Msc in CatalysisCatalyst Design Project

Edidiong Asuquo, Charlotte Corcoran and Jessica SegnanThe Dehydrogenation of Isopropanol to form Hydrogen gas

Abstract The design of a catalyst for the dehydrogenation of isopropanol to generate hydrogen was studied. The dehydrogenation is favoured thermodynamically at a temperature range of 465-473k and by evaporation of the products. A tri-metallic catalyst composed of Cu-Pt-Re was proposed on a silica support in a CSTR reactor .The heat source for the endothermic reaction is through a microwave heater. A Pd-membrane separator is used to separate the hydrogen. Two distillation columns are used to separate the acetone/isopropanol/water mixture. The isopropanol-water mixture is also recycled as feed, therefore making the reaction environmentally viable.

Introduction

The advancement in industrialisation has placed man in an energy driving economy. Thereby putting a heavy demand on processes that will maximize energy production, whilst reducing the cost. This trend has been the wheel in the energy transition.

One of the most likely sources of renewable energy is hydrogen. Hydrogen as an energy vector was discussed in the energy alternative plan in the 1970s. The hydrogen economy has been proposed as a viable option as a solution to the impending energy problem that may arise from fossil reserve decline and environmental implications of fossil energy sources.

One of the immediate uses of hydrogen in the energy applications is in the development of fuel cells. Fuel cells are used to convert hydrogen to electricity. However, hydrogen basically is an energy carrier and not a fuel

The major thermal advantage of hydrogen, over both gasoline and natural gas, is its specific energy, which is almost 3 times the combustion energy of the other 2 fuels

Hydrogen can be produced by the dehydrogenation of alcohols. Isopropanol is a secondary alcohol and its dehydrogenation products are hydrogen and acetone. However, the design of an efficient catalyst to optimise the production of hydrogen from isopropanol has been a limiting factor in the process of commercialising hydrogen production using this route.

The aim of this present study is to develop an efficient catalyst and process system for the dehydrogenation of isopropanol to optimise hydrogen production

Hydrogen Production Techniques3 main routes:

steam reforming of hydrocarbons (natural gas) Steam reforming of natural gas is a well-understood process that is being used in existing commercial plants. Methane in natural gas reacts with water (steam) to produce carbon monoxide and hydrogen using a nickel catalyst on a ceramic support. CH4 + H2O CO + 3H2 H=+251 KJmol-1 The carbon monoxide is put through a water-gas shift reaction, where it combines with water to produce hydrogen and carbon monoxide. CO + H2O CO2 + H2 H=-42 KJmol-1

electrolysis of water by electricity The system uses an electrolyser powered by electricity. The required electricity may come from conventional power plants such as coal or nuclear, but can also use renewable energy resources such as wind, solar thermal, photovoltaic and hydropower.

biomass gasification Biomass gasification can be considered as a form of pyrolysis, which takes place at higher temperatures and produces a mixture of gases with hydrogen content ranging from 6-6.5% Hydrogen Utilization Routes

Industrially, hydrogen is used chemically as a reducing agent in the mineral industry, as a hydrogenation agent in the petroleum industry and as a bonding agent in the chemical industry

Fuel cell (hydrogen to electricity) technology is one of the most hydrogen utilising routes in the transportation sector through fuel cell vehicles. Fuel cells have high efficiencies and potentially substantially lower negative externalities than current energy systems, which has made them an attractive future option in micro, stationary and automotive applications.

Large-scale future applications are possible as a fuel for power plant fuel-cell generation of electricity, as a coolant in super conductor technology, and as a fuel in transportation applications such as motor vehicles with internal-combination engines, motor vehicles with fuel-cell electrical engines, marine vessels, aviation jet engines and space travel.

Literature Review

Homogeneous Catalysis

Recent studies have shown that ruthenium, in the presence of easily available amine ligands, constitutes an active catalyst for the dehydrogenation of alcohols to generate hydrogen, under mild conditions.

It was shown that the addition of trialkylamines gave much better results compared to secondary and primary amines. Also, applied coordinating ligands showed a significant inhibition of the reaction.

