catalysts

Review Review OlefinsOlefins fromfrom BiomassBiomass Intermediates:Intermediates: AA ReviewReview

1 1,2, VasilikiVasiliki Zacharopoulou Zacharopoulou1 andand AngelikiAngeliki A.A. LemonidouLemonidou 1,2,** 1 Department of Chemical Engineering, Aristotle University of Thessaloniki, University Campus, 1 Department of Chemical Engineering, Aristotle University of Thessaloniki, University Campus, Thessaloniki 54124, Greece; [email protected] 54124 Thessaloniki, Greece; [email protected] 2 Chemical Process Engineering Research Institute (CERTH/CPERI), P.O. Box 60361 Thermi, 2 Chemical Process Engineering Research Institute (CERTH/CPERI), P.O. Box 60361 Thermi, Thessaloniki 57001, Greece 57001 Thessaloniki, Greece * Correspondence: [email protected]; Tel.: +30‐2310‐996‐273 * Correspondence: [email protected]; Tel.: +30-2310-996-273 Received: 16 November 2017; Accepted: 19 December 2017; Published: Received: 16 November 2017; Accepted: 19 December 2017; Published: 23 December 2017

Abstract:Abstract: Over the last decade, increasing demand for olefinsolefins and theirtheir valuablevaluable productsproducts hashas promptedprompted researchresearch onon novelnovel processesprocesses andand technologiestechnologies forfor theirtheir selectiveselective production.production. As olefinsolefins are predominatelypredominately dependent on fossil resources, their production is limited by thethe finitefinite reservesreserves and the associated economic and environmental concerns.concerns. The need for alternative routes for olefinolefin productionproduction isis imperativeimperative inin orderorder toto meetmeet thethe exceedinglyexceedingly highhigh demand,demand, worldwide.worldwide. BiomassBiomass isis consideredconsidered aa promising alternativealternative feedstockfeedstock thatthat cancan bebe converted into the valuable olefins,olefins, among other chemicalschemicals andand fuels.fuels. Through processes such as fermentation,fermentation, gasification,gasification, crackingcracking andand deoxygenation, biomass derivatives can be effectively converted into C2–C4 olefins. This short deoxygenation, biomass derivatives can be effectively converted into C2–C4 olefins. This short review focusesreview focuses on the conversion on the conversion of biomass-derived of biomass‐ oxygenatesderived oxygenates into the most into valuablethe most olefins,valuable e.g., olefins, ethylene, e.g., propylene,ethylene, propylene, and butadiene. and butadiene.

Keywords: olefins;olefins; biomass; ethylene; propylene;propylene; butadiene;butadiene; catalysiscatalysis

1. Introduction

The importanceimportance ofof CC22–C–C44 olefinsolefins (i.e.,(i.e., CC22HH44,C, C33H6,, butenes, and CC44H6)) has been highlightedhighlighted because of their numerous applications as key building blocks in the chemical industry, linkedlinked withwith thethe increasingincreasing needs needs of of the the expanding expanding global global population population [1]. These [1]. lowerThese olefinslower areolefins the most are prevalentthe most organicprevalent compounds, organic compounds, with the highest with the production highest production volumes, worldwide, volumes, worldwide, highly dependent highly on dependent crude oil andon crude natural oil gasand products natural gas [2]. products It is estimated [2]. It is that estimated 400 million that 400 tons million of olefins tons are of annuallyolefins are produced, annually usingproduced, one billion using tonsone asbillion hydrocarbon tons as feedstock,hydrocarbon via feedstock, processes suchvia processes as fluid-catalytic such as cracking, fluid‐catalytic steam cracking,cracking, andsteam dehydrogenation cracking, and dehydrogenation [3]. Almost 60% [3]. of the Almost global 60% feedstocks of the global are used feedstocks in FCC are units, used and in approximatelyFCC units, and 40% approximately in steam cracking 40% in processes.steam cracking (Figure processes.1) Produced (Figure olefins 1) Produced can be used olefins in a can wide be spectrumused in a wide of high-end spectrum applications of high‐end such applications as packaging, such construction, as packaging, solvents, construction, coatings, solvents, and synthetic coatings, fibersand synthetic [4]. fibers [4].

FigureFigure 1. 1.Olefin Olefin production production methods methods using using hydrocarbonhydrocarbon feedstocks. Reproduced Reproduced from from [3]. [3 ].2014, 2014, WILEY-VCH Verlag GmbH & Co.WILEY‐VCH Verlag GmbH & Co.

Catalysts 2018, 8, 2; doi: 10.3390/catal8010002 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 2; doi:10.3390/catal8010002 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 2 2 of 18

