 611–612, March, 1993.     

NiMo/Al_2O_3 catalyst containing nano-sized zeolite Y for deep hydrodesulfurization and hydrodenitrogenation of diesel

Two mixed-matrix NiMo/Al2O3 catalysts containing nano-and micro-sized zeolite Y have been prepared to explore the size effect of zeolite Y particle on the hydrodesulfurization(HDS)and hydrodenitrogenation(HDN)activities of fluid catalytic cracking(FCC)diesel.They were characterized by SEM,BET,XRD,H2-TPR,NH3-TPD and HRTEM.The results show that the catalyst containing nano-sized zeolite Y possesses larger average pore diameter,higher pore volume,weaker and lesser acid sites,more easily reducible metal phases,shorter MoS2 slabs and more slab layers than the catalyst containing micro-sized zeolite Y.The catalysts were also evaluated with a high-pressure fixed-bed reactor using real FCC diesel as feed.The results display that the catalyst containing nano-sized zeolite Y bears higher HDS and HDN activities and exhibits higher relative rate constant for the removal of total sulfur or nitrogen than the one containing micro-sized zeolite.     

Role of solvation energy in starch adsorption on oxide surfaces

The effect of potassium and sodium cations on the adsorption of starch onto hematite and quartz was investigated. The role of these ions was analyzed in terms of their water structure-making or -breaking capabilities. In the presence of Na⁺, a structure maker, the polymer adsorption density did not change compared to the adsorption levels observed in distilled water. However, in the solutions of K⁺, a structure-breaking cation, the adsorption density of starch significantly increased. Assuming hydrogen bonding and chemical interaction to be the driving adsorption mechanism, it was proposed that the starch–oxide interactions can be envisioned as the competition between chemical interaction/hydrogen bonding and solvation energy:

K⁺ reduces solvation energy by disturbing interfacial water structure and thus increases the free energy of adsorption, allowing the polymer to more closely approach the oxide surfaces. In contrast, Na⁺ which is indifferent to solvation energy does not interfere with the free energy of adsorption.     

Thiol-functionalized hierarchical zeolite nanocomposite for adsorption of Hg2+ from aqueous solutions

A thiol-functionalized hierarchical zeolite nanocomposite was synthesized and investigated with a view to remove mercury from aqueous solutions. The hierarchical zeolite was prepared by the use of a beta zeolite and of cetyltrimethylammoniumbromide (CTAB). The ligand, 3-mercaptopropyltrimethoxysilane containing thiol (–SH) groups, was then immobilized on the surface of the hierarchical zeolite through grafting with surface silanol groups. FTIR, XRD, SEM, TG-DTG, and N₂ adsorption–desorption techniques were used to characterize the nanocomposite before and after functionalization. Adsorption experiments showed that this adsorbent was an excellent one to bind mercury with high selectivity; an adsorption capacity of 8.2 mequiv·g⁻¹ of adsorbent was obtained. Furthermore, the adsorbent retained most of its capacity after regeneration with nitric acid and thiourea solutions. The adsorption data was fitted to the Freundlich isotherm.     

Acid properties of the HZSM-12 zeolite with different Si/Al ratio by thermo-programmed desorption

In this work was studied the acid properties of a series of HZSM-12 zeolites with different Si/Al molar ratio. The samples of ZSM-12 were synthesized by the hydrothermal method starting from a gel with the following molar composition: 20MTEA:10Na₂O:xAl₂O₃:100SiO₂:200H₂O, where: x=2, 1, 0.67 and 0.50, respectively. After the synthesis, the samples of ZSM-12 were ion-exchanged NH₄Cl solution to obtain zeolite in the acid form (HZSM-12). The acid properties were evaluated by n-butylamine thermodesorption in a TG equipment at three different heating rates. The model-free kinetic model was applied in the TG integral curves to estimate the apparent activation energy (E _a) of the n-butylamine desorption process. The results obtained showed that the HZSM-12 zeolite presents two kinds main of acid sites: one with E _a in the range of 115-125 kJ mol^-1 classified as weak and other kind with E _a varying of 230-250 kJ mol^-1 classified as strong. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption of benzene from benzene/n-C6 and n-C7 mixture by nano Beta zeolite with different Si/Al ratios

The adsorption of benzene from benzene/n-alkane mixtures was studied by two types of nano Beta zeolite with Si/Al ratios of 11.5 and 24.5. Benzene was adsorbed into benzene/n-hexane and n-heptane mixtures which had 0.5% up to 10% mole fraction of benzene using batch technique in the ambient temperature. The nano Beta zeolite has active sites on its surface, which have interaction with π electron in benzene, and this can increase the heat of adsorption. The Si/Al ratio defines the number of active sites in the zeolite surface and the heat of adsorption. However, an increase in the active sites of Beta zeolite declines the entropy of adsorption. Therefore, free energy of mixing specifies the potential of adsorption in Beta zeolite.As the results indicated in all mixtures, benzene is adsorbed more than n-hexane and n-heptane into the Beta zeolite surface, which suggests that this type of zeolite has a high separation factor (∼50) for benzene in Beta zeolite (Si/Al = 24.5). Also, Beta zeolite with Si/Al = 24.5 had a greater separation factor than Beta zeolite with Si/Al = 11.5 in similar mixtures.     

Ionic liquid modification of zeolite and its removal of chromate from water

This study focused on using imidazolium of different chain lengths to modify the negatively charged zeolite. The modification involves a cation exchange process of the organic cations of ionic liquids (ILs) for the alkali and alkaline earth elements on zeolite surfaces. X-ray diffraction analyses revealed that the uptake of ILs was on the external surfaces. Fourier transform infrared analyses showed that different mechanisms were attributed to IL uptake on zeolite at below and above the external cation exchange capacity. After modification, the zeolite reversed its surface charge to positive, thus enhancing adsorptive removal of anionic contaminants such as chromate from water. At the same time, the modified zeolite increased their total organic carbon content, and thus could promote better adsorptive removal of hydrophobic organic contaminants from water, too. These features enable IL-modified zeolite to be used as inexpensive sorbents for the removal of multi-types of contaminants from water simultaneously.     

Removal of heat of adsorption from adsorbent by forced convection

An analytical mathematical model is used to investigate the effectiveness of forced convection for removal of the heat of adsorption from an adsorbent mass undergoing a differential adsorption process in a flow system. An example of such a process is measurement of gas adsorption kinetics using a differential adsorption bed. Isothermal operation may not be achieved even when a high gas flow rate is used, particularly if the sorption kinetics is relatively fast. Very small changes in the adsorbent temperature can cause significant departure from the isothermal uptake behavior when the heat of adsorption is moderately large. A criterion for validity of isothermal data analysis is given.