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Industrial Aerobic Oxidation of Hydrocarbons
No full textIn this chapter, two industrial processes of gas-phase, hydrocarbon catalytic oxidation are examined: (a) the oxychlorination of ethylene to 1,2-dichloroethane; and (b) the oxidation of n-butane to maleic anhydride. In the former case, the use of ethane as an alternative reactant has been studied, still without success, whereas in the latter case the alkane has replaced the corresponding C4 alkenes because of the better selectivity achieved. Recent developments are discussed for the two reactions, especially in terms of catalysts’ chemical-–physical and reactivity properties
Surface properties of VOx-SiO2 and VOx-Al2O3 catalysts: a spectroscopic study by FT-IR, Micro-Raman, XPS and EPR techniques
No full textRecently, the flame pyrolysis method (FP) has been proposed for the preparation of VOx-SiO2 and VOx-Al2O3 catalysts (nominal content of V2O5 5-50 % by weight) effective in the oxidative dehydrogenation (ODH) of propane to propylene [1,2]. As a whole, catalytic tests revealed that VOx-Al2O3 samples were more active whereas VOx-SiO2 exhibited higher selectivity to propylene. Specifically, the most selective catalyst resulted to be that with a 10 % nominal content of V2O5 (V10Si). As expected, the catalytic performances of such catalysts may be correlated to their physico-chemical properties, as studied by FT-IR, Micro-Raman, XPS and EPR techniques. IR spectra of VOx-SiO2 samples (Figure 1) showed a band at 930 cm-1, due to the vibration of SiO44- groups strongly polarized by interaction with vicinal vanadium atoms: such [SiOδ-...Vδ+] species were not observed with a sample prepared by impregnation (V10Si-i), indicating that V-incorporation into the silica framework takes place during FP. No evidence of V introduction in the framework of alumina was instead detected in VOx-Al2O3 systems. Micro-Raman analysis showed the presence of isolated VOx species (signals 1027 and 512 cm-1) only with sample V10Si, whereas with all other VOx-SiO2 and VOx-Al2O3 systems, and with samples prepared by impregnation, only bands of crystalline V2O5 were detected. Adsorption of CO at nominal -196 °C on V10Si sample showed the presence of different OH species, with the following relative acidity scale: isolated SiOH < H-bonded SiOH < V-OH. Likewise, the Brønsted acidity was studied by means of NH3 adsorption on samples outgassed at 500 °C. The blue-shift (ΔνNH4+) of the band due to the bending vibration of NH4+ species with respect to that of free NH4+ (1410 cm-1) can be used as a semi-quantitative measure of the acidic strength of Brønsted sites: the smaller the shift the higher the Brønsted acidity. A good correlation has been observed between the acidic strength of Brønsted sites and the selectivity to propylene for both VOx-SiO2 and VOx-Al2O3 systems: for both sets of catalysts, the selectivity increases with the decreasing of the Brønsted acidic strength, which favors side-reactions catalyzed by acids leading to a higher amount of COx [3]. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of VOx species. Indeed, XPS can provide information on the V oxidation states by considering the V 2p3/2 binding energies (BE). Interestingly, the V10Si sample exhibited a broad peak at higher V 2p3/2 BE as compared to other VOx-SiO2 systems, probably due to the presence of highly dispersed V5+-OH groups partially incorporated into the silica matrix and interacting poorly with one another [4,5]. A similar spectra was recorded on V5Si, although the latter exhibited an additional signal at ca. 516 eV, suggesting also the presence V3+ species at the surface. The latter result is in fair agreement with the worse catalytic performances of V5Si towards the ODH of propane, kinetically modeled through a MvK mechanism based on V5+ reduction and subsequent reoxidation. On the contrary, at higher V-contents and with V10Si-i sample peaks at low V 2p3/2 BE appeared, typically assigned to extra-framework V2O5. EPR spectra revealed weaker V=O bond in VOx- Al2O3 systems: being V=O bond strength an index of oxygen availability, this could explain their higher activity towards this reaction. In conclusion, for FP-prepared catalysts: (i) a better dispersion of the active phase can be obtained with silica rather than with alumina; (ii) selectivity to propylene depends on Brønsted acidity; (iii) stronger Brønsted acidity leads to higher COx production. Moreover, the best catalyst (V10Si) was characterized by highly dispersed (isolated) V species, less acidic Brønsted sites, higher V oxidation states and stronger V=O bonds.
