Biomass to fuels The role of zeolite and mesoporous materials

Biomass to fuels:The role of zeolite and mesoporous materials qCarlo Perego,Aldo Bosetti ⇑Eni S.p.A.,Research Center for Non-Conventional Energies,Istituto Eni Donegani,Via G.Fauser 4,28100Novara,Italya r t i c l e i n f o Article history:Received 16September 2010Received in revised form 19November 2010Accepted 26November 2010Available online 16December 2010Keywords:Biomass Biofuels Catalysis Zeolitea b s t r a c tBiomass is an abundant and carbon–neutral renewable energy resource for the production of biofuels,moving the market dependence away from fossil-based energy sources.The main problem is how to effi-ciently remove the abundant oxygen content from biomass-derived products and convert it into a hydro-phobic molecule with the appropriate combustion or chemical properties.Many efforts have been devoted to the search of heterogeneous catalytic systems,more selective,safe and environmentally friendly.In this scenario zeolites and mesoporous compounds may help chemists to develop new biofuel generation processes.The development of new catalysts in the field of conversion of biomass to biofuels requires knowledge of the complex nature of the substrates to be converted.Starting from the main chemical aspects of the different biomass platforms,an overview of some of the zeolite and mesoporous materials technologies currently used commercially or tested at pilot and laboratory scale,is presented in this paper.Ó2010Elsevier Inc.All rights reserved.1.IntroductionExpansion and progress,particularly in emerging countries,will boost the need for energy in all the end-use sectors,in particular in the transportation utilization [1].As reported by 2009Energy Outlook of Energy Information Administration of DOE,the world liquid fuel supply forecast is still increasing [2].In 2030106.6million barrels per day is the expected demand of liquid fuel (Fig.1).The transportation sector accounts for the largest increment in total liquids demand,at nearly 80%of the total world increase.Unconventional liquids play an increasingly important role in meeting demand for liquid fuels over the course of the IEO2009projections.Fuels derived from shale oils,oils sands,extra heavy oils,coal to liquids and gas to liquids processes,and biofuels are considered unconventional liquids.In the reference case,12.6%of world liquids supply in 2030comes from unconventional sources,including 1.5million barrels per day from OPEC and 11.9million from non-OPEC sources.As illustrated in Fig.2,biofuels represent a significant part of unconventional supplies,i.e.increasing as absolute value and as percentage with respect to the other uncon-ventional sources.Biofuels are liquids or gases for transport pur-poses that are produced from biomass.The increasing demand of biofuels is due to several key issues,the first one is that bio-based resources are renewable and CO 2neutral in contrast with fossil fuels.In fact,while electricity and heat can be generated from a wide spectrum of alternatives (sun,wind,hydro,geothermal heat,etc.)the production of transporta-tion fuels can just rely on carbon-biomass,as only alternative car-bon source to fossils ones.However,the main driver is the strong political focus on renewable biofuel alternatives together with the increasing severity of regulations everywhere in the World.The Renewable Energy Directive (RED)of European Union requires biofuels to reach 10%of total automotive fuel consumptions by the year 2020.Biofuels must contribute to CO 2reduction of 35%at the introduction of the new Directive,till to reach 50%CO 2reduction by 2017(60%for the new production plants).US Renewable Fuel Standard (RFS)requires around 2.2MBPD biofuels by 2022(30%of the transport pool),with corn ethanol capped at 1MBPD.First-generation biofuels,produced primarily from agricultural crops,traditionally grown for food and animal feed purposes,are the initial step in this direction.The main first-generation biofuels are bioethanol,used as a gas-oline substitute,produced from sugar containing plants or cereals crops,and biodiesel,produced from vegetable oils after conversion into the corresponding fatty acid methyl esters.However,most of first-generation biofuels have several draw-backs,including the competition with food crops,the competition for water,the potentially negative impact on biodiversity,the lim-ited greenhouse gas emission reduction (with the exception of sug-arcane ethanol)and the high production cost [3].Many of these problems could be addressed by the production of the second generation biofuels,manufactured from agricultural1387-1811/$-see front matter Ó2010Elsevier Inc.All rights reserved.doi:10.1016/j.micromeso.2010.11.034qKeynote lecture presented at the 16th International Zeolite Conference (16th IZC),jointly organised with the 7th International Mesostructured Materials Symposium (7th IMMS),held in Sorrento,Italy,July 4–9,2010.