C2H4 constitutes the most predominant olefin in the global market and is primarily produced Catalysts 2018, 8, 2 2 of 19 via naphtha steam cracking, among various hydrocarbon feedstocks, as well as through ethane thermal cracking. Globally, 57% (Figure 2) of the C2H4 volume is produced via naphtha and gas oil steam crackingC2H4 constitutes and 38% the through most predominant ethane and olefin LPG in the(Liquefied global market Petroleum and is primarilyGas) steam produced cracking. Naphthavia naphtha is a liquid steam fraction cracking, obtained among from various petroleum hydrocarbon refining feedstocks, processes, as well such as as through catalytic ethane cracking thermal cracking. Globally, 57% (Figure2) of the C H volume is produced via naphtha and gas and hydrocracking. Depending on its origin, it contains2 4 variable amounts of paraffins, aromatic, and oil steam cracking and 38% through ethane and LPG (Liquefied Petroleum Gas) steam cracking. olefinicNaphtha compounds. is a liquid The fraction ratio obtainedof these components from petroleum can refining indicate processes, the process such that as catalytic the specific cracking fraction can beand used hydrocracking. for the optimum Depending results. on its At origin, high temperatures it contains variable (i.e., amounts 650–750 of °C), paraffins, naphtha aromatic, and gas and oil can yieldolefinic 30 and compounds. 25 wt. % of The C2 ratioH4, respectively. of these components In the case can indicate of ethane, the processadded thatalong the with specific naphtha fraction in the ◦ feed canstream, be used yields for the of optimum C2H4 can results. reach At 80 high wt. temperatures % [5]. Its (i.e.,eminent 650–750 industrialC), naphtha uses and cause gas oilthe can world demandyield for 30 andC2H 254 to wt. incessantly % of C2H4, increase, respectively. as Init can the casebe used of ethane, for significant added along applications, with naphtha such in the as the productionfeed stream, of yieldsintermediate of C2H4 canchemicals, reach 80 wt.mainly % [5]. Itsin eminentthe industry industrial of uses plastics. cause thei.e., world polymers demand (e.g., polyfor‐ethylene), C2H4 to incessantlypropionaldehyde—via increase, as it can hydroformylation, be used for significant vinyl applications, chloride—via such as halogenation the production and of intermediate chemicals, mainly in the industry of plastics. i.e., polymers (e.g., poly-ethylene), de‐hydrohalogenation, alpha‐olefins—via oligomerization, and C2H4 oxide and acetaldehyde—via oxidationpropionaldehyde—via [4,6,7]. In recent hydroformylation, years, bio‐ethanol vinyl has chloride—via been extensively halogenation studied and de-hydrohalogenation,as an alternate feedstock alpha-olefins—via oligomerization, and C2H4 oxide and acetaldehyde—via oxidation [4,6,7]. In recent for C2H4 production [8]. Other bio‐derived compounds such as methanol and dimethyl‐ether, can years, bio-ethanol has been extensively studied as an alternate feedstock for C2H4 production [8]. also be used as a feedstock for C2H4, via Methanol to Olefins (MTO) and Dimethyl‐ether to Olefins Other bio-derived compounds such as methanol and dimethyl-ether, can also be used as a feedstock (DMTO) processes [9,10]. Bio‐ethylene can also be produced via bio‐synthesis from various enzymes for C2H4, via Methanol to Olefins (MTO) and Dimethyl-ether to Olefins (DMTO) processes [9,10]. or microorganismsBio-ethylene can [11]. also be produced via bio-synthesis from various enzymes or microorganisms [11].

FigureFigure 2. 2. CC2H2H4 production4 production methods methods using using hydrocarbon hydrocarbon feedstocks.feedstocks. Reproduced Reproduced from from [3 [3].]. 2014, 2014, WILEY-VCH Verlag GmbH & Co.WILEY‐VCH Verlag GmbH & Co.

C3HC6 3isH 6theis thesecond second most most significant significant olefin, olefin, conventionally produced produced via via steam steam cracking, cracking, as a as a co‐product,co-product, or orthrough through fluidfluid catalytic catalytic cracking cracking (FCC). (FCC). (Figure (Figure3)C 2H 4 3)and C gasoline2H4 and production gasoline severelyproduction severelyaffect affect C3H6 production;C3H6 production; recently, recently, steam crackers steam switch crackers to ethane switch feedstocks, to ethane suppressing feedstocks, concurrent suppressing production of C3H6, while its demand outpaces existing steam and fluid catalytic cracking capacity [12]. concurrent production of C3H6, while its demand outpaces existing steam and fluid catalytic C3H6 is mainly used for the production of polypropylene, as well as for the synthesis of numerous cracking capacity [12]. C3H6 is mainly used for the production of polypropylene, as well as for the platform chemicals (e.g., cumene, acrylonitrile, propylene-oxide) [13]. The importance of C H in synthesis of numerous platform chemicals (e.g., cumene, acrylonitrile, propylene‐oxide)3 [13].6 The the C3 value chain addresses the need for alternative processes; using the conventional technology, importance of C3H6 in the C3 value chain addresses the need for alternative processes; using the C3H6 can also be produced through C4-C8 olefin cracking, or through FCC under severe conditions conventionalin order to technology, increase produced C3H6 can volume also [be12 ].produced On-purpose through production C4‐C8 methods olefin cracking, that include or through propane FCC underdehydrogenation severe conditions (PDH) [in14 ],order olefin metathesisto increase [15 ],produced or methanol volume to olefins [12]. (MTO) On [16‐purpose,17], have recentlyproduction methodsbeen implemented,that include propane as well as dehydrogenation using other unconventional (PDH) [14], feedstocks olefin [metathesis18], such as [15], bio-alcohols or methanol and to olefinsvegetable (MTO) oils. [16,17], have recently been implemented, as well as using other unconventional feedstocks [18], such as bio‐alcohols and vegetable oils.

Catalysts 2018, 8, 2 2 of 18

C2H4 constitutes the most predominant olefin in the global market and is primarily produced via naphtha steam cracking, among various hydrocarbon feedstocks, as well as through ethane thermal cracking. Globally, 57% (Figure 2) of the C2H4 volume is produced via naphtha and gas oil steam cracking and 38% through ethane and LPG (Liquefied Petroleum Gas) steam cracking. Naphtha is a liquid fraction obtained from petroleum refining processes, such as catalytic cracking and hydrocracking. Depending on its origin, it contains variable amounts of paraffins, aromatic, and olefinic compounds. The ratio of these components can indicate the process that the specific fraction can be used for the optimum results. At high temperatures (i.e., 650–750 °C), naphtha and gas oil can yield 30 and 25 wt. % of C2H4, respectively. In the case of ethane, added along with naphtha in the feed stream, yields of C2H4 can reach 80 wt. % [5]. Its eminent industrial uses cause the world demand for C2H4 to incessantly increase, as it can be used for significant applications, such as the production of intermediate chemicals, mainly in the industry of plastics. i.e., polymers (e.g., poly‐ethylene), propionaldehyde—via hydroformylation, vinyl chloride—via halogenation and de‐hydrohalogenation, alpha‐olefins—via oligomerization, and C2H4 oxide and acetaldehyde—via oxidation [4,6,7]. In recent years, bio‐ethanol has been extensively studied as an alternate feedstock for C2H4 production [8]. Other bio‐derived compounds such as methanol and dimethyl‐ether, can also be used as a feedstock for C2H4, via Methanol to Olefins (MTO) and Dimethyl‐ether to Olefins (DMTO) processes [9,10]. Bio‐ethylene can also be produced via bio‐synthesis from various enzymes or microorganisms [11].

Figure 2. C2H4 production methods using hydrocarbon feedstocks. Reproduced from [3]. 2014, WILEY‐VCH Verlag GmbH & Co.