References:
[1] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan, L. Forni, J. Catal. 256 (2008) 45-61.
[2] I. Rossetti, L. Fabbrini, N. Ballarini, C. Oliva , F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, Catal. Tod., 141 (2009) 271-281.
[3] K. Chen, A. Khodakov, J. Yang, A. T. Bell, E. Iglesia, J. Catal. 186 (1999) 325-333.
[4] C. Hess, R. Schlögl, Chem. Phys. Lett. 432 (2006) 139-145.
[5] C. Hess, J. Catal. 248 (2007) 120-123
Oxidation and Ammoxidation of Toluene Over Vanadium Titanium-oxide Catalysts - A Fourier-transform Infrared and Flow Reactor Study
No full textThe oxidative dehydrogenation of propane over V-containing mesoporous silicas: the effect of vanadium dispersion, surface acidity and support properties on the catalytic activity
No full textAmong the many materials considered for propane ODH, V-containing mesoporous silicas characterized by high surface area, large pores and well-dispersed VOx species prove to be most effective [1,2]. V-based catalysts are usually obtained by impregnation: it has been observed, though, that V-containing mesoporous systems (i.e. V-SBA-15 and V-MCF) prepared by direct synthesis exhibit better V-dispersion and superior catalytic performances in both selective and total oxidation reactions, reactions both based on V reduction and subsequent reoxidation (MvK mechanism) [3,4]. As it is known, isolated V-species are beneficial in terms of propene selectivity, whereas higher activity is usually associated to polyvanadates (up to the formation of VOx monolayer) [1-5].In this work, a V-SBA-15 and a V-MCF sample [3,4] (V cont. 2.5 wt.%) prepared by direct synthesis were tested in propane ODH. Their physico-chemical and catalytic properties were compared with those of materials with the same V-content but prepared by different ways: i) two impregnated samples (V-SBA-15-i and V-MCF-i) and ii) a non-porous sample obtained by flame pyrolysis (V-SiO2), a high temperature synthesis technique allowing high V-dispersion to be achieved [2]. As a whole, both V-SBA-15 and V-MCF exhibited better textural properties (i.e. higher SSA) and higher V dispersion (isolated V-species), as compared to impregnated ones, in which polymeric VOx and micro-crystalline V2O5 were detected by TEM, H
2-TPR micro-Raman and DR UV-Vis [4,5]. On the other hand, both impregnated samples showed higher Brønsted acidity (related to V-OH groups), as shown by IR-spectroscopy of adsorbed CO and NH3
Better catalytic performances in terms of selectivity to propene were achieved with both samples directly synthesized, as consequence of higher V dispersion and lower surface acidity. Brønsted acidity seems indeed to be negatively correlated with the selectivity to propene, since stronger Brønsted acidic sites favor the formation of CO.. In contrast, samples directly synthesized show supported V species that can be converted into Lewis acidic sites by dehydroxylation much more easily than impregnated samples (Figure 1).
x and other by-products. A similar correlation was previously observed with a set of non-porous VOx-SiO2 samples prepared by flame pyrolysis (results to be published). Moreover, higher selectivity to propene was obtained over V-MCF as compared to V-SBA-15 (a similar trend was observed with impregnated ones). This behavior was related to the different porous network of the two catalysts: the structure of MCF, featuring a 3-D network with ultra-large cells (20-40 nm), favors molecular diffusion along each direction, unlike cylindrical channels of SBA-15 (3-6 nm) occurring in one dimension mainly (axial diffusion). Molecules can be much more retained inside channels of SBA-15 (deeper oxidation) as compared to MCF. On the other hand, the non-porous sample, with V-species well dispersed and incorporated into the silica framework, exhibited worse catalytic results than mesoporous samples, confirming the important role of mesoporosity in oxidation processes.
References
[1] F. Cavani, N. Ballarini, E. Cericola, Catal. Today 127 (2007) 113.
[2] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, J. Catal. 256 (2008) 45.
[3] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, L. Gaberova, M. Armandi, E. Garrone, Catal. Today, 176 (2011) 458.
[4] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, M. Armandi, C. Thomas, E. Garrone, Catal. Today doi:10.1016/j.cattod.2011.06.028
[5] H. Dai, A.T. Bell, E. Iglesia, J. Catal 221 (2004) 49
A newly designed process for the production of acetonitrile from renewable sources
No full text1. Introduction
The chemical importance of acetonitrile comes from its very particular polarity, affinity with both organic liquids and water and relatively high boiling point. Its main use is as a solvent for pharmaceutical and laboratory applications (nearly 70%) [1], but is also used in the extractive separation of butadiene from C4 alkanes and in other similar processes.
