Styrene Selectivity Research Articles

Transformation of Styrene Oxide on Bi-Functional Ni, Co and Cu Catalysts Supported on Zirconia-Alumina: Effect of Depositing Zirconia on Alumina

2-phenyl ethanol (2-PEA) and 2-phenylacetaldehyde (PAA) are important perfumery chemicals which are prepared by hydrogenation and isomerization of styrene oxide (SO) respectively. Activities of Ni, Co and Cu supported on a set of zirconia-alumina supports with varying ZrO2:Al2O3 molar ratio are compared for these reactions. Zirconia was deposited on γ-alumina by deposition-precipitation. All catalysts were characterized by XRD, NH3-TPD, H2-TPR, N2 physisorption, XPS and Raman spectroscopy. Increasing the zirconia content significantly influenced catalyst characteristics. These are correlated with reactivity: i) surface concentration of Ni, Co and Cu (XPS) decreased, which affected conversion of styrene oxide and selectivity to 2-PEA ii) 1 1 1 plane of Cu (XRD) was suppressed which enhanced selectivity of styrene by-product relative to 2-PEA iii) acidity (NH3-TPD) decreased, which adversely affected yield of PAA. Product space time yields (STY) of 2-PEA and PAA correlated well with surface concentration of active metals (XPS) and acidity (NH3-TPD) respectively. Various by-products were identified. Styrene appeared to form by deoxygenation of SO. TPR and Raman spectroscopy indicated strong interaction of Co and Ni with alumina while Cu showed strong interaction with zirconia. This reflected in their XRD crystallite size. Cu has different electronic properties than Co and Ni, which reflected in differences in their behavior. This study directly compares the performance of these transition metals in continuous flow studies and correlates catalyst characteristics with reactivity.

Dual-site cooperation mediated by synergy in Mg-Co bimetallic oxide for high-performance styrene epoxidation reaction

Styrene epoxidation under mild reaction conditions is an important reaction to produce styrene oxide. Herein a synergistic intermetallic catalysis between Mg and Co in MgCo2O4 is revealed to be a facile strategy to facilitate the epoxidation performance, with the roles of CoO and MgO sites in the OMgOCoO structure responsible for activity and selectivity, respectively. Both experimental studies and theoretical calculations verify that the introduction of Mg into the Co3O4 framework contributes to the construction of alkaline environment on the catalyst surface, realizing a remarkable enhancement of the selectivity towards styrene oxide by suppressing its further transformation in a base-free condition. As a result, this strategy breaks the trade-off between activity and selectivity, contributing to both enhanced styrene conversion and selectivity to styrene oxide. The dual-site cooperation strategy in bimetallic oxides composed by earth-abundant elements is expected to provide important guidance and impetus for moving academically appealing chemistry such as styrene epoxidation to an industrial reality.

Unravelling the role of boron dopant in borocarbonitirde catalytic dehydrogenation reaction

Borocarbonitride (BCN) materials are newly developed metal-free catalytic materials exhibiting high selectivity in oxidative dehydrogenation (ODH) of alkanes. However, the in-depth understandings on the role of boron (B) dopants and the intrinsic activities of –C=O and –B–OH still remain unknown. Herein, we report a series of BCN materials with regulable B content and surface oxygen functional groups via self-assembly and pyrolysis of guanine and boric acid. We found that the B/C ratio is the key parameter to determine the activity of ODH and product distribution. Among them, the high ethylbenzene conversion (∼57%) and styrene selectivity (∼83%) are achieved in ODH for B1CN. The styrene selectivity can be improved by increasing of B/C ratio and this value reaches near 100% for B5CN. Structural characterizations and kinetic measurements indicate that –C=O and –B–OH dual sites on BCN are real active sites of ODH reaction. The intrinsic activity of –C=O (5.556 × 10−4 s−1) is found to be 23.7 times higher than –B–OH (0.234 × 10−4 s−1) site. More importantly, we reveal that the deep oxidation to undesirable CO2 occurs on –C=O rather than –B–OH site, and B dopant in BCN materials can reduce the nucleophilicity of –C=O site to eliminate the CO2 emission. Overall, the present work provides a new insight on the structure–function relationship of the BCN catalytic systems.