⇑Corresponding author.Tel.:+390321447348;fax:+390321447241.E-mail address:aldo.bosetti@ (A.Bosetti).and forest residues and from ligno-cellulosic non-food energy crops.Second generation biofuels are expected to be superior to many of the first-generation biofuels in terms of energy balances,green-house gas emission reduction,land requirement and competition for land,food,fiber and water.The main reason they have not yet been taken up commercialization,despite their potential advantages,is that the involved production technologies are not technically proven at a commercial scale and their costs are at the moment estimated to be significantly higher than that of most first-generation biofuels.Therefore,there is still much work to be done for the improve-ment of the existing processes and for the development of new efficient technologies.Acid and base catalysis plays a crucial role in most of the processes currently used for the production of bio-fuels,such as the transesterification of vegetable oils with metha-nol in the biodiesel process (catalysed by alkali,i.e.NaOH,MeONa,KOH)and the hydrolysis of cellulose to fermentable sugars for bio-ethanol production (catalysed by H 2SO 4).The homogeneous cata-lysts,used in both these processes,have some drawback,mainly due to the neutralization step needed at the end of the reaction.In order to avoid these problems,many efforts have been de-voted to the search of solid catalysts,more selective,safe and envi-ronmentally friendly.In this scenario zeolites and mesoporouscompounds may help chemists to develop new biofuel generation processes,due to their important properties,namely,high concen-tration of active sites,high thermal/hydrothermal stability and en-hanced shape selectivity.The aim of this contribution is to summarise some challenges facing zeolite and mesoporous cata-lysts in new industrial processes.2.The role of zeolite catalyst in biomass conversionZeolites have found wide application as solid acid catalysts or catalyst carriers in the oil refining and petrochemical industries,where they have been gradually replacing conventional homoge-neous and heterogeneous catalysts (e.g.free liquid acids;amor-phous mixed oxides).In most of the processes,zeolite and mesoporous materials are involved in acid-catalyzed reactions that proceed through the formation of carbocation-like intermediates.Therefore the chemistry of the catalytic transformations is closely related to the chemistry of the carbocations in the restricted micro-porous environment.Normally,the reactions are performed in apolar gas or liquid phases where the substrate to be converted is an hydrocarbon oil or an hydrocarbon-like molecule.To produce biofuels,we should take in account that the composition and the structure of biomass raw materials are totally different from petro-leum.Lignocellulose (wood and its derivatives)is the cheapest and most abundant source of biomass and is essentially composed of cellulose (38–50%),hemicelluloses (23–32%)and lignin (15–25%)[4].All these compounds are polymeric molecule insoluble in most of conventional organic solvents.The transport of heavy biomass molecules into catalyst pores is very cumbersome and severe mass transfer limitations should be expected.To be transformed the bio-mass require a dispersant as reaction medium and the so obtained20406080100120Non-OPEC ConventionalOPEC ConventionalTotal UnconventionalTotalMillion bpdTable 1Chemical composition of various biomass feedstock.Biomass substrate (dry base)Elemental analysis (%)CH O N S Sawdust 45.8 6.142.7<0.1<0.1Switchgrass 47.8 5.845.0 1.10.1Arundo donax 47.1 5.846.40.60.1Algae46.17.441.3 4.80.4Organic urban waste 44.3 6.744.5 3.50.9Fossil heavy oil86.111.8<0.10.12.0C.Perego,A.Bosetti /Microporous and Mesoporous Materials 144(2011)28–3929sluggish medium is itself heterogeneous.The recovery and reuse of the solid catalyst at the end of the reaction is another critical issue.As illustrated in Table1in which several typical bio-starting material are compared to fossil heavy oil,biomass composition is not often homogeneous and elemental composition includes oxy-gen(the most abundant),nitrogen and sulphur[5].The presence of heteroatoms reduce heat content of molecules and hinder the blending with existing fossil fuels[1].As for heavy oil,in general to produce a fuel we have to breakdown large molecules,but in case of biomass we have to removelarger amount of heteroatoms too.Actually it is necessary to re-move the abundant oxygen content from biomass-derived prod-ucts and