Font Size: a A A

A Study Of Novel Ru Catalysts For Liquid-Phase Hydrogenation Of Benzene To Cyclohexene

Posted on:2015-10-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:G B ZhouFull Text:PDF
GTID:1221330464961484Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Cyclohexene is an important chemical, as its reactive C=C bond facilitates facile conversion to value-added cyclohexanol, caprolactam, and adipic acid via typical olefin reactions. The production of cyclohexene by benzene partial hydrogenation features exceptional superiority to processes including dehydration of cyclohexanol, dehalogenation of cyclohexane halide, and dehydrogenation of cyclohexane in terms of feedstock accessibility, atomic economy, succinct reaction route, and consequently, operational simplicity. However, the standard Gibbs free energy change for cyclohexane formation from benzene hydrogenation (-98 kJ mol-1) is much more negative than that for cyclohexene formation (-23 kJ mol-1), i.e., the formation of cyclohexene is thermodynamically less favorable than that of cyclohexane. Hence, elaborately designed catalyst that is able to kinetically tailor the rates of the benzene to cyclohexene step and the cyclohexene to cyclohexane step is essential to a high selectivity of cyclohexene from benzene hydrogenation.Catalytic hydrogenation of benzene to cyclohexene has been carried out in gas or liquid phase. The main advantage of the liquid-phase reaction is that it is accessible to a much higher selectivity to cyclohexene at a high conversion level of benzene. Among the various metals screened, Ru is the most selective. Although numerous literature pointed out that the size of Ru could influence the catalytic performance, the relationships between the size of Ru and the activity and selectivity to cyclohexene are unclear. Many examples have presented that enhancing the hydrophilicity of the Ru catalysts is an effective way to a high selectivity to cyclohexene, but no quantitative information is available concerning the relationship between the hydrophilicity of the Ru catalysts and the selectivity to cyclohexene. In addition, the roles of the acidity, pore size and crystallographic form of support in the partial hydrogenation of benzene remain obscure. It is expected that such knowledge will deepen our understanding of the governing factors of selectivity, and consequently, enable the rational design of more selective and stable catalysts. Therefore, we studied the following aspects about the catalysts for liquid-phase benzene partial hydrogenation:(1) Ru nanoparticles (NPs) with a tunable particle size were synthesized by a polyol reduction method and deposited on ZrO2 to investigate the particle size effect; (2) A series of Ru/ZrO2 catalysts with various hydrophilic properties were prepared by alkaline post-treatment of a binary Ru-Zn/ZrO2 catalyst with NaOH aqueous solutions of different concentrations. The relationship between the hydrophilicity of these catalysts and the selectivity to cyclohexene was identified and discussed; (3) A series of B-doped ZrO2 (B-ZrO2) with different doping levels of B with the aim to tailor the acidic property of ZrO2 were synthesized. The relationships between the acid amounts of the supports and the catalytic performances of the Ru/B-ZrO2 catalysts were unveiled and discussed; (4) The ZrO2 samples with the same crystallographic form but different pore sizes were synthesized by precipitation and solvothermal methods. The effect of pore size of the support on the catalytic performances of the Ru-Zn/ZrO2 catalysts was investigated; (5) In order to find more efficient strategy to prepare Ru catalysts, we prepared Ru-based binary catalysts via galvanic replacement, using the active metals as both reducing agents and promoting agents, and optimized the preparation conditions; (6) The effect of crystallographic phase of TiO2 on the catalytic performances of the Ru/TiO2 catalysts was investigated. The above investigations deepened our understanding of the structure-performance relationship, enriched the preparation strategy of the catalysts, and paved the way to develop more efficient catalyst for benzene partial hydrogenation.1. The size effect of the Ru/ZrO2 catalystsBenzene hydrogenation is a structure sensitive reaction, i.e., the particle size of Ru exhibited a remarkable effect on the catalytic performance. The polyol reduction method is an efficient way to prepare Ru NPs with different sizes. By changing the types of polyol (ethylene glycol, glycerol, and 1,2-propanediol) and the concentrations of the additive (sodium acetate) (14.64,9.76,4.88, and 0 mmol L-1), uniform Ru NPs with tunable particle size from 2.4 to 5.4 nm were synthesized by the polyol reduction method and deposited on ZrO2. It was found that the type of polyol and the concentration of additive imposed remarkable effect on the particle size of Ru. A distinct particle size effect occurred in partial hydrogenation of benzene. With the increase of the size of the Ru NPs, the turnover frequency (TOF) of benzene increased, and the initial selectivity to cyclohexene (So) showed a volcanic-type variation tendency, revealing that the optimal