Lattice Oxygen Regulation of Perovskites for Chemical Looping Oxidative Dehydrogenation of Ethylbenzene to Styrene

The direct dehydrogenation process produces over 90% of global styrene. However, it is hampered by high energy consumption due to equilibrium limitation, a high steam/ethylbenzene feeding ratio, and complicated separation procedures. In this study, we propose the synthesis of a novel perovskite-structured metal oxide as the redox catalyst for chemical looping oxidative dehydrogenation (CL-ODH) of ethylbenzene into styrene. Lower ethylbenzene activation temperature and higher oxygen storage capacity contributing to high ethylbenzene conversion are achieved by Mn doping of the B site in SrFeO3−δ, and styrene yield is further enhanced by Ba doping of the A site. The optimized Sr0.8Ba0.2Fe0.2Mn0.8O3−δ catalyst exhibits favorable dehydrogenation activity but is deactivated in the first five redox cycles due to the severe carbonation and perovskite structure decomposition. The decarbonization treatment on the catalyst at 950 °C in an O2 atmosphere restores the perovskite structure and dehydrogenation activity, resulting in 85% ethylbenzene conversion and 89% styrene selectivity at 550 °C. Ethylbenzene on the catalyst undergoes full combustion via adsorbed oxygen, ODH of ethylbenzene into styrene and H2O using lattice oxygen, and direct dehydrogenation without available lattice oxygen. The proposed redox catalyst demonstrates efficient conversion of ethylbenzene into styrene, showing great potential for the energy conservation and process intensification of styrene production.

A 3D porous Fe-MoS2/CMF catalyst for high-efficient catalytic reforming of lignin vapor by microwave irradiation

Improving the yield of target monophenols plays an important role in increasing the economic competitiveness of lignin valorization. In this study, a 3D porous Fe-MoS2/CMF catalyst with nanoflower microstructure was successfully fabricated by a simple one-step hydrothermal method. The catalytic depolymerization of the lignin β-O-4 model showed that Fe-doping increased the efficiency of the catalytic conversion reaction and promoted the cleavage of the β-O-4 ether bond, in agreement with theoretical calculations. Reforming of lignin depolymerization vapor revealed that the Fe-MoS2/CMF catalysts significantly improved the selectivity of phenol (30.01 wt%) and styrene (16.34 wt%) with the decrease of oligomers. Moreover, finite element simulation showed that the Fe-MoS2/CMF catalysts can also enhancing the local thermal effect by creating hot spots on the surface, thereby facilitate the lignin vapor reforming. Consequently, this work provides a promising method for the efficient production of phenol and styrene from lignin using bimetallic nanoarray microwave responsive catalyst.

Efficient Oxidative Dehydrogenation of Ethylbenzene over K/CeO2 with Exceptional Styrene Yield

Oxidative dehydrogenation (ODH) is an alternative for styrene (ST) production compared to the direct dehydrogenation process. However, ODH with O2 or CO2 suffers from either over-oxidation or endothermic property/low ethylbenzene conversion. Herein, we proposed an ODH process with a CO2-O2 mixture atmosphere for the efficient conversion of ethylbenzene (EB) into styrene. A thermoneutral ODH is possible by the rationalizing of CO2/O2 molar ratios from 0.65 to 0.66 in the temperature range of 300 to 650 °C. K modification is favorable for ethylbenzene dehydrogenation, and 10%K/CeO2 achieved the highest ethylbenzene dehydrogenation activity due to the enhanced oxygen mobility and CO2 adsorbability. The catalyst achieved 90.8% ethylbenzene conversion and 97.5% styrene selectivity under optimized conditions of CO2-4O2 oxidation atmosphere, a temperature of 500 °C, and a space velocity of 5.0 h−1. It exhibited excellent catalytic and structural stability during a 50 h long-term test. CO2 induces oxygen vacancies in ceria and promotes oxygen exchange between gaseous oxygen and ceria. The ethylbenzene dehydrogenation in CO2-O2 follows a Mars-van Krevelen (MvK) reaction mechanism via Ce3+/Ce4+ redox pairs. The proposed ODH strategy by using oxygen vacancies enriched catalysts offers an important insight into the efficient dehydrogenation of ethylbenzene at mild conditions.