convert it into a hydrophobic molecule with theappropriate combustion or chemical properties.By this treatment,the hydrophilic substrates start to become more and more hydro-phobic and the interactions with the active centre of the catalystsare probably changing during the course of the reaction.The reduc-tion of the oxygen content in biomass by dehydration and hydrog-enolysis is associated with the formation of the water in themedium.The water,sometime used as convenient dispersant sol-vent for biomass or naturally present in the starting material,canpoison active sites of the catalyst or modify their activities.Thereis also the possibility that the polymeric molecules react on theexternal active sites and the reaction continues in the internal ac-tive pores,maybe with smaller compounds and with a differentmechanism.Therefore,in biotransformation the interactions be-tween biomass components with solid catalysts and zeolites aredifferent with respect to hydrocarbon ones and are not based onlyon apolar carbocation-like relationships in the active centers.Zeo-lites and zeolite like materials seem to have not the same impor-tance for biomass conversion as they have in petroleum industryso far.However,as soon as the ligno-cellulosic biomass is decon-structed to a sufficient extent,the further transformations of theobtained platform chemicals can take advantage of the uniqueproperties of zeolites and related materials[6].In the following sections the main chemical aspects of the dif-ferent biomass platforms,vegetable oils,sugars,lignin and bio-oils,will be presented together with some examples of recent lab andpilot scale or industrial processes based on the aforementionedcatalysts.3.Vegetable oilsVegetable oils are triglyceride esters of fatty acids.In principlethey could be used directly as diesel fuel,however,due to severaldrawbacks(e.g.high viscosity,encrustations in the internal com-bustion chambers,blockage of the injectors,and dilution of the lu-bricant)it is advisable to convert them into biodiesel[7].Biodieselis a mixture of fatty acid methyl esters(FAME)produced by transe-sterification of triglycerides with methanol in presence of a propercatalyst(Fig.3).Glycerol is the main by-product,whose amount isequivalent to approximately10%of the total FAME production.While various biodiesel feedstock are renewable,its competi-tion with food source is a major concern.Non-edible oil shouldbe preferred(castor oil,waste vegetable oils,algae based oil,etc.).Any vegetable(or animal)fat can be used to prepare biodie-sel,but the source of feedstock should fulfil two requirements:acheap price(more than80%production cost of biodiesel is thefeedstock price)and local availability(in terms of large and con-stant volume production or waste reuse).The process and proper-ties of thefinal produced biodiesel are strongly influenced by thelength of the fatty acid chains,the degree of un-saturation of thestarting triglyceride,the presence of other chemical functionalgroups in the chain,the free fatty acids(FFA)contents,the meltingpoint of fat and the degree of moisture and impurities[8].Vegeta-ble oil is composed from over100substances,and different oilshave different compositions that can vary even for the same oilduring the year.As a consequence,a vegetable oil process shouldbeflexible versus feed.In principle,both acid and base catalysts could be used in thetransesterification process,however,base catalysts are generallypreferred for their superior activity(approximately4000timeshigher that that of acid catalysts)[9].Today the commercial biodie-sel production plants are utilizing homogeneous alkaline catalysts(usually sodium hydroxide or sodium methylate)[10].The reactionis usually carried out with an excess of methanol(6/1w/w)at60°C.The resulting reaction mixture is a biphasic system consisting in apolar phase,containing most of the glycerol,the catalyst and a partof co-produced soaps,and an apolar phase containing the FAMEs,afraction of methanol,traces of the catalyst and most of the soaps.Actually,the current technology for biodiesel production hastwo main shortcomings.Firstly,the presence of free fatty acidsand water in the feedstock causes the lost of the catalyst and theformation of soap.Secondly,the alkaline catalyst must be neutra-lised at the end of the reaction,and the resulting salt is difficult toremove from the glycerol to get a high purity grade product.Toavoid these problems,many heterogeneous catalysts have beenproposed and reported in the literature.Different heterogeneous catalysts have been developed to cata-lyze the transesterification of vegetable oils,including zeolite andmesoporous materials[11].A comprehensive list of zeolite,mi-cro-mesoporous material considered as transesterification catalysthas been recently reported in a review by Miertus et al.