Membrane assisted propane dehydrogenation: Experimental investigation and mathematical modelling of catalytic reactions

Support Screening to Shape Propane Dehydrogenation SnPt-Based Catalysts

Propane dehydrogenation reaction (PDH) is an extremely attractive way to produce propylene; however, the catalysts often lead to byproduct formation and suffer from deactivation. This research focuses on the development of efficient Pt/Sn-based shaped catalysts by utilizing Mg-modified mesoporous silica, sepiolite (natural SiMgO x mesoporous clay), and sepiolite/bentonite/alumina as supports with the aim of achieving superior stability and selectivity for industrial propylene production by PDH. The catalysts were prepared by sequential impregnation of the supports with the corresponding solutions of tin chloride and platinum chloride, by obtaining a nominal loading of 0.7 wt % of Sn and 0.5 wt % of Pt. A range of analytical techniques were used to characterize the catalysts, including X-ray diffraction, nitrogen physisorption isotherms, Hg intrusion porosimetry, thermogravimetric analyses, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The basicity of the catalysts was assessed using carbon dioxide temperature-programmed desorption (CO2-TPD). The results confirm that the support material plays a critical role in catalyst performance; in particular, the presence of weak basic sites, due to magnesium addition, improved selectivity to propylene and reduced coke formation. Catalytic pellets of Sn-Pt supported on macroporous sepiolite or sepiolite and bentonite-modified mesoporous alumina performed comparably with propane conversion very close to thermodynamic equilibrium and selectivity to propylene above 95%. The latter support led to improved stability and was regenerated at milder temperatures, making it suitable for industrial applications.

Modeling the Selectivity of Hydrotalcite-Based Catalyst in the Propane Dehydrogenation Reaction

The propylene production processes currently used in the petrochemical industry (fluid catalytic cracking and steam cracking of naphtha and light diesel) are unable to meet the increase of propylene demand for industrial applications. For this reason, alternative processes for propylene production have been investigated, and among the others, the propane dehydrogenation (PDH) process, allowing the production of propylene as a main product, has been industrially implemented (e.g., Catofin and Oleflex processes). The main drawback of such processes is closely linked to the high temperature required to reach a sustainable propane conversion that affects catalyst stability due to coke formation on the catalyst surface. Accordingly, the periodic regeneration of the catalytic bed is required. In this work, the performance in the PDH reaction of different Sn-Pt catalysts, prepared starting by alumina- and hydrotalcite-based supports, is investigated in terms of propane conversion and selectivity to propylene in order to identify a more stable catalyst than the commercial ones. The experimental tests evidenced that the best performance was obtained using the catalyst prepared on commercial pellets of hydrotalcite PURALOX MG70. This catalyst has shown, under pressure conditions of 1 and 5 bar (in order to evaluate the potential future application in integrated membrane reactors), propane conversion values close to the thermodynamic equilibrium ones in all of the investigated temperature ranges (500-600 °C) and the selectivity was always higher than 95%. So, this catalyst was also tested in a stability run, performed at 500 °C and 5 bar: the results highlighted the loss of only 12% in the propane conversion with no changes in the selectivity to propylene. Properly designed experimental tests have also been performed in order to evaluate the kinetic parameters, and the developed mathematical model has been optimized to effectively describe the system behavior and the catalyst deactivation.

The mathematical catalyst deactivation models: a mini review

Catalyst deactivation is a complex phenomenon and identifying an appropriate deactivation model is a key effort in the catalytic industry and plays a significant role in catalyst design. Accurate determination of the catalyst deactivation model is essential for optimizing the catalytic process. Different mechanisms of catalyst deactivation by coke and metal deposition lead to different deactivation models for catalyst activity decay. In the rigorous mathematical models of the reactors, the reaction kinetics were coupled with the deactivation kinetic equation to evaluate the product distribution with respect to conversion time. Finally, selective and nonselective deactivation kinetic models were designed to identify catalyst deactivation through the propagation of heterogeneous chemical reactions. Therefore, the present review discusses the catalyst deactivation models designed for CO₂ hydrogenation, Fischer-Tropsch, biofuels and fossil fuels, which can facilitate the efforts to better represent the catalyst activities in various catalytic systems.

Fluidized Bed Membrane Reactor for the Direct Dehydrogenation of Propane: Proof of Concept

In this work, the fluidized bed membrane reactor (FBMR) technology for the direct dehydrogenation of propane (PDH) was demonstrated at a laboratory scale. Double-skinned PdAg membranes were used to selectively remove H₂ during dehydrogenation tests over PtSnK/Al₂O₃ catalyst under fluidization. The performance of the fluidized bed membrane reactor was experimentally investigated and compared with the conventional fluidized bed reactor (FBR) by varying the superficial gas velocity over the minimum fluidization velocity under fixed operating conditions (i.e., 500 °C, 2 bar and feed composition of 30vol% C₃H₈-70vol% N₂). The results obtained in this work confirmed the potential for improving the PDH performance using the FBMR system. An increase in the initial propane conversion of c.a. 20% was observed, going from 19.5% in the FBR to almost 25% in the FBMR. The hydrogen recovery factor displayed a decrease from 70% to values below 50%, due to the membrane coking under alkene exposure. Despites this, the hydrogen extraction from the reaction environment shifted the thermodynamic equilibrium of the dehydrogenation reaction and achieved an average increase of 43% in propylene yields.