Synthesis of ultrafine Pd nanoparticles encapsulated in imidazolium-based porous polymers for semi-hydrogenation of alkynes

Interaction between metal and support can dramatically affect the activity and selectivity of supported metallic nanoparticles. Therefore, the development of novel support is a critical path to achieve advanced catalysis. Here, we report that imidazolium-based porous polymers were prepared through a free-radical cross-linking reaction. These imidazolium-based porous polymers with high specific surface area and abundant imidazonium units were utilized as supports to synthesize ultra-small and narrow dispersed Pd nanoparticles . The optimized catalyst displayed 99% selectivity for styrene at 98% conversion in alkyne semi-hydrogenation under ambient conditions with a general application. Comprehensive characterizations demonstrated that the interaction between cationic imidazolium-based porous polymers and Pd enhanced the alkene selectivity by weakening the adsorption on Pd, as well as stabilized Pd nanoparticles by the anchoring and spatial confining effects. The pivotal roles of the Pd-imidazonium interaction and the mesoporous structure provide valuable guidance for the manufacture of high-performance metallic nanoparticles catalyst for selective hydrogenation.

Insight into single- and bi-objective optimization of industrial problems

ABSTRACT Evolutionary single- and bi-objective optimization of industrial problems, namely, naphtha cracking and styrene reactor are considered. Bi-objective optimization is solved using the HMODE-DLS algorithm, whereas an improved black hole optimizer (BHO) is employed for the single-objective optimization (SOO) problem. For the naphtha cracking process, ethylene selectivity (SE) and severity index (SI) are selected as objectives. Similarly, styrene selectivity (SST) and styrene flow rates (FST) are considered objectives for styrene reactor. Pareto ranking is carried out by using the net flow method (NFM) and the best solution is compared with the single- and multi-objective optimization results. In a single objective optimization study, the optimum solution corresponds to the lowest value of the SI (1.571) at the cost of the worst value of SE (0.269). Similarly, the highest values of FST (16.642 kmol/h) and SST (96.158%) are resulted in the SOO study of styrene reactor.

Ce-promoted Fe-K-Mg catalyst and its application in dehydrogenation of ethylbenzene

The continuous pursuit of green, low-carbon, and efficient processes in catalysis necessitates a deep understanding of catalytic reaction mechanisms and the structure-performance relationship of catalysts. Styrene, a vital monomer in the chemical industry, is primarily produced through the catalytic dehydrogenation of ethylbenzene over Fe-based catalysts. This study prepared a series of Fe-K-Mg catalysts promoted with different CeO2 contents. Various techniques such as mercury intrusion, particle strength tests, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, Mössbauer spectroscopy, and temperature-programmed techniques were employed to characterize the catalysts' physical properties, surface, and bulk structures. The results indicate that the bulk density and crush strength of the catalyst are closely related to the pore structure and grain size of potassium β-ferrite. The surface of the Ce-promoted catalyst contains potassium β-ferrite KFe11O17, cerium dioxide, and potassium carbonate, while the bulk structure consists of potassium β-ferrite and cerium dioxide. The addition of Ce enhances the catalytic activity of ethylbenzene dehydrogenation, but the selectivity of styrene shows the opposite trend. The Fe-K-Mg catalyst containing 12% CeO2 exhibited the highest single-pass yield of styrene, reaching 70.9%. This study also discusses the possible effect of CeO2 on the performance of the Fe-K-Mg catalyst for ethylbenzene dehydrogenation.