C3H6 is the second most significant olefin, conventionally produced via steam cracking, as a co‐product, or through fluid catalytic cracking (FCC). (Figure 3) C2H4 and gasoline production severely affect C3H6 production; recently, steam crackers switch to ethane feedstocks, suppressing concurrent production of C3H6, while its demand outpaces existing steam and fluid catalytic cracking capacity [12]. C3H6 is mainly used for the production of polypropylene, as well as for the synthesis of numerous platform chemicals (e.g., cumene, acrylonitrile, propylene‐oxide) [13]. The importance of C3H6 in the C3 value chain addresses the need for alternative processes; using the conventional technology, C3H6 can also be produced through C4‐C8 olefin cracking, or through FCC under severe conditions in order to increase produced volume [12]. On‐purpose production methods that include propane dehydrogenation (PDH) [14], olefin metathesis [15], or methanol to olefins (MTO) [16,17], have recently been implemented, as well as using other unconventional Catalysts 2018, 8, 2 3 of 19 feedstocks [18], such as bio‐alcohols and vegetable oils.

Catalysts 2018, 8, 2 3 of 18

Figure 3. C3H6 production methods using hydrocarbon feedstocks. Reproduced from [3]. 2014, Figure 3. C3H6 production methods using hydrocarbon feedstocks. Reproduced from [3]. 2014, WILEY-VCHWILEY‐VCH Verlag Verlag GmbH GmbH & & Co. Co. (*XTP (*XTP refers refers to to propylene propylene production production from from any any kind kind of of feedstock). feedstock)

C4 olefins are mostly produced via fluid catalytic cracking, as well as through steam cracking C4 olefins are mostly produced via fluid catalytic cracking, as well as through steam cracking (Figure 4). Most common C4 olefins are C4H6, isobutylene, and butenes; C4H6 is a key chemical, (Figure4). Most common C olefins are C H , isobutylene, and butenes; C H is a key chemical, currently used for the production4 of polymers4 6 (i.e., rubbers), butylenes are4 expended6 in the fuel currently used for the production of polymers (i.e., rubbers), butylenes are expended in the fuel industry for the production of blending components and octane enhancers, and n‐butenes are used industry for the production of blending components and octane enhancers, and n-butenes are used as co‐monomers of polyethylene and for the synthesis of higher olefins. C4H6 is the prevalent C4 as co-monomers of polyethylene and for the synthesis of higher olefins. C4H6 is the prevalent C4 olefin, conventionally co‐produced via naphtha and gas oil cracking, along with C3H6 and C2H4, olefin, conventionally co-produced via naphtha and gas oil cracking, along with C3H6 and C2H4, among others; C4H6 yield via cracking is substantially low, highlighting the need for more targeted among others; C4H6 yield via cracking is substantially low, highlighting the need for more targeted production methods [4]. Demand for C4H6 is expected to increase, as it can be used as a bio‐based production methods [4]. Demand for C4H6 is expected to increase, as it can be used as a bio-based feedstock for greener rubbers. Several bio‐based routes have based proposed for C4H6‐ production; feedstock for greener rubbers. Several bio-based routes have based proposed for C H - production; bio‐ethanol constitutes the most promising feedstock for this purpose. 4 6 bio-ethanol constitutes the most promising feedstock for this purpose.

Figure 4. C4 olefin production methods using hydrocarbon feedstocks. Reproduced from [3]. 2014, Figure 4. C4 olefin production methods using hydrocarbon feedstocks. Reproduced from [3]. 2014, WILEY-VCH Verlag GmbH & Co. WILEY‐VCH Verlag GmbH & Co.

Overall, existing olefinolefin production is highlyhighly dependent on fossil resources thatthat areare expendedexpended intointo energy-consumingenergy‐consuming processes, processes, heavily heavily contributing contributing to the environmental to the environmental affliction. Continuously affliction. increasingContinuously demand increasing has turned demand the petrochemical has turned the industry petrochemical towards optimization industry towards of existing optimization processes inof orderexisting to meetprocesses challenging in order capacities, to meet challenging while lowering capacities, the vast while production lowering costs. the Recently,vast production investigation costs. ofRecently, alternative investigation feedstocks of hasalternative been considerably feedstocks has studied been considerably in order to offerstudied an in attractive order to solutionoffer an untrammeledattractive solution from untrammeled the limited crude from oilthe reserves;limited crude coal, oil natural reserves; gas, coal, and biomassnatural gas, are and some biomass of the examplesare some of [2 ,the4]. examples Coal has been[2,4]. usedCoal has in coal-rich been used countries in coal‐rich as a countries feedstock as for a feedstock chemicals’ for production; chemicals’ evenproduction; though even it reduces though dependence it reduces ondependence fossil resources, on fossil the resources, resulting COthe 2resultingemissions CO limit2 emissions the extent limit of itsthe applications extent of its [4 ].applications Methane prices [4]. haveMethane recently prices dropped have duerecently to technological dropped due advances, to technological enabling theadvances, use of shaleenabling gas the as anuse attractive, of shale gas economical as an attractive, feedstock. economical Thus, cost-effective feedstock. Thus, olefin cost production,‐effective viaolefin steam production, cracking, via has steam been cracking, enabled, has already been implemented enabled, already in the implemented olefin market, in the primarily olefin market, for the productionprimarily for of Cthe3H production6 and higher of olefins C3H6 [and12,19 higher,20]. Renewable olefins [12,19,20]. feedstocks Renewable offer great feedstocks advantages offer in terms great ofadvantages sustainability, in terms energy of sustainability, consumption, energy environmental consumption, pollution, environmental CO2 emissions, pollution, and cost; CO2 biomassemissions, is anand abundant cost; biomass carbon-source is an abundant with potential carbon to‐source replace with fossil potential resources to [21 replace]. Through fossil processes resources such [21]. as fermentation,Through processes hydro-deoxygenation such as fermentation, or gasification, hydro‐deoxygenation light olefins can or begasification, produced fromlight bio-feedstocks,olefins can be produced from bio‐feedstocks, or from bio‐intermediates (e.g., ethanol, butanol, naphtha, methanol, and propane) via dehydration, metathesis, and steam‐cracking, among others [22]. In this context, the alternative production of olefins from biomass intermediates is reviewed and compared to the conventional processes, with emphasis on the most promising chemical technologies for future applications that progress biomass valorization.