Propylene Synthesis: Recent Advances in the Use of Pt-Based Catalysts for Propane Dehydrogenation Reaction

Propylene is one of the most important feedstocks in the chemical industry, as it is used in the production of widely diffused materials such as polypropylene. Conventionally, propylene is obtained by cracking petroleum-derived naphtha and is a by-product of ethylene production. To ensure adequate propylene production, an alternative is needed, and propane dehydrogenation is considered the most interesting process. In literature, the catalysts that have shown the best performance in the dehydrogenation reaction are Cr-based and Pt-based. Chromium has the non-negligible disadvantage of toxicity; on the other hand, platinum shows several advantages, such as a higher reaction rate and stability. This review article summarizes the latest published results on the use of platinum-based catalysts for the propane dehydrogenation reaction. The manuscript is based on relevant articles from the past three years and mainly focuses on how both promoters and supports may affect the catalytic activity. The published results clearly show the crucial importance of the choice of the support, as not only the use of promoters but also the use of supports with tuned acid/base properties and particular shape can suppress the formation of coke and prevent the deep dehydrogenation of propylene.

Flux-Reducing Tendency of Pd-Based Membranes Employed in Butane Dehydrogenation Processes

We report on the effect of butane and butylene on hydrogen permeation through thin state-of-the-art Pd-Ag alloy membranes. A wide range of operating conditions, such as temperature (200-450 °C) and H2/butylene (or butane) ratio (0.5-3), on the flux-reducing tendency were investigated. In addition, the behavior of membrane performance during prolonged exposure to butylene was evaluated. In the presence of butane, the flux-reducing tendency was found to be limited up to the maximum temperature investigated, 450 °C. Compared to butane, the flux-reducing tendency in the presence of butylene was severe. At 400 °C and 20% butylene, the flux decreases by ~85% after 3 h of exposure but depends on temperature and the H2/butylene ratio. In terms of operating temperature, an optimal performance was found at 250-300 °C with respect to obtaining the highest absolute hydrogen flux in the presence of butylene. At lower temperatures, the competitive adsorption of butylene over hydrogen accounts for a large initial flux penalty.

Latest Developments in Membrane (Bio)Reactors

The integration of membranes inside a catalytic reactor is an intensification strategy to combine separation and reaction steps in one single physical unit. In this case, a selective removal or addition of a reactant or product will occur, which can circumvent thermodynamic equilibrium and drive the system performance towards a higher product selectivity. In the case of an inorganic membrane reactor, a membrane separation is coupled with a reaction system (e.g., steam reforming, autothermal reforming, etc.), while in a membrane bioreactor a biological treatment is combined with a separation through the membranes. The objective of this article is to review the latest developments in membrane reactors in both inorganic and membrane bioreactors, followed by a report on new trends, applications, and future perspectives.

Microwaves and Heterogeneous Catalysis: A Review on Selected Catalytic Processes

Since the late 1980s, the scientific community has been attracted to microwave energy as an alternative method of heating, due to the advantages that this technology offers over conventional heating technologies. In fact, differently from these, the microwave heating mechanism is a volumetric process in which heat is generated within the material itself, and, consequently, it can be very rapid and selective. In this way, the microwave-susceptible material can absorb the energy embodied in the microwaves. Application of the microwave heating technique to a chemical process can lead to both a reduction in processing time as well as an increase in the production rate, which is obtained by enhancing the chemical reactions and results in energy saving. The synthesis and sintering of materials by means of microwave radiation has been used for more than 20 years, while, future challenges will be, among others, the development of processes that achieve lower greenhouse gas (e.g., CO2) emissions and discover novel energy-saving catalyzed reactions. A natural choice in such efforts would be the combination of catalysis and microwave radiation. The main aim of this review is to give an overview of microwave applications in the heterogeneous catalysis, including the preparation of catalysts, as well as explore some selected microwave assisted catalytic reactions. The review is divided into three principal topics: (i) introduction to microwave chemistry and microwave materials processing; (ii) description of the loss mechanisms and microwave-specific effects in heterogeneous catalysis; and (iii) applications of microwaves in some selected chemical processes, including the preparation of heterogeneous catalysts.

Application of Pd-Based Membrane Reactors: An Industrial Perspective

The development of a chemical industry characterized by resource efficiency, in particular with reference to energy use, is becoming a major issue and driver for the achievement of a sustainable chemical production. From an industrial point of view, several application areas, where energy saving and CO₂ emissions still represent a major concern, can take benefit from the application of membrane reactors. On this basis, different markets for membrane reactors are analyzed in this paper, and their technical feasibility is verified by proper experimentation at pilot level relevant to the following processes: (i) pure hydrogen production; (ii) synthetic fuels production; (iii) chemicals production. The main outcomes of operations in the selected research lines are reported and discussed, together with the key obstacles to overcome.