Enhanced Alkene Selectivity for Transfer Semihydrogenation of Alkynes over Electron-Deficient Pt Nanoparticles Encapsulated in Hollow Silica Nanospheres.

In this work, we report that Pt nanoparticles confined in hollow porous silica nanospheres (Pt@HPSNs) function as highly selective catalysts for the transfer hydrogenation of phenylacetylene to styrene with ammonia borane. Relative to the deep hydrogenation of phenylacetylene to ethylbenzene over the supported Pt/SiO2, Pt@HPSNs exhibit above 88% of styrene selectivity at nearly 100% of phenylacetylene conversions, and the high selectivity of Pt@HPSNs can be maintained even at high ammonia borane/phenylacetylene ratios and longer reaction time. The Pt 4f X-ray photoelectron spectrum of Pt@HPSNs shows a remarkable ∼1.5 eV shift to high binding energy, proving the nature of electron deficiency of such encapsulated Pt nanoparticles. Combined with extremely minor transfer hydrogenation of styrene to ethylbenzene when styrene as substrates, the enhanced styrene selectivity of Pt@HPSNs is ascribed to the electron deficiency of encapsulated Pt nanoparticles, which leads to the fast desorption of styrene and thus avoids deep hydrogenation.

Continuous Flow Liquid‐Phase Semihydrogenation of Phenylacetylene over Pd Nanoparticles Supported on UiO‐66(Hf) Metal‐Organic Framework

AbstractCatalysis under continuous flow operation is industrially more relevant compared to reactions carried out in batch reactors. But catalysis with metal‐organic frameworks (MOFs) is largely restricted to batch reactions, especially in case of semi hydrogenation reactions. We anchored Pd nanoparticles (NPs) on robust UiO‐66(Hf) MOF to develop effective catalyst for semi hydrogenation of phenylacetylene into styrene. We investigated catalytic performance of Pd/UiO‐66(Hf) in a continuous‐flow liquid‐phase semi‐hydrogenation of phenylacetylene (PA). In this study, Pd/UiO‐66(Hf) catalyst exhibited good catalytic performance with a rare combination of high PA conversion (99.0 %) and high styrene selectivity (90.0 %). Additionally, Pd/UiO‐66(Hf) demonstrated lower deactivation rate in contrast to many reported materials to date. The higher stability of metal NPs over UiO‐66(Hf) aroused from the strong metal‐support interaction (between Brønsted acidic μ3‐OH of MOF and Pd). The Pd/UiO‐66(Hf) retained its catalytic activity even after four recycles.

Folic Acid-Derived Low-dimensional carbons for efficient oxidative dehydrogenation of ethylbenzene

The oxidative dehydrogenation (ODH) of alkane is one of the most attractive routes in alkane production because of its favourable thermodynamic characteristic. Nitrogen-doped nanocarbons have demonstrated great potential in this reaction due to its cost-effective, high catalytic activity and stability. However, the influence of nitrogen on the catalytic properties of carbon materials is poorly understood due to the complexities of surface oxygen and nitrogen functional groups. Here we derive the performance descriptor that account for the nitrogen-dependent carbocatalysis in ODH reaction. To achieve this, we designed a set of nitrogen-doped nanocarbon materials with tunable nitrogen species by hydrothermal carbonization (HTC) treatment of the biomass folic acid (FA), which are applied in ODH of ethylbenzene. Among them, FA-180-1000 catalyst can achieve high ethylbenzene conversion (up to ∼62 %) and styrene selectivity (∼87 %), outperforming other HTC carbon-based catalysts. Structural characterizations and kinetic analyses revealed that nitrogen doping strongly interferes the charge polarization of CO site via electron transfer between CO, and nitrogen (mainly pyridine nitrogen and graphitic nitrogen) thus enhancing the reactivity of CO. Furthermore, the induction period during reaction process can be shortened by applying of sulfuric acid-assisted HTC method for constructing nitrogen-doped carbon catalyst with low crystallinity. The present work provides new insights into the contribution of nitrogen doping to the ODH reaction of carbon nanocatalysts, as well as guidance for the rational design of carbon catalysts for the conversion of hydrocarbons to high-value chemicals.