[8].López et al.reported an interesting comparison among differentsolid catalyst and liquid homogeneous catalyst(NaOH and H2SO4)in the transesterification of triacetin as model compound for largertriglycerides as found in vegetable oils and fats[12].The reactionwere performed at60°C using a6:1(methanol:triacetin)initialmolar ratio and2wt.%of solid catalyst in comparison with0025%NaOH and0.25%H2SO4(final reaction time500min).Inthese conditions the catalytic activity decreases as follows:NaOH>H2SO4>ETS-10(Na,K)>Amberlyst-15>Sulfated Zirco-nia>Nafion NR50>MgO$Tungstated Zirconia>Supported Phos-phoric Acid>H-Beta>ETS-10(H).The maximum conversion oftriacetin reached with the different catalysts at500min of reactiontime are summarized in Fig.4.The data indicate that homogeneous catalysts were more activethan heterogeneous ones on a weight basis.Furthermore micropo-rous acid zeolite is not a valid alternative to homogeneous30 C.Perego,A.Bosetti/Microporous and Mesoporous Materials144(2011)28–39甘油三酯酯交换脂肪酸甲酯catalysts in mild conditions.López et al.suggest internal mass transfer limitations as explanation of the poor performances.How-ever,basic heterogeneous catalyst ETS-10exhibited almost the same activities as H2SO4,but still lower than NaOH.Corma et al. evaluated glycerolysis of triglycerides using basic Cs-MCM-41, Cs-sepiolite and hydrotalcites[13].The reaction was carried out at240°C and the best conversion was obtained with hydrotalcite (92%)without any significant loss of ing soybean oil as substrate,a series of zeolite catalysts was tested by Suppes et al.[14],including NaX and ETS-10zeolites exchanged with potassium and cesium.The ETS-10catalysts provided higher con-versions than the zeolite-X type catalysts.The increased conver-sion was attributed to the higher basicity of the ETS-10material and larger pore structures that improved intra-particle diffusion. At100°C,the ETS-10provided a conversion of92%in3h.Unfortu-nately,the presence of FFA(oleic acid)quenched the reaction due to inhibition of basic sites.Currently,refined vegetable oils(containing less than0.5%of free fatty acids,FFA)are the major feedstock for biodiesel produc-tion.However,waste greases,such as yellow grease from used cooking oils and animal fats,can also be employed because of their availability and low cost.The high concentration of FFAs(up to15%)present in these inexpensive feedstocks,make them inappropriate for the conven-tional base-catalyzed transesterification route to biodiesel due to soap formation.Although the acid catalysts require a longer reaction time and a higher temperature than the alkali-catalysts,they are more effi-cient when the amount of free fatty acids in the oil exceeds0.5%. In this case,a single-step,acid catalysed esterification/transesteri-fication process is more economical than the alkali-catalyzed pro-cess,which requires an extra step to convert free fatty acids to methyl esters,thus avoiding soap formation.Homogeneous catalysts(H2SO4,HCl,BF3,H3PO4,and organic sulfonic acids)[15]although effective,lead to serious contamina-tion problems,due to the formation of decomposition products that make essential the implementation of good separation and product purification protocols,which translate into higher produc-tion costs.Heterogeneous catalysts were tested.Solid sulfated oxi-des,such as SO4/ZrO2and SO4/SnO2proved to be active catalysts due to their high acid strength,but deactivation phenomena and sulphate leaching were detected under the transesterification con-ditions[16].Esterification has been carried out using ion-exchange resins(Amberlyst-15)[17]and Nafion[18]as heterogeneous cata-lysts.However,most ion-exchange resins are not stable at temper-atures above140°C,which prohibits their application to reactions that require higher temperatures.For this kind of application,inor-ganic acid catalysts,such as zeolites,are generally more suitable.Using microporous zeolites catalysts the mass-transfer resis-tance becomes a critical issue if large molecules,are used as sub-strates.In this case the reaction takes place mainly on the external surface of zeolite crystals.For this reason,only large-pore zeolites(faujasite,mordenite)have been successfully used in fatty acid esterifications[19].Using waste frying oils,various zeolite cat-alysts with different acidity and pore structure were tested in the transesterification[20].H+-exchanged MOR,MFI,FAU and BEA zeolites were employed.The yield increased linearly with enhanc-ing of acid strength and increasing of amount of acid sites.The syn-thesis of biodiesel from soybean oil and methanol catalyzed by zeolite Beta modified with La3+was described[21].Results