Literature Review

Heterogeneous Catalysis Supported Metal Catalysts

Dehydrogenation of isopropanol over a family of carbon-supported Pt, Cu, and bimetallic Cu-Pt catalysts revealed that Pt was more active than copper and all catalysts were 100% selective to acetone when supported on a high temperature treated carbon (possessing no acidic surface groups.) bimetallic Cu-Pt catalysts were less active than their monometallic counterparts.

Cu-SiO2 catalysts have also been reported, the activity of the fresh Cu/SiO2 catalyst initially increases with time until a maximum is reached, and then rapid deactivation follows. The results showed that simultaneous reduction and sintering processes are responsible for the activation-deactivation behaviour observed for a Cu/SiO2 shell catalyst during the dehydrogenation of isopropyl alcohol to acetone.

Cu/CNF and Cu/CeO2/CNF catalysts have been synthesized on different carbon nanofibers (CNF) . The presence of CeO2 enhanced the reduction and dispersion of Cu, and lead to higher turnover frequencies. However, excess CeO2 enhanced hydration activity, and thereby, reduced the selectivity. Cu/CNF activities were similar to that found for active carbon supported Cu catalysts

Literature Review

Heterogeneous CatalysisInfluence of Rhenium additives It has previously been found by [33] that the introduction of the 1% Re additive enhances the activity of the Cu (1%)/Sib catalyst in the transformation of isopropyl alcohol into acetone. The addition of 0.25% Re favours an increase in the dehydrogenating activity and stability of the Cu (4%)/Sib catalyst.

Rhenium, being a metal with a comparatively high melting point does not undergo recrystallisation and therefore favours an increase in the stability of the metal-supported catalyst

Literature ReviewHeterogeneous CatalysisRecyclable Ruthenium Catalyst Here, a recyclable and easily synthesized heterogeneous catalyst for acceptor-free alcohol dehydrogenation is described:

Literature Review

Heterogeneous CatalysisMetal oxides Heterogeneous CatalysisSimple metal oxides

Objective of Study This work sets out to design a dehydrogenation catalyst for the generation of hydrogen from isopropanol.The goal of thus study is to propose a catalytic system that will be highly active, very selective and stable for the reaction to optimise hydrogen generation at a specified reaction condition. Therefore, it will be designed to achieve the following:

Reaction thermodynamics: Examination of the energetics and feasibility of the reaction at specified conditions.Kinetics and mechanism of the reaction.Catalyst properties and mode of operation: The catalyst feedstock phase system that will enhance activity, selectivity and stability of the catalyst. In addition to conversion, mass transfer effect, deactivation and regeneration considerations.Chemical reactor design considerations.Process economics/environmental considerations.

Thermodynamics of the ReactionCatalysis is basically a kinetic phenomenon. This is because it does not affect the thermodynamics of the reaction. As a catalyst does not affect the equilibrium constant for the overall reaction , it is therefore necessary that an examination of the thermodynamics of the dehydrogenation reaction be carried out using the thermodynamic properties of the reactants and products

The energetic quantities in the thermodynamic system of the chemical reaction are:Enthalpy of reaction, Hr Entropy of reaction, Sr Gibbs Free Energy, Gr Equilibrium Constant, K.

The standard enthalpy and entropy of the reactant and products are :

Thermodynamics of the ReactionStandard enthalpy = Hr = Hf298(products) Of reaction - Hf298(reactants) = (-248.1 + 0) (318.2) = (-248.1 + 318.2) Hr298 = 70.1 kJ/mol

Standard entropy = Sr298 = Sm298 (products) Of reaction - Sm298 (reactants) = [(200.4 + 130.6) 180] = [331 180] Sr298 = 151 J/K mol

Standard Gibbs = Gr298 = Hr298 - T Sr298 free energy of reaction = 70.1 kJ (298.15 x 151) J = (70100 45020.65) J = 25079.35 J Gr298 = 25.079 kJ/mol