The promoting effect of boron oxide on the FeOx/Si3N4 catalyst for oxidative dehydrogenation of ethylbenzene

Stronger interaction created by B2O3 modification mitigates the oxygen reactivity of FeOx, promoting styrene selectivity in the oxidative dehydrogenation of ethylbenzene.

ZIF-8 derived N-doped porous carbon confined ultrafine PdNi bimetallic nanoparticles for semi-hydrogenation of alkynes

Catalytic semi-hydrogenation of alkynes to alkenes plays an important role in the polymer, fragrance, medicine, and other chemical industrial fields. However, increasing the selectivity of alkenes during the semi-hydrogenation process is a huge challenge. Herein, N-doped porous carbon support (NC) was fabricated from zinc-based zeolite imidazole frameworks (ZIF-8). Ultrafine PdNi bimetallic nanoparticles (NPs) were uniformly dispersed, stably anchored on the NC support, and used as an efficient alkynes semi-hydrogenation catalyst. The synthesized Pd2Ni2/NC catalyst had a selectivity of 95.1% toward styrene and high catalytic activity (TOF=977.7 h−1) for the semi-hydrogenation of phenylacetylene under mild reaction conditions. The catalyst was mainly through metal-support interaction and electron transfer between Pd and Ni metal, modulating the electronic structure of the active sites, thus improving the selectivity of styrene. Selective hydrogenation of various substituted alkynes on Pd2Ni2/NC catalyst has also proved excellent applicability, and the obtained catalyst can be reused with favorable stability. This study adjusted the catalytic performance by adding a secondary metal, which provided a useful strategy for designing highly efficient catalysts for the semi-hydrogenation of alkynes.

Aerosol Spray Drying Guided Synthesis of Ultrasmall Alloyed Bimetallic Nanoparticles Supported on Silica for Catalytic Semihydrogenation.

Supported bimetallic nanoparticles (NPs) with ultrasmall sizes and homogeneous alloying are attractive for catalysis. However, facile synthesis of this type of material remains very challenging. Here, the aerosol drying impregnation method for rapid, scalable, and general synthesis of silica-supported bimetallic NPs is proposed. The method relies on aerosol spray drying to promote the mixing and dispersing of binary metal precursors on SiO2 . It is capable of controlling the composition and size of bimetallic NPs and avoids the use of expensive metal complex salts and complicated experiment procedures. Twelve permutations combining a noble metal (Pd, Ru, and Pt) and a base one (Fe, Co, Ni, and Cu) with ultrasmall sizes (1.4-2.2nm in average size), uniform dispersion, and good alloying are synthesized. Interesting activity and selectivity trends in catalytic semihydrogenation of phenylacetylene over the supported Pd-based NPs can be observed. The silica-supported PdNi NPs deliver both high activity and styrene selectivity. Spectroscopic and density functional theory calculation results reveal the improved chemoselectivity originated from the suitably down-shifted d-band center of the PdNi NPs inducing an increased energy barrier for overhydrogenation and a weakened styrene adsorption.