Ru size for obtaining the highest So is 4.4 nm. The Ru/ZrO2 catalyst reduced by 1,2-propanediol exhibited the highest So (82%) and the yield of cyclohexene (39%). We suggested that with the increment of the Ru size, the terrace sites are increased, while the corner and edge sites are decreased, thus influencing the adsorption probabilities of benzene and cyclohexene, the variations of the activity and selectivity.2. Structural and catalytic properties of alkaline post-treated Ru/ZrO2 catalystsThe hydrophilicity of the Ru catalysts is beneficial for the selectivity to cyclohexene. However, no quantitative information is available concerning the relationship between the hydrophilicity of the Ru catalysts and the selectivity to cyclohexene. It is expected that such knowledge is important for the rational design of more selective catalysts. In this part, a series of Ru/ZrO2 catalysts were prepared by post-treatment of a binary Ru-Zn/ZrO2 catalyst using 5-30 wt% NaOH aqueous solutions. Alloying between Ru and Zn was evidenced for the catalyst post-treated only by water (Ru/ZrO2-0). Alkaline post-treatment removed metallic Zn, forming smaller Ru NPs. Concomitantly, the hydrophilicity of the catalysts increased and maximized on the 10 wt% NaOH-treated catalyst (Ru/ZrO2-10). In partial hydrogenation of benzene, the Ru/ZrO2-0 catalyst displayed the highest TOF (8.1 s-1), whereas the TOFs over the Ru/ZrO2-5, Ru/ZrO2-10, and Ru/ZrO2-30 catalysts were lower and similar to each other (-3.3 s-1). Among the catalysts investigated, the Ru/ZrO2-0 catalyst exhibited the lowest So (73%) and yield of cyclohexene (38%), whereas the Ru/ZrO2-10 catalyst exhibited the highest So (86%) and yield of cyclohexene (51%). Therefore, such a post-treatment is adverse to the activity but advantageous to the selectivity to cyclohexene. The decrease in the activities of the catalysts after alkaline post-treatment was probably associated with the removal of Zn and/or the decrease in the particle size of Ru. The more hydrophilic the Ru/ZrO2 catalyst surface is, the higher is the amount of water adsorbed on the catalyst, and consequently, the higher is the selectivity to cyclohexene. By plotting So against the amount of water adsorbed on the catalysts, an excellent linear relationship was established, which manifests the important role of the hydrophilicity of the Ru/ZrO2 catalysts for obtaining high selectivity of cyclohexene. We further found that the surface hydroxyl groups can directly block one type of the chemisorption sites of cyclohexene, which may be another important factor for pronounced selectivity enhancement by alkaline post-treatment.3. Doping effects of B in the supports of the Ru/B-ZrO2 catalystsDespite of the wide acceptance of the stepwise hydrogenation mechanism of benzene (benzene to cyclohexene followed by cyclohexene to cyclohexane), the roles of the catalyst surface properties in the partial hydrogenation of benzene, such as the surface acid sites of the supports, remain obscure. In this part, the B-doped ZrO2 (B-ZrO2) supports with different B/Zr ratios aiming at tailor the acidic property of ZrO2 were synthesized. The infrared spectra of adsorbed pyridine (Py-IR) indicated that only Lewis acid sites existed on the ZrO2 and B-ZrO2 samples. However, both Py-IR and the temperature-programmed desorption of NH3 (NH3-TPD) indicated the same sequence of the acid amount of ZrO2< B-ZrO2(1/20)< B-ZrO2(1/15)< B-ZrO2(1/10) (the values in the parentheses represent the nominal B/Zr molar ratio) with the improvement of the doping level of B. The Ru/B-ZrO2 catalysts were then prepared, and their electronic and structural properties were systematically characterized by spectroscopic techniques. It is identified that the Ru NPs supported on these B-ZrO2 samples exhibited similar size, chemical state, and microstructure. In the partial hydrogenation of benzene, with the increase of the doping level of B, the TOF of benzene increased, while So and the yield of cyclohexene increased firstly, and then decreased. The S0 and yield on the Ru/B-ZrO2(1/15) catalyst are both the highest (88% and 48%, respectively). By plotting the TOFs against the acid amounts of the supports (nNH3), an excellent linear relationship emerged for TOFs, while a volcano-type relationship was established for the So, indicating that there is an optimal acid amount to maximize the selectivity to cyclohexene on the Ru/B-ZrO2 catalysts. Kinetic analysis indicated that the acid sites on the supports improved the rate constants of the benzene to cyclohexene step (k1) and the cyclohexene to cyclohexane step (k2) to different degrees, thus altering the activity and selectivity of cyclohexene of the Ru/B-ZrO2 catalysts.4. Effect of pore size of the support on the catalytic properties of the Ru-Zn/ZrO2 catalystsIn this part, ZrO2 supports with the same tetragonal crystallographic form (t-ZrO2) but different pore sizes were synthesized by the precipitation and the solvothermal methods.t-ZrO2 supports with pore sizes of 11.7 and 10.2 nm were obtained by the NH3-H2O precipitation method and calcination at 873 and 1073 K, respectively, while that with pore size of 3.2 nm was prepared by the