2. Olefins from Biomass Following the incessantly increasing demand worldwide, technological advances have enabled the production of light olefins from various biomass‐derived feedstocks, obtaining mainly mixtures of C2–C4 olefins. Specific processes can be selective to the production of a certain product, as will be

Catalysts 2018, 8, 2 4 of 19 or from bio-intermediates (e.g., ethanol, butanol, naphtha, methanol, and propane) via dehydration, metathesis, and steam-cracking, among others [22]. In this context, the alternative production of olefins from biomass intermediates is reviewed and compared to the conventional processes, with emphasis on the most promising chemical technologies for future applications that progress biomass valorization.

2. Olefins from Biomass Catalysts 2018, 8, 2 4 of 18 CatalystsFollowing 2018, 8, 2 the incessantly increasing demand worldwide, technological advances have enabled4 of 18 thediscussed production in this of chapter. light olefins Overall, from olefins various from biomass-derived biomass are predominantly feedstocks, obtaining produced mainly from mixtures biomass ofintermediates,discussed C2–C4 olefins. in this and Specificchapter. more specifically, processesOverall, olefins can from be fromalcohols, selective biomass diols, to the are and production predominantly other oxygenates. of a certain produced These product, fromintermediates as biomass will be discussedare,intermediates, in most in cases, this and chapter. formed more specifically, via Overall, fermentation, olefins from fromalcohols, hydro biomass‐deoxygenation, diols, are and predominantly other or oxygenates.gasification produced processesThese fromintermediates (Figure biomass 5). intermediates,are, in most cases, and formed more specifically, via fermentation, from alcohols, hydro‐deoxygenation, diols, and other or oxygenates. gasification Theseprocesses intermediates (Figure 5). are, in most cases, formed via fermentation, hydro-deoxygenation, or gasification processes (Figure5).

Figure 5. Biomass to olefins olefins primary routes. Figure 5. Biomass to olefins primary routes. 2.1. Ethylene (C2H4) 2.1. Ethylene (C2H4) 2.1. Ethylene (C2H4) As mentionedmentioned above, above, steam steam cracking cracking (mainly (mainly using using naphtha naphtha as a feedstock) as a feedstock) is the most is extensively the most extensivelyAs mentioned used process above, forsteam C2H cracking4 production (mainly from using hydrocarbons. naphtha as Steama feedstock) cracking is operatingthe most used process for C2H4 production from hydrocarbons. Steam cracking operating conditions require vastconditionsextensively amounts requireused of energy, process vast thus amounts for increasing C2H of4 production theenergy, production thus from increasing cost, hydrocarbons. and heavily the production contributing Steam crackingcost, to environmental and operating heavily issues.contributingconditions Even require to though environmental vast the amounts installed issues. of technology Evenenergy, though thus for thesethe increasing installed processes thetechnology hasproduction already for these beencost, processes modifiedand heavily has for increasedalreadycontributing been efficiency, tomodified environmental novel for productionincreased issues. efficiency,Even methods though havenovel the been productioninstalled explored technology methods in order forhave to these reduce been processes explored production has in orderalready to been reduce modified production for increased cost and efficiency, to substitute novel the production finite fossil methods resources. have Apartbeen exploredfrom C2 Hin4 cost and to substitute the finite fossil resources. Apart from C2H4 conventional production from conventionalorder to reduce production production from cost petrochemicals, and to substitute C2H4 thecan finitebe effectively fossil resources. produced Apart from fromrenewable C2H4 petrochemicals, C2H4 can be effectively produced from renewable feedstocks, such as plants, microorganisms,feedstocks,conventional such production as and plants, bio-alcohols from microorganisms, petrochemicals, (Figure6). and bioC2H‐alcohols4 can be (Figureeffectively 6). produced from renewable feedstocks, such as plants, microorganisms, and bio‐alcohols (Figure 6).

FigureFigure 6.6. Schematic chartchart ofof CC22H4 productionproduction methods.

Figure 6. Schematic chart of C2H4 production methods. C2H4 biosynthesis is a natural pathway, as it constitutes an important hormone that plants recognizeC2H4 andbiosynthesis produce is[11]. a natural ACC synthase pathway, and as oxidaseit constitutes are two an vitalimportant enzymes hormone that enable that plants C2H4 productionrecognize and from produce ACC (1 [11].‐aminocyclopropane ACC synthase and‐1carboxylic oxidase acid)are two and vital SAM enzymes (S‐adenosyl that methionine),enable C2H4 alongproduction with fromcarbon ACC dioxide (1‐aminocyclopropane and HCN, via the‐1carboxylic Yang cycle, acid) starting and SAM from (S‐ adenosylmethionine methionine), via three consecutivealong with carbonreaction dioxide steps: (a)and methionine HCN, via isthe converted Yang cycle, into startingSAM by fromSAM methioninesynthetase; via(b) threeACC synthetaseconsecutive converts reaction SAM steps: to (a)ACC; methionine and finally, is (c)converted ACC is convertedinto SAM intoby SAMC2H4 bysynthetase; ACC oxidase (b) ACC[23]. (Schemesynthetase 1) convertsMicroorganisms, SAM to ACC; such andas bacteria finally, and(c) ACC fungi is haveconverted also beeninto Creported2H4 by ACC to produce oxidase C[23].2H4 starting(Scheme from1) Microorganisms, methionine via such the KMBAas bacteria (2‐keto and‐4 ‐fungimethylthiobutyric have also been acid) reported formation to produce pathway C2 Hor4 throughstarting from2‐oxoglutarate methionine viaconversion the KMBA [24–26]. (2‐keto ‐Ethylene4‐methylthiobutyric production acid) rate formation can reach pathway 2859.2 or through 2‐oxoglutarate conversion [24–26]. Ethylene production rate can reach 2859.2