Synergistic trimetallic Ni–Cu–Sn catalysts for efficient selective hydrogenation of phenylacetylene

Selective hydrogenation of phenylacetylene is the most effective way to remove a small amount of phenylacetylene from styrene, but it is still a challenge to develop catalysts with high activity and selectivity. Herein, a series of non-precious bimetallic Ni3M/SiO2 (M = Zn, Sn, Cd, Fe, Co, Cu, or Mo) and trimetallic Ni3Cu–Sn/SiO2 catalysts were prepared and applied to selective hydrogenation of phenylacetylene. A trade-off between activity and selectivity was observed on Ni3M/SiO2, where Ni3Cu/SiO2 showed the best hydrogenation activity because of the hydrogen spillover effect of the Ni–Cu alloy, while Ni3Sn/SiO2 exhibited the highest selectivity to styrene due to the active-site isolation effect of the Ni–Sn alloy. Impressively, trimetallic Ni3Cu–Sn/SiO2 offered superior catalytic performance (94 % styrene selectivity at >99 % phenylacetylene conversion) as compared to bimetallic catalysts, owing to the synergistic effect among Ni, Cu, and Sn. In the case of Cu and Sn co-doping, the hydrogen spillover on the Ni–Cu alloy was deeply suppressed by Sn, which enabled an effective modification of the geometric and electronic structures of Ni to favor styrene desorption. The hydrogenation activity of Ni3CuSn0.3/SiO2 can be readily enhanced by increasing the metal loading and further improved by elevating the reaction temperature and pressure, but meanwhile the high selectivity is little affected. This work demonstrates a multi-metal synergistic strategy to break the activity–selectivity trade-off of Ni-based bimetallic catalysts in the selective hydrogenation of alkynes.

An efficient nanodiamond-based monolithic foam catalyst prepared by a facile thermal impregnation strategy for direct dehydrogenation of ethylbenzene to styrene

Nanodiamond (ND) has long been recognized as an effective carbocatalyst for synthesizing styrene via direct dehydrogenation (DDH). However, the induced drastic pressure drop of its powder form limits its industrial application in heterogeneous catalytic process. In this work, we report a facile hexamethylenetetramine nitrate (HN)-assisted thermal impregnation (HNTI) strategy for fabricating a novel nanodiamond-based monolithic foam (ND/CNT-SiC-ms-HN) catalyst through a two-step approach: One is to soak the carbon nanotube-modified SiC foam (CNT-SiC) with the slurry composed of HN, KCl, LiCl, and dispersed ND, and the other is to heat the slurry-soaked CNT-SiC (ND-HN-KCl-LiCl/CNT-SiC) in N2 atmosphere at 750 °C. The as-synthesized ND/CNT–SiC-ms-HN monolithic foam features the enriched surface kenotic CO by promoted ND dispersion and O-doping, abundant stuctural defects, and improved nucleophilicity by N-doping, originating from the promoted ND dispersion by thermal impregnation (TI) in KCl-LiCl molten salt (MS) and the presence of HN in the annealing process. As a result, the ND/CNT–SiC-ms-HN monolithic foam catalyst by HNTI strategy exhibits 1.5 folds higher steady-state styrene rate (5.49 mmol g−1 h−1) associated with 98.4% of styrene selectivity compared to the ND-based monolithic foam catalyst (ND/CNT-SiC). Moreover, the ND/CNT–SiC-ms-HN monolithic foam shows excellent long-term stability for the direct dehydrogenation of ethylbenzene to styrene. This work also comes up with a novel way of preparing other highly-dispersed nanocarbons-based monolithic foam catalysts with promising catalytic performance for diverse transformations.