solvothermal method using methanol as the solvent. Using the above ZrO2 samples, Ru-Zn/ZrO2 catalysts were prepared by the deposition-precipitation method and reduction in ZnSO4·7H2O aqueous solution. It turned out that the TOFs of the Ru-Zn/ZrO2 catalysts are similar, while So increased with the increment in the pore size of ZrO2. Among the catalysts investigated, ZrO2 with pore size of 11.7 nm supported Ru catalyst (Ru-Zn/ZrO2(11.7)) exhibited the highest So (88%) and cyclohexene yield (54%). TEM indicated that the pore size of ZrO2 had no impact on the Ru size, leading to similar TOFs. We suggested that ZrO2 with larger pore size is beneficial for the diffusion and mass transfer of cyclohexene, promoting the desorption of cyclohexene from the catalyst surface to avoid consecutive hydrogenation, and consequently a higher So.The optimal ZnSO4·7H2O concentrations during reduction and reaction are both 0.52 mol L-1. The optimal reduction time is 3 h. The optimal stirring rates during reduction and reaction are both 1200 rpm. The γ40 (numbers of grams of benzene converted per gram of catalyst per hour at benzene conversion of 40%) and S40 (cyclohexene selectivity at benzene conversion of 40%) of the Ru-Zn/ZrO2(11.7) catalyst were 123 h-1 and 82%, respectively, which meet the standard of industrial production.5. Synthesis of Ru-based binary catalysts via galvanic replacement and their catalytic propertiesA promoter is frequently added into the Ru catalyst to improve the selectivity to cyclohexene. In this part, we prepared Ru-based binary catalysts via galvanic replacement, using the active metals as reducing agents as well as promoting agents. The effects of the amount of hydrochloric acid during preparation, replacement metal, and support on the catalytic performances were investigated in detail. It turns out that when using Zn as the replacement metal, with the increment in the amount of hydrochloric acid during preparation from 1.00 g to 1.40 g, the TOFs of the Ru-Zn/ZrO2(x) (x represents the mass of hydrochloric acid during preparation) catalysts increased, while So increased firstly and then decreased. The Ru-Zn/ZrO2(1.11) catalyst exhibited the highest So (72%) and yield of cyclohexene (39%). X-ray photoelectron spectroscopic (XPS) spectra confirmed the existence of Zn(OH)2 on the Ru-Zn/ZrO2(x) catalysts. The characterization results indicated that the amount of Zn(OH)2 decreased with the increment in the amount of hydrochloric acid during preparation. We suggested that the adsorption of benzene was facilitated with the decrease in the amount of Zn(OH)2, and consequently, an increased TOF. The temperature-programmed desorption of cyclohexene revealed that Zn(OH)2 can suppress the adsorption of cyclohexene on the Ru-Zn/ZrO2(x) catalysts, while Zn(OH)2 may also cover some active sites on Ru for cyclohexene formation. These effects led to the variation of So with the increment in the amount of hydrochloric acid during preparation, namely, first increased and then decreased. Al exhibited the best promoting effect for So and the yield of cyclohexene among the replacement metals investigated including Zn, Mg, Al, Fe, Co, Ni, Cu, and Sn. So and the yield of cyclohexene on the Ru-Al/ZrO2(1.67) catalyst are 75% and 41%, respectively.6. Effect of crystallographic phase of the support on the catalytic properties of the Ru/TiO2 catalystsSince the Ru-Zn/P25 catalyst in the fifth part exhibited excellent catalytic performance, in this part, we prepared commercial available P25 TiO2-, anatase TiO2-, and rutile TiO2-supported Ru catalysts by the wetness impregnation-chemical reduction method to investigate the effect of crystallographic phase of TiO2 on the catalytic performances of the Ru/TiO2 catalysts. It was found that the Ru NPs on the Ru/P25, Ru/anatase, and Ru/rutile catalysts exhibited similar size and composition. The Ru/P25 catalyst exhibited the highest cyclohexene yield (61%). The TOF (2.0 s-1) and So (90%) on the Ru/P25 catalyst are also higher than those on the Ru/anatase and Ru/rutile catalysts. High angle annular dark field imaging in the scanning transmission electron microscopy (HAADF-STEM) and high-resolution transmission electron microscopy (HRTEM) images evidenced the location of the Ru NPs at the anatase/rutile interface on the Ru/P25 catalyst. XPS and extended X-ray absorption fine structure (EXAFS) spectra confirmed the existence of electron-deficient Ru species (Ruδ+) at the anatase/rutile interface of the Ru/P25 catalyst. We suggested that such Ruδ+ species can facilitate the adsorption of benzene and enhance the hydrophilicity of the catalyst, thus promoting the desorption of cyclohexene and inhibiting the readsorption of cyclohexene. Therefore, the TOF and So of the Ru/P25 catalyst are both higher than those of the Ru/anatase and Ru/rutile catalysts. The γ40 (192 h-1) and S40 (85%) of the Ru/P25 catalyst reached the standard of industrial production, indicating the potential of P25 in this reaction.
Keywords/Search Tags:Ruthenium, benzene, hydrogenation, cyclohexene, zirconia, titania, polyol reduction method, size effect, alkaline post-treatment, galvanic replacement
PDF Full Text Request
Related items