Acetonitrile is mainly a byproduct of the acrylonitrile synthesis (6 Mton in 2010) and its yield depends on how the main process is operated. This intrinsic dependence is the underlying reason for the recognized mismatch between its demand and availability worldwide.
More recently, routes to acetonitrile as the main reaction product have been sought [1] and an efficient atom-economy could be achieved by using C2 substrates, such as ethanol, ethane and ethylene. In brief, all these reactions are characterized by the alkylation of ammonia. Ethanol as a reactant is a promising alternative being a renewable resource, readily available from established fermentation processes and usable for this process without particular purification requirements.
A new fully integrated ethanol-to-acetonitrile production plant has been designed here from the grass roots. The system is designed to produce acetonitrile on a pilot scale (10 kg/h) from ethanol, ammonia and air (ammoxidation). Besides the reaction section, the full separation train for pure acetonitrile recovery (> 99%) has been optimized and integrated with the recovery of all the byproducts (CO2, HCN) and unreacted NH3. The recovery and valorisation of the marketable byproducts (cyanide salts and NH4HCO3) is also discussed. Finally, the process consumes more CO2 than what constitutes the reactor byproduct, allowing the further sequestration of this greenhouse gas.
2. Methods
The overall process design has been carried out using the software Aspen PLUS® V 8.8, with the APV88 and NISTV88 components databanks for components properties. The thermodynamic system used is the ENRTL (Electrolyte Non Random Two Liquids) to compute the non-ideality in the liquid phase. It was chosen since salts are present overall the process and it allows to model their thermodynamic properties in a more reliable way than NRTL. The Redlich-Kwong equation of state was used to model non-ideality for the gaseous phase. Some species were also treated as Henry components (properties from the same databases) to account for their solubility.
3. Results and discussion
After selecting the most appropriate thermodynamic package to correctly compute phase equilibria and the relative duties, a new integrated process has been designed as conceptually sketched in the Figure. Basically, ethanol, ammonia and air are mixed in the reactor, which operates according to the specifications experimentally derived in a previous work [2]. The product mixture is composed of unreacted and newly formed N2, NH3, HCN, CO2, CH3CN and H2O. All the byproducts are separated by washing and precipitation as marketable NH4HCO3 (even using additional CO2 as external input) and NaCN. The separation of the former salt is accomplished straigforwardly by using ethanol, which recovered in the drier, thus achieving preheating and partial vaporisation of the reactant and its mixing with air. Further heat recovery is allowed between the reactor fedd/products lines (not drawn). Finally, the separation of pure acetonitrile is accomplished by pressure swing distillation, for intensified resolution of the azeotrope [3]. 92% recovery of the produced acetonitrile (99.5% purity) is the final yield of the process, which includes marketable byproducts, heat recovery options and further CO2 sequestration.
4. Conclusions
A process for acetonitrile production through ethanol ammoxidation has been designed on a pilot-plant scale. This represents a fully new process, with complete materials recovery, that allows the independent production of acetonitrile exploiting renewable sources. We consider the relatively low ammonia consumption, the full byproducts recovery and marketability and the close connection between the process sections as the most promising features of this newly designed plant.
In order to evaluate the potential benefits of the bio-based synthesis of acetonitrile a Life Cycle Assessment (LCA) approach was applied by comparing the environmental scores of the renewable route with those achieved by the traditional fossil-based pathway. A cradle to gate perspective, from raw material extraction up to the acetonitrile production, has confirmed the lower impacts in terms of resources depletion and environmental burdens for the innovative and renewable synthetic process.
References
[1] I.F. Mcconvey, D. Woods, M. Lewis, Q. Gan, P. Nancarrow, Org. Process Res. Dev. 16 (2012) 612-624.
[2] F. Folco, J.V. Ochoa, F. Cavani, L. Ott, M. Janssen, Catal. Sci. Technol. 7 (2017) 200–212.
[3] A. Tripodi, M. Compagnoni, G. Ramis, I. Rossetti, Chem. Eng. Res. Des., in press.
Keywords
Acetonitrile; Life cycle assessment; Pressure swing distillation; Plant design
Surface-structure and Reactivity of Vanadium-oxide Supported On Titanium-dioxide - V2O5/TiO2 (rutile) Catalysts Prepared By Hydrolysis
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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