Styrene epoxidation catalyzed by polyoxometalate/quaternary ammonium phase transfer catalysts: The effect of cation size and catalyst deactivation mechanism

Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry, and the deep insight into catalyst deactivation will help to develop new epoxidation process. In this work, series of quaternary ammoniums bearing different cationic sizes, i.e. MTOA+ (methyltrioctylammonium, [(C8H17)3CH3N]+), HTMA+(hexadecyltrimethylammonium, [(C16H33)(CH3)3N]+) and DMDOA+ (dimethyldioctadecylammonium, [(C18H37)2(CH3)2N]+) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs), which were used in the styrene epoxidations. Among them, (MTOA)3PW4O24 exhibits the best catalytic performance judged from the highest styrene conversion rate (52%) and styrene oxide selectivity (93%), during which the styrene epoxidation conditions were optimized. Meanwhile, the deactivation mechanism of this kind of PTCs was proposed firstly, i.e. in the case of low H2O2 content, the oxidant can only be used in the styrene epoxidation, in which the catalyst can transform into stable Keggin-type POM. But when the content of H2O2 is higher, the excess H2O2 can re-activate the Keggin-type POM into active (PW4O24)3− anions, which can trigger the ring-opening polymerization of styrene oxide. Consequently, the catalyst is deactivated by adhered poly(styrene oxide) irreversibly, which was determined by NMR spectra. In this situation, the active moiety {PO4[WO(O2)2]4}3− in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides. This work will be beneficial for the design of new PTCs in alkene epoxidation in fine chemical industry.

Superior performance of K/Co2AlO4 catalysts for the oxidative dehydrogenation of ethylbenzene to styrene with N2O as an oxidant

This study explored the feasibility of coupling N2O decomposition with ethylbenzene (EB) oxidative dehydrogenation, as an alternative approach for greenhouse gas elimination and styrene (ST) production, on the Co-Al mixed oxides and K-modified catalysts. It was found that N2O could decompose completely over the K/Co2AlO4 catalyst, accompanied with 62.0% of EB conversion and 85.1% of styrene selectivity, which were much better than the existing catalyst systems for EB oxidative dehydrogenation. Characterization results showed that despite the decreased specific surface area of the catalysts with increasing the Co/Al molar ratio, the improved reducibility, the reduced acid properties as well as the higher ratio of Co3+/Co2+ were responsible for the enhanced performance. The K modification not only changed the electronic properties of active metal, resulting from the charge transfer from K cation to the Co species, but also weakened the binding energy of Co3+-O−, leading to the complete decomposition of N2O. Furthermore, the optimized strong acid properties inhibited the dealkylation or ring-opening reactions and significantly reduced the coke deposition on the catalyst surface, thus improving the ST selectivity.

Selectivity-induced conversion model explaining the coke-catalysed O2-mediated styrene synthesis over wide-pore aluminas

Explaining the conversion and selectivity of a heterogeneous catalyst is often done separately. The general rule is that with high temperatures conversion increases but the catalyst becomes less selective to the desired product, as other pathways to side products are activated. However, this study shows that conversion and selectivity can be correlated. Ethylbenzene (EB) can be converted into styrene under oxidative dehydrogenation conditions. This process is catalysed by the coke formed on an acidic solid material (alumina in this study) at the beginning of the reaction. The process also produces unwanted CO and CO2 (denoted as COx). In a previous study, the alumina calcination temperature influenced the coke selectivity; it increased the styrene selectivity. The EB conversion was also enhanced and it was explained in qualitative terms. In this work, we developed a quantitative model explaining the changes in EB conversion related to the selectivity. The model was obtained by setting an O2 mass balance and showed to explain the experimental reaction data well. The enhanced EB conversion, observed in the alumina series, was explained by the fact that the coke becomes less selective to the unwanted COx, and more to styrene. The COx-forming reactions consume more O2 than the styrene reaction, and have a highly negative impact on the EB conversion. The study provides evidence for the difficulty in achieving a high EB conversion under ODH conditions when COx is formed. For instance, to achieve an EB conversion above 80%, the selectivity to COx should be below 4% at O2/EB of 0.6 (mole) for a typical alumina catalyst. The model proves the link between conversion and selectivity and, secondly, the demanding conditions to achieve a high EB conversion for this reaction. To the best of our knowledge, this is the first example showing a quantitative model relating selectivity and conversion for a heterogeneously catalysed reaction.