Equilibrium = Keqm = e (- G) Constant RT = e (- 25079) 298.15 x 8.314 = e (-25079) 2478.81 Keqm = e (-10.11) Keqm = 4.03 x 10-5

Thermodynamics of the ReactionFor the dehydrogenation reaction of isopropanol to generate hydrogen, the following thermodynamic results were obtained at standard conditions (298 K): Hr298 0 reaction is endothermic Sr298 0 net formation of gas in reaction Gr298 0 Reverse reaction is spontaneous Forward reaction not favourable at standard conditions. Keqm = 4.03 x 10-5

Thermodynamics of the ReactionSince the Gibbs free energy is positive at standard conditions (298 K), then the reaction is not thermodynamically favourable at this temperature. Therefore, heat must be applied to make it favourable.

At a temperature of 465 K the Gibbs free energy will become negative. Hence for the reaction to be thermodynamically favourable it should be operated at temperatures above 465 K.

Since the reaction is limited by its thermodynamics in the liquid phase, the equilibrium can be shifted by evaporation of either one or both of the products, thereby reducing the concentration of the products. If both hydrogen and acetone are continuously expelled, then the equilibrium will shift towards the formation of the products according to Le Chateliers principle. Therefore a high temperature is needed.

Kinetics and Mechanism of Isopropanol Dehydrogenation The study of the kinetics and mechanism of any reaction is useful in the selection of a suitable catalyst and appropriate reaction conditions that will enhance the selectivity and conversion of the process towards the desired products.

The generally accepted, simplified mechanism of solid-catalysed fluid (gas or liquid) phase reaction is outlined below :

Transport of reactants from fluid phase to exterior surface of catalyst pellet.Transport of reactants to active sites inside the pores.Adsorption of reactant onto the surface at active sites.Chemical reaction of reactants on surface of catalyst with formation of products.Desorption of products from the surface.Transport of products out of the pores to the pellet surface.Transport of products from the catalyst pellet surface to the bulk of the fluid.

Kinetics and Mechanism of Isopropanol DehydrogenationFig 2 Mechanisms proposed for 2-propanol decomposition

Catalyst design for Reaction The suitability of a catalyst for an industrial process depends mainly on the following three properties: ActivitySelectivityStability (deactivation) behaviour. The activity is a measure of how fast one of more reactions proceeds in the presence of a catalyst ad can be expressed using:

Reaction rateRate constantActivation energy, Ea.

Catalyst/support Interaction

Since selectivity in alcohol dehydrogenation depends on the strength and distribution of acidic or basic sites, recent efforts have been made in order to design a catalyst with controlled acidity and a number of mixed oxide catalysts have been designed for this purpose.

The catalyst supports can be basic, acidic or both. The acidity of the support determines whether the dehydration or the dehydrogenation reaction will be favoured. Basic supports favour the dehydrogenation reaction, while acidic supports favour the dehydration reactions. Silica (basic) and alumina (acidic) have very high surface areas compared to the other oxides.

Catalyst Choice

The type of catalyst we propose for the dehydrogenation of isopropanol to give hydrogen and acetone is a tri-metallic catalyst, composed of Cu-Pt-Re. This is because of the need to increase the overall catalytic activity, selectivity and stability of the catalyst under reaction conditions in the reactor.

Copper was chosen because if exhibits the highest selectivity to the dehydrogenation reaction.

Platinum has been reported to have a high activity in the dehydrogenation of isopropanol and shows less sensitivity to the acetone inhibition of the rate.

Rhenium exhibits the highest stability and is resistant to thermal sintering, this helps to stabilize the copper which is very susceptible to sintering, thereby increasing the overall catalytic conversion.

Choice of Support

The nature of the support for the heterogeneous Cu-Pt-Re catalyst is also an important factor. Dehydrogenation activity is decreased when active sites contain a high number of acidic sites, due to the presence of a dehydration reaction. However, dehydrogenation activity of a catalyst can be improved by the incorporation of amphoteric or basic metal oxides into the catalyst.

MgO is very basic. It needs high temperatures to give dehydrogenation. At 200-300C propene is the major product.

Al2O3 is very acidic and gives propene/Di-isopropyl Ether(DIE).

SiO2 is amphoteric. Has very low propene formation due to very weak basic sites, so there is no ether formation

Ce3+, Mg2+, Al3+ - mixed oxides form the condensation product (MIBK) with copper.

TiO2- ZrO2 mixed oxides catalyse di-isopropyl ether formation.

Activated carbon has the tendency to lead to the formation of carbonaceous species on the surface of the catalyst. Silica has been shown to contain weak basic sites, which means it is capable of catalysing the dehydrogenation reaction. The catalytic reaction is also sensitive to the number of active sites on the catalyst. So a high surface area of silica is necessary.

Catalyst Preparation

Reactor Design Chemical reactors are the most important part of a chemical process plant. In most cases, improvement in reaction rates and selectivities to desired products can be attained by changes to the reactor design. The reactor must be designed in such a way that it produces the desired products, hydrogen and acetone, in a safe, reliable and economic way. The choice of reactor depends on the nature of the reactants, products and reaction conditions. The factors which influence these are given below:

Catalyst form (pellet, extruders, monolithic)Reactor volumeLevel of agitation between catalyst and reactantHeat transfer and controlTemperature of reactionPressure of reaction

The choice of reactor had to take into account the thermodynamics and selectivity of the reaction. A batch reactor would not be favourable in this case as the products would remain inside the reactor and cause inhibition of the catalyst and subsequent side reactions such as self-condensation of the acetone to give MIBK. Therefore the selectivity would be low.

Reactor DesignFor this project we chose a continuous stir tank reactor. This is a reactor in which there is continuous agitation of the catalyst bed. The products are also continuously removed and so it favours formation of the desired products.The reaction conditions for our reactor are as follows:Temperature- 465-473KPressure- 20 bar N2Solvent- Water

The choice of water as a solvent was made due to the fact that the side reaction of isopropanol dehydrogenation to give propene and isopropyl ether gives water as a co-product. In order to inhibit the presence of unwanted side reactions, water was use as a solvent.

The reactor is kept in continuous operation and the continuous removal of the products, acetone and hydrogen, favours equilibrium. The heat source is from a microwave heater, as it provides many advantages to conventional heating routes therefore it has the ability to provide heat throughout the catalyst bed.

A schematic representation of the reactor process:

Process EconomicsUsing either, palladium, ruthenium, rhodium or platinum catalysts supported on carbon or amberlyst, isopropanol can be obtained from a renewable feedstock. This will greatly enhance the economics and viability of this route for production of hydrogen. This will also ensure that the production capacity of isopropanol is increased to meet the new and old uses of the feedstock.

Furthermore, to improve the economics of the isopropanol route for hydrogen production, the use of the by-product of the process is also very commercially attractive. Acetone is obtained as the by-product of the process. Acetone is manufactured primarily as a co-product of phenol production via cumene peroxidation.

Methyl isobutyl ketone (MIBK) is used as a solvent for paint and protective coatings, an extracting agent for production of antibiotics and commercial lubricating oils.

The most important factor in the economics of this isopropanol conversion process is the generation of hydrogen. Its production capacity must increase and be made more sustainable if it can be obtained from renewable resources.

ConclusionConcerns about the depletion of fossil fuel energy sources and the pollution caused by continuous energy demands make hydrogen an attractive alternative energy source.

Hydrogen is currently being derived from non-renewable natural gas and petroleum however the dehydrogenation of alcohols is an alternative and renewable source of hydrogen.

The dehydrogenation of isopropanol to give acetone and hydrogen is one route that offers the possibility of hydrogen production for various uses.

A tri-metallic catalyst composed of Cu-Pt-Re supported on silica has been designed in this project in order to maximise hydrogen and acetone production in an economic and environmentally friendly way.

The thermodynamics of the process indicate a temperature above 465K must be used and that the reaction is favourable when the gaseous products are removed thus driving the reaction to favour product formation.

A continuous stirred flow tank reactor (CSTR) has been proposed for the reaction.

Thank you!