of the study showed that La-zeolite Beta shows higher conversion and stability than zeolite Beta for the production of biodiesel,which may be correlated to the higher quantity of external BrØnsted acid sites available for the reactants.It should be noted that the optimal reaction conditions were4h of reaction,60°C and a conversion of triglyceride equal to about49%.The catalytic performances reported so far by zeolites,both in acidic or basic form,are however lower than that of conventional catalyst(NaOH and Na methylate),and also that of other heteroge-neous catalysts.This is probably the reasons why up to now,no zeolite catalysts have been considered for commercial biodiesel production processes.An additional improvement of biodiesel process deals with the utilization of the coproduced glycerol for the synthesis of oxygen-ated fuel components(e.g.glycerine etherification with olefins)[1]. At present,glycerol has already a great number of utilizations lead-ing to a large number of products for non-fuel application,such as: propane diols(monomers for polyester or polyurethane materials), oligoglycerols(cosmetics,food additives and biodegradable lubricants),glycerol carbonate(solvent),glyceric acid(pharmaceu-ticals),epichlorohydrine,and glycidol(polymers and pharmaceuti-cals).In the future,the availability of glycerol could exceed the demand for traditional use and its price will decrease,making it a cost effective raw material for the preparation of fuel compo-nents and additives.This approach not only makes a valuable use of the by-product but could also increase the fuel yield in the over-all biodiesel processes.In European Union,the leading worldwide producer of biodiesel,the glycerine price and production was sta-ble at the beginning of the2010and followed an ongoing trend in the second part due to the imminent implementation of Renew-able Energy Directive.Player in the market have predicted optimis-tic scenario for the second half of2011.Glycerol cannot be added directly to fuels because of its low sol-ubility and poor thermal stability that raise to engine problems at high temperatures.Therefore,glycerol must be transformed into derivatives that are compatible with diesel and biodiesel,prior to being added to the fuel[22–25].Due to their high oxygen content, glycerol derivatives can be used as ignition accelerators,antiknock additives,viscosity and melting point enhancers and particle emis-sion reducers.Among these products,glycerol ethers have been extensively studied for their promising physical,chemical and blending and properties.The reaction of glycerol with isobutene yields a mixture of the corresponding mono-,di-and tri-tertiary butyl ethers,as shown in Fig.5.The mono-ethers are soluble in polar solvents[26],whereas the mixture of di-and tri-ethers are miscible with apolar media and can be used in the formulation of diesel fuels(as particulate matter emission reducers)or gasoline(as octane-booster in substitution of methyl-tertiarybutyl ether MTBE).The etherification reaction is efficiently promoted by both homogeneous and heterogeneous catalysts such as para-toluene sulfonic acid,acid ion exchangers resins like Amberlyst15,and acid zeolites such as H–Y or H-Beta [27,28].Typically,the reaction is carried out in liquid phase at 70–90°C with a3/1isobutene/glycerol molar ratio.A process,in which glycerol and isobutene were reacted in presence of tert-butyl alcohol using zeolite Beta,was recently patented[29].The zeolite Beta have a silicon to aluminum ratio greater than150 and the presence of the alcohol reduces the formation of isobutyl-ene oligomers(less than5%)producing a more selective glycerol di-tert-butyl ethers mixture yield.An alternative reaction of glyc-erol in the biofuel production is the catalytic cracking pathway. Corma et al.[30]studied six FCC catalysts for the cracking of50wt.%glycerol-water solution at500°C,using glycerol asC.Perego,A.Bosetti/Microporous and Mesoporous Materials144(2011)28–3931油酸猝灭反应representative of biomass derived oxygenates.Among the catalyst tested,a ZSM-5based FCC additive and USY zeolites.The activity of the catalysts resulted different and USY was the highest active cat-alyst in the formation of a mixture of olefins C 2–C 4,aromatics and paraffins.Glycerol was also fed together with vacuum gasoil (VGO)without significantly altering in the product selectivity,suggesting that biomass-derived products could be co-fed with petroleum-de-rived streams in FCC process.In a similar way,triglyceride oils can be used to produce liquid fuels that contain linear and cyclic par-affin,olefins,aldehydes,ketones and carboxylic acids.Historically,pyrolysis products of vegetable oils were used as fuel during both World Wars.Vegetable oils are thermally unstable and a solid cat-alyst is compulsory to improve product yields.Many zeolites were tested in catalytic cracking,including ZSM-5,zeolite Beta and USY,using vegetable oils as only stream or as co-fed with fossil ones (VGO)[4].Catalytic cracking of vegetable oils appears to be a pro-cess for the production of good RON gasoline and olefins,however,some problems should be resolved (coke formation,more stable catalytic system,etc.).A more radical innovation in bio-based diesel fuels considers the complete hydrogenation of the triglyceride feedstocks to hydrocarbon mixture,avoiding the side-production of glycerol and allowing a better integration of the process and the product in the exiting refinery infrastructure and fuel distribution system.Several companies have been developing such a kind of triglyc-erides hydroprocessing (e.g.Neste Oil,BP,Conoco-Phillips,Petro-bras,Dynamic Fuels,Haldor Topsoe,Axens and UOP-Eni)[10,31].The UOP/Eni Ecofining TM process is based on catalytic hydro-deoxygenation,decarboxylation and isomerization reactions (Fig.6)to produce a diesel fuel rich in isoparaffins [32,33].This alternative product is called greendiesel.As this kind of process is very flexible to the feedstocks,it can be considered also for ined-ible (e.g.jatropha and camelina)and unconventional (ed and cooking oils,animal fats)triglycerides.In this concern greendiesel can be considered as a bridge between first and second generationbiodiesel [34].The main improvement of the Ecofining technology compared to the conventional FAME biodiesel,is that it allows refiners to ob-tain a synthetic fuel that has a similar chemical composition and similar chemical-physical properties compared to petroleum die-sel.For this reason the product can be easily blended with conven-tional refinery streams.In addition,all of the Ecofining by-products are already present during normal refinery operation and do not require any special handling.The greendiesel advantages,in comparison to mineral diesel fuels and FAME,are high cetane number (CN >80);negligible O content,that means to have the same energy content as mineral diesel fuel and higher than FAME;better stability and blending properties,due to the absence of double bonds and oxygenated molecules.Besides,the hydrogenation is less sensitive to the qual-ity of vegetable oil,in particular the fatty acid distribution and the degree of un-saturation that affect the properties of FAME biodiesel.A simplified flow diagram of Ecofining process is shown in Fig.7.In the first stage,vegetable oil is combined with hydrogen and brought to reaction temperature,then it is sent to a reactor section where it is converted to greendiesel.The reactor section can con-sist of either a deoxygenation reactor or a combination of a hydro-processing and an isomerization reactors,to achieve better cold flow properties in the green diesel product.The resulting mixture is separated from the recycle gas in the separator and the liquid stream sent to a fractionation section,producing propane,naphtha,and diesel products.In the hydroprocessing stage oxygen is removed from the tri-glyceride molecules via three competing reactions:hydrodeoxy-genation,decarbonylation and decarboxylation (Fig.6).The three carbon ‘‘backbone’’yields propane that can be recovered easily when the process is integrated into a refinery.The oxygen con-tained in the feed is removed from the fatty acid chain either as CO/CO 2or water.In addition,all olefinic bonds are saturated,resulting in a product consisting of only n -paraffins.The hydroprocessing is carried out at moderate temperature (310°C)using a bimetallic hydrotreating catalyst (e.g.Ni–Mo or Co–Mo catalyst),specifically tailored for the selected feedstock.Despite to the high cetane number,the high cloud point of the liquid stream coming out from the hydrotreating reactor has a great impact in limiting the volume that can be blended with min-eral diesel.In order to overcome this restriction,this linear paraf-finic stream is isomerized in a second stage.For such a purpose a proper hydroisomerization catalyst,based on a precious metal loaded on a mild acidic carrier has been developed.The scope of this second stage is to control the cold flow properties of the final green diesel.As well explained in the open literature [35–38],the diesel yield from the process will depend on the severity required in the isomerization reactor to meet cold flow specifications.Typical acidic supports for bifunctional cata-lysts used in the hydroisomerization reaction are:amorphous oxides or mixture of oxides (i.e.HF-treated Al 2O 3,SiO 2–Al 2O 3,32 C.Perego,A.Bosetti /Microporous and Mesoporous Materials 144(2011)28–39一代生物柴油主要是指脂肪酸低碳醇酯丆以甲酯为主丟二代主要是脂肪烃。