Catalysts 2018, 8, 2 5 of 19

C2H4 biosynthesis is a natural pathway, as it constitutes an important hormone that plants recognize and produce [11]. ACC synthase and oxidase are two vital enzymes that enable C2H4 production from ACC (1-aminocyclopropane-1carboxylic acid) and SAM (S-adenosyl methionine), along with carbon dioxide and HCN, via the Yang cycle, starting from methionine via three consecutive reaction steps: (a) methionine is converted into SAM by SAM synthetase; (b) ACC synthetase converts SAM to ACC; and finally, (c) ACC is converted into C2H4 by ACC oxidase [23]. (Scheme1) Microorganisms, such as bacteria and fungi have also been reported to produce C2H4 starting Catalystsfrom methionineȱ2018,ȱ8,ȱ2ȱ via the KMBA (2-keto-4-methylthiobutyric acid) formation pathway or through5ȱofȱ18ȱ 2-oxoglutarate conversion [24–26]. Ethylene production rate can reach 2859.2 µmol/gCDW/h (CDW-Cellΐmol/gCDW/h Dryȱ Weight)(CDWȬCell fromȱ DryPseudomonasȱ Weight)ȱ putidafromȱ[27Pseudomonas]. Despiteȱ theputida factȱ that[27]. bio-synthesisȱ Despiteȱ the technologyȱ factȱ thatȱ biois atȬsynthesis its earlyȱ technology stages, recentȱ isȱ techno-economicatȱ itsȱ earlyȱ stages, analysesȱ recentȱ highlighttechnoȬeconomic the potentialȱ analyses of theseȱ highlight processesȱ theȱ potentialfor on-purposeȱofȱthese Cȱprocesses2H4 production.ȱforȱonȬpurpose However,ȱC2H further4ȱproduction. studiesȱHowever, are requiredȱfurther in orderȱstudies toȱare reduceȱrequired costȱ inandȱ order overcomeȱ toȱ reduce or improveȱ costȱ and aspectsȱ overcome such asȱ or productivityȱ improveȱ aspects and productȱ suchȱ as separation.ȱ productivity Advancesȱ andȱ product in theȱ separation.biotechnologicalȱAdvances processesȱinȱthe couldȱbiotechnological improve productivityȱprocesses andȱcould reduceȱimprove cost,ȱproductivity enabling futureȱand developmentȱreduceȱcost,ȱ enablingof C2H4 biosynthesisȱfutureȱdevelopment methodsȱof [28ȱC].2H4ȱbiosynthesisȱmethodsȱ[28].ȱ

ȱ

Schemeȱ 1.1.ȱStepsȱ forȱ C22H4ȱbiobio-synthesis.Ȭsynthesis.ȱ

Ethanol,ȱ derived ȱ from ȱ biomass, ȱ i.e., ȱ cellulose, ȱ corn, corn,ȱ and andȱ sugarcane ȱ [8] [8]ȱ is ȱ the ȱ mostȱ commonȱ bioȬfeedstockȱforȱtheȱproductionȱofȱC2H4.ȱSeveralȱcompaniesȱworldwide,ȱsuchȱasȱBraskemȱ(SaoȱPaulo,ȱ bio-feedstock for the production of C2H4. Several companies worldwide, such as Braskem (SaoBrazil), Paulo,ȱ Axens Brazil),ȱ (Rueil AxensȬMalmaison, (Rueil-Malmaison,ȱ France),ȱ Solvay France),ȱ (Brussels, Solvayȱ (Brussels,Belgium), Belgium),ȱ andȱ BPȱ (London, and BP (London,ȱ Unitedȱ Kingdom),ȱfocusȱonȱresearchȱandȱplantȱoperationȱforȱethanolȱdehydrationȱintoȱC2H4ȱ[13–15],ȱasȱtheȱ United Kingdom), focus on research and plant operation for ethanol dehydration into C2H4 [13–15], globalȱ demandȱ forȱ C2H4ȱ isȱ continuouslyȱ increasingȱ [29,30].ȱ Evenȱ thoughȱ eachȱ companyȱ hasȱ as the global demand for C2H4 is continuously increasing [29,30]. Even though each company has developedȱ andand installedȱ installed specificȱ specific technologiesȱ technologies forȱ thisforȱ reaction,thisȱ reaction, ethanolȱ ethanol dehydrationȱ dehydration typicallyȱ typically includesȱ includestwo steps;ȱtwo reactionȱsteps;ȱreaction of ethanolȱofȱ dehydrationethanolȱdehydration and purificationȱandȱpurification of productsȱofȱproducts [29]. Bio-ethanolȱ[29].ȱBioȬethanol can beȱ canproducedȱbeȱproduced via fermentationȱviaȱfermentation processesȱprocesses causingȱcausing sugarsȱ tosugars selectivelyȱtoȱselectively break downȱbreak intoȱdown ethanol,ȱintoȱethanol, at highȱ atrates,ȱ high withȱ rates, limitedȱ with by-productȱ limitedȱ byȬ formationproductȱ formation [31]. However,ȱ[31].ȱ However, the strongȱ the dependenceȱ strongȱ dependence on sugar productionȱ onȱ sugarȱ productioncosts limit theirȱ costs applicationȱ limitȱ their [ȱ30application,32]. Moreȱ [30,32]. complexȱ More feedstocksȱ complex withȱ feedstocks higher marketȱ withȱ availabilityhigherȱ market andȱ availabilitythus lowerȱ costandȱ (i.e.,thusȱ cellulose,lowerȱcost hemicellulose,ȱ(i.e.,ȱcellulose, andȱhemicellulose, lignocellulose)ȱandȱ arelignocellulose) more attractiveȱareȱmore but theirȱattractive directȱ butconversionȱtheirȱdirect intoȱconversion ethanol isȱinto challengingȱethanolȱis andȱchallenging has notȱ beenandȱhas reportedȱnotȱbeen toȱreported be satisfactorilyȱtoȱbeȱsatisfactorily viable forȱ viableapplicationsȱforȱapplications in the industryȱinȱthe [ȱ33industry]. ȱ[33].ȱ BioBio-ethanolȬethanolȱ dehydrationdehydration isȱ anisȱ endothermicanȱ endothermic reaction,ȱ reaction, requiringȱ relativelyrequiringȱ moderaterelatively temperaturesȱ moderateȱ (i.e.,temperatures 180–500 ◦ȱC)(i.e., andȱ 180–500 the presenceȱ °C)ȱ and of aȱ catalyst.theȱ presence The mechanismȱ ofȱ aȱ catalyst. of bio-ethanolȱ Theȱ mechanism dehydrationȱ ofȱ bio consistsȬethanol ofȱ dehydrationthe followingȱconsists steps: (a)ȱof protonationȱtheȱfollowing ofȱsteps: the hydroxylȱ(a)ȱprotonation group byȱof anȱthe acidȱhydroxyl catalyst,ȱgroup (b) deprotonationȱbyȱanȱacidȱcatalyst, of the ȱ (b)ȱdeprotonationȱofȱtheȱmethylȱgroupȱbyȱtheȱconjugateȱbaseȱofȱtheȱcatalyst,ȱandȱ(c)ȱrearrangementȱtoȱ methyl group by the conjugate base of the catalyst, and (c) rearrangement to form C2H4 (Scheme2)[ 30]. formȱC2H4ȱ(Schemeȱ2)ȱ[30].ȱTheȱselectionȱofȱtheȱmostȱsuitableȱcatalystȱforȱbioȬethanolȱdehydrationȱ The selection of the most suitable catalyst for bio-ethanol dehydration into C2H4 is of key importance, intoin orderȱC2H4 toȱis lowerȱofȱkey reactionȱimportance, temperatureȱinȱorderȱ andtoȱlower overcomeȱreaction commonȱtemperature issues,ȱand suchȱovercome as catalystȱcommon deactivationȱissues,ȱ suchdue toȱas cokeȱ catalyst formation,ȱ deactivation particleȱ due collision,ȱ toȱ coke andȱ formation, agglomerationȱ particle [ȱ29collision,,30]. Acidȱ and catalysts,ȱagglomeration such asȱ zeolites[29,30].ȱ Acidand silicoaluminophosphatesȱcatalysts,ȱsuchȱasȱzeolites (SAPO),ȱandȱsilicoaluminophosphates have been widely usedȱ(SAPO), for thisȱhave reaction,ȱbeen overȱwidely theȱused last decades,ȱforȱthisȱ reaction,as they promoteȱoverȱthe selectiveȱlastȱdecades, conversionȱasȱthey intoȱpromote the desiredȱselective product,ȱconversion with extremelyȱintoȱtheȱdesired high conversionȱproduct,ȱwith andȱ extremelyselectivityȱhigh values,ȱconversion despiteȱ theand factȱselectivity that catalystsȱvalues, needȱdespite to beȱthe frequentlyȱfactȱthatȱcatalysts regeneratedȱneed [ȱ30toȱ,be34ȱ].frequently Actually,ȱ regeneratedSAPO catalystsȱ[30,34]. are significantlyȱActually,ȱSAPO activeȱcatalysts in this reaction,ȱareȱsignificantly reachingȱ 98.0%activeȱ selectivityinȱthisȱreaction, to ethylene,ȱreaching at 250ȱ98.0%◦C,ȱ selectivityover SAPO-11-4ȱtoȱethylene, and 98.4%,ȱatȱ250 atȱ°C, 340ȱover◦C,ȱ overSAPO Mn-SAPO-34Ȭ11Ȭ4ȱandȱ98.4%, [35,36ȱat].ȱ340 However,ȱ°C,ȱover overȱMn modifiedȬSAPOȬ34 HZSM-5ȱ[35,36].ȱ However,ȱ overȱ modifiedȱ HZSMȬ5ȱ andȱ MCMȬ41ȱ catalysts,ȱ selectivityȱ toȱ C2H4ȱ isȱ evenȱ higherȱ (i.e.,ȱ 99.0%)ȱ[37].ȱMoreover,ȱaluminaȬbasedȱcatalysts,ȱsuchȱasȱAl2O3ȬMgO/SiO2,ȱexhibitedȱhighȱconversionȱ andȱselectivityȱvaluesȱ(i.e.,ȱ99.0ȱandȱ97.0%,ȱrespectively)ȱrequiringȱregenerationȱoverȱlongerȱperiodsȱ ofȱ operationȱ [29].ȱ TungstenȬbasedȱ heteropolyacids,ȱ supportedȱ onȱ variousȱ substancesȱ areȱ highlyȱ selectiveȱ atȱ relativelyȱ lowȱ reactionȱ temperaturesȱ (i.e.,ȱ 180–250ȱ °C)ȱ [38].ȱ Itȱ hasȱ beenȱ provenȱ thatȱ acidȬbaseȱ sitesȱ onȱ theȱ catalystȱ surfaceȱ areȱ linkedȱ withȱ theȱ productȱ distributionȱ ofȱ thisȱ reaction;ȱ ethanolȱdehydrationȱmostȱprobablyȱproceedsȱviaȱtheȱformationȱofȱintermediateȱspeciesȱandȱhighlyȱ dependsȱonȱtemperature,ȱethanolȱpartialȱpressure,ȱandȱtheȱnatureȱofȱacidȬbaseȱsitesȱ[30,39–43].ȱInȱfact,ȱ asȱethanolȱdehydrationȱisȱanȱendothermicȱreaction,ȱtheȱreactionȱtemperatureȱstronglyȱaffectsȱC2H4ȱ yield.ȱBasicȱcatalysts,ȱsuchȱasȱMgOȱorȱCaOȱhaveȱalsoȱbeenȱusedȱwithȱresultsȱthatȱresembledȱthose,ȱ overȱacidicȱcatalysts.ȱPhosphoricȱacid,ȱoxides,ȱmolecularȱsieves,ȱandȱotherȱheteropolyȱacidȱcatalystsȱ

Catalysts 2018, 8, 2 6 of 19

and MCM-41 catalysts, selectivity to C2H4 is even higher (i.e., 99.0%) [37]. Moreover, alumina-based catalysts, such as Al2O3-MgO/SiO2, exhibited high conversion and selectivity values (i.e., 99.0 and 97.0%, respectively) requiring regeneration over longer periods of operation [29]. Tungsten-based heteropolyacids, supported on various substances are highly selective at relatively low reaction temperatures (i.e., 180–250 ◦C) [38]. It has been proven that acid-base sites on the catalyst surface are linked with the product distribution of this reaction; ethanol dehydration most probably proceeds via the formation of intermediate species and highly depends on temperature, ethanol partial pressure, and the nature of acid-base sites [30,39–43]. In fact, as ethanol dehydration is an endothermic reaction, the reaction temperature strongly affects C2H4 yield. Basic catalysts, such as MgO or CaO have also been usedCatalysts withȱ2018 results,ȱ8,ȱ2ȱ that resembled those, over acidic catalysts. Phosphoric acid, oxides, molecular sieves,6ȱofȱ18ȱ and other heteropoly acid catalysts can also be used for this purpose [29]. Syndol is a commercial catalyst,canȱalsoȱbe usedȱused byȱfor Halconȱthisȱpurpose SD (USA),ȱ[29]. toȱSyndol obtainȱ highisȱaȱcommercial ethanol conversionȱcatalyst,ȱ andused selectivityȱbyȱHalcon toȱSD ethylene,ȱ(USA),ȱ attoȱ 400–500obtainȱ high◦C[ȱ29ethanol,44–46ȱ].conversion Overall, aȱ selectionandȱ selectivity of the mostȱ toȱ ethylene, promisingȱ atȱ catalysts400–500ȱ for°Cȱ ethanol[29,ȱ 44–46]. conversionȱ Overall, intoȱ aȱ 2 4 Cselection2H4 areȱof presentedȱtheȱmost inȱpromising Table1. ȱcatalystsȱforȱethanolȱconversionȱintoȱC H ȱareȱpresentedȱinȱTableȱ1.ȱ

ȱ

SchemeSchemeȱ 2.2.ȱMechanismMechanismȱ ofofȱ bio-ethanolbioȬethanolȱ dehydrationdehydrationȱ totoȱ CC22H44.ȱ

Tableȱ1.ȱSelectedȱcatalystsȱforȱbioȬethanolȱdehydrationȱtoȱC2H4.ȱ Table 1. Selected catalysts for bio-ethanol dehydration to C2H4. Ethanolȱ Selectivityȱtoȱ Catalystȱ Ethanol Selectivity to Temperatureȱ(°C) WHSVȱa/LHSVȱbȱ(hƺ1)ȱ Referenceȱ Catalyst Conversionȱ(%)ȱ C2H4ȱ(%)ȱ Temperature (◦C) WHSV a/LHSV b (h−1) Reference Conversion (%) C H (%) MnȬSAPOȬ34ȱ 99.4ȱ 298.44 ȱ 340ȱ 2.0ȱaȱ [35]ȱ a 0.5%LaMn-SAPO-34Ȭ2%PȬHZSMȬ5ȱ 100.0 99.4ȱ 99.9 98.4ȱ 240–280 340ȱ 2.0 2.0ȱaȱ [[47]35] ȱ a 0.5%La-2%P-HZSM-5TPAȬMCMȬ41ȱ*ȱ 100.098.0ȱ 99.9 99.9ȱ 240–280300ȱ 2.9 2.0ȱaȱ [[48]47] ȱ TPA-MCM-41 * 98.0 99.9 300 2.9 a [48] SynDolȱ**ȱ 99.0ȱ 96.8ȱ 450ȱ 26–234ȱbȱ [29,ȱ44–46]ȱ SynDol ** 99.0 96.8 450 26–234 b [29,44–46] STAȬMCMȬ41ȱ***ȱ 99.0ȱ 99.9ȱ 250ȱ 2.9ȱaȱ [49]ȱ STA-MCM-41 *** 99.0 99.9 250 2.9 a [49] *ȱtungstophosphoricȱacidȱ(TPA),ȱ**ȱMgOȬAl2O3/SiO2,ȱ***ȱsilicotungstica ȱacidȱ(STA).ȱ * tungstophosphoric acid (TPA), ** MgO-Al2O3/SiO2, *** silicotungstic acid (STA). Weight hourly space velocity b (WHSV), LiquidaȱWeight hourlyȱhourly spaceȱspace velocityȱvelocity (LHSV).ȱ(WHSV),ȱbȱLiquidȱhourlyȱspaceȱvelocityȱ(LHSV)ȱ Methanolȱ isȱ anotherȱ alcoholȱ thatȱ canȱ beȱ convertedȱ intoȱ olefins,ȱ throughȱ theȱ wellȬknownȱ Methanol is another alcohol that can be converted into olefins, through the well-known methanolȬtoȬolefinsȱ(MTO)ȱprocess.ȱTheȱMTOȱreactionȱisȱoneȱofȱtheȱmostȱimportantȱprocessesȱforȱ methanol-to-olefins (MTO) process. The MTO reaction is one of the most important processes producingȱ olefinsȱ fromȱ aȱ C1ȱ feedstockȱ [50].ȱ MTOȱ wasȱ initiallyȱ proposed,ȱ inȱ 1977,ȱ byȱ Mobilȱ for producing olefins from a C1 feedstock [50]. MTO was initially proposed, in 1977, by Mobil Corporationȱ [51],ȱ followedȱ byȱ numerousȱ researchȱ studies,ȱ focusingȱ onȱ theȱ developmentȱ ofȱ aȱ Corporation [51], followed by numerous research studies, focusing on the development of a commerciallyȱavailableȱtechnologyȱ[52,17].ȱWithinȱthisȱcontext,ȱtheȱfirstȱMTOȱplantȱwasȱinstalledȱinȱ commercially available technology [17,52]. Within this context, the first MTO plant was installed 2010,ȱinȱChina,ȱforȱtheȱproductionȱofȱlightȱolefinsȱfromȱcoalȱ[53].ȱ in 2010, in China, for the production of light olefins from coal [53]. BioȬmethanolȱ canȱ beȱ producedȱ viaȱ severalȱ processesȱ includingȱ pyrolysis,ȱ bioȬsynthesis,ȱ Bio-methanol can be produced via several processes including pyrolysis, bio-synthesis, gasification,ȱandȱelectrolysis,ȱusingȱaȱwideȱrangeȱofȱbiomassȱwaste,ȱsuchȱasȱagricultural,ȱforest,ȱandȱ gasification, and electrolysis, using a wide range of biomass waste, such as agricultural, forest, municipalȱ [17,ȱ 53–55].ȱ Althoughȱ mostȱ ofȱ theseȱ processesȱ areȱ currentlyȱ underȱ development,ȱ theȱ and municipal [17,53–55]. Although most of these processes are currently under development, the installationȱ ofȱ bioȬmethanolȱ productionȱ unitsȱ requiresȱ furtherȱ studiesȱ onȱ designȱ andȱ energyȱ installation of bio-methanol production units requires further studies on design and energy efficiency efficiencyȱinȱorderȱtoȱensureȱfeasibilityȱofȱlargeȬscaleȱapplicationȱ[56].ȱ in order to ensure feasibility of large-scale application [56]. MTOȱisȱanȱautocatalyticȱreactionȱthatȱconventionallyȱtakesȱplaceȱatȱmoderateȱtemperatureȱ(i.e.,ȱ MTO is an autocatalytic reaction that conventionally takes place at moderate temperature 300–450ȱ°C),ȱoverȱacidicȱcatalysts.ȱAȱnumberȱofȱsuggestionsȱonȱtheȱmechanismȱofȱMTOȱhasȱbeenȱ (i.e., 300–450 ◦C), over acidic catalysts. A number of suggestions on the mechanism of MTO has proposed,ȱshowingȱthatȱmostȱconclusionsȱagreeȱthatȱtheȱreactionȱnetworkȱconsistsȱofȱatȱleastȱthreeȱ been proposed, showing that most conclusions agree that the reaction network consists of at least mainȱ pathways:ȱ (a)ȱ directȱ methanolȱ conversion;ȱ (b)ȱ directȱ conversionȱ ofȱ ethylene;ȱ andȱ (c)ȱ ethaneȱ three main pathways: (a) direct methanol conversion; (b) direct conversion of ethylene; and (c) ethane methylationȱbyȱmethanolȱ[52].ȱDependingȱonȱtheȱcatalystȱused,ȱMTOȱcanȱselectivelyȱyieldȱmainlyȱ methylation by methanol [52]. Depending on the catalyst used, MTO can selectively yield mainly C2H4 C2H4ȱ andȱ C3H6;ȱ mostȱ studiesȱ focusȱ onȱ methanolȱ conversionȱ intoȱ ethylene,ȱ asȱ theȱ mainȱ product.ȱ and C3H6; most studies focus on methanol conversion into ethylene, as the main product. (Scheme3) (Schemeȱ3)ȱOverȱSAPOȱcatalystsȱandȱzeolites,ȱbioȬmethanolȱcanȱbeȱselectivelyȱconvertedȱintoȱlightȱ olefins;ȱoverȱSAPOȬ34,ȱselectivityȱtoȱC2H4ȱandȱC3H6ȱisȱ60.0%,ȱatȱ350–425ȱ°Cȱ[57].ȱModificationȱonȱtheȱ catalystȱsynthesisȱprocedureȱstronglyȱaffectsȱproductȱdistribution.ȱDimethylȱetherȬtoȬolefinsȱ(DMTO)ȱ isȱ aȱ similarȱ processȱ thatȱ yieldsȱ theȱ sameȱ products.ȱ BioȬdimethylȱ etherȱ canȱ beȱ producedȱ fromȱ lignocellulosicȱbiomass,ȱviaȱpyrolysisȱandȱgasification,ȱresultingȱinȱtheȱformationȱofȱolefinsȱ(i.e.,ȱC2H4ȱ andȱC3H6)ȱandȱsyngas,ȱfollowingȱtheȱbioliq®ȱconceptȱdevelopedȱinȱKarlsruheȱInstituteȱofȱTechnology,ȱ whichȱisȱalreadyȱimplementedȱinȱaȱlargeȱscaleȱunitȱinȱGermanyȱ[58–60].ȱDMTOȱcanȱtakeȱplaceȱatȱhighȱ temperatureȱ(i.e.,ȱ723ȱ°C)ȱandȱlowȱpressureȱ(i.e.,ȱ4ȱbar),ȱfullyȱconvertingȱdimethylȱetherȱ(DME)ȱintoȱ C2H4ȱ(45.0%),ȱC3H6ȱ(39.0%),ȱbutenesȱ(8.0%),ȱandȱotherȱlightȱgasesȱ[10].ȱDMEȱtoȱolefinsȱconversionȱisȱ drivenȱbyȱaȱsubstantiallyȱcomplexȱreactionȱmechanismȱbasedȱonȱmethylation,ȱoligomerization,ȱandȱ hydrocarbonȱformationȱandȱcrackingȱreactions,ȱoverȱzeoliteȱcatalystsȱ[61].ȱ

Catalysts 2018, 8, 2 7 of 19

Over SAPO catalysts and zeolites, bio-methanol can be selectively converted into light olefins; over ◦ SAPO-34, selectivity to C2H4 and C3H6 is 60.0%, at 350–425 C[57]. Modification on the catalyst synthesis procedure strongly affects product distribution. Dimethyl ether-to-olefins (DMTO) is a similar process that yields the same products. Bio-dimethyl ether can be produced from lignocellulosic biomass, via pyrolysis and gasification, resulting in the formation of olefins (i.e., C2H4 and C3H6) and syngas, following the bioliq® concept developed in Karlsruhe Institute of Technology, which is already implemented in a large scale unit in Germany [58–60]. DMTO can take place at high temperature ◦ (i.e., 723 C) and low pressure (i.e., 4 bar), fully converting dimethyl ether (DME) into C2H4 (45.0%), C3H6 (39.0%), butenes (8.0%), and other light gases [10]. DME to olefins conversion is driven by a substantially complex reaction mechanism based on methylation, oligomerization, and hydrocarbon formationCatalystsȱ2018 and,ȱ8,ȱ2ȱ cracking reactions, over zeolite catalysts [61]. 7ȱofȱ18ȱ

ȱ Schemeȱ 3.ȱBioBio-methanolȬmethanolȱconversion ȱinto ȱolefins olefinsȱ (MTO).ȱ

AnȱoverallȱcomparisonȱofȱtheȱmainȱC2H4ȱproductionȱprocessesȱisȱshownȱinȱTableȱ2.ȱDespiteȱofȱ An overall comparison of the main C2H4 production processes is shown in Table2. Despite of the theȱ technologicalȱ advancesȱ ofȱ theȱ lastȱ decades,ȱ bioȬethyleneȱ productionȱ processesȱ cannotȱ replaceȱ technological advances of the last decades, bio-ethylene production processes cannot replace those thoseȱdependentȱonȱfossilȱresources.ȱAsȱbioȬsynthesisȱprocessesȱareȱaȱrecentȱresearchȱsubject,ȱfurtherȱ dependent on fossil resources. As bio-synthesis processes are a recent research subject, further studies studiesȱ onȱ theȱ reductionȱ ofȱ theȱ costȱ andȱ onȱ increasingȱ theȱ productivityȱ areȱ essentialȱ priorȱ toȱ on the reduction of the cost and on increasing the productivity are essential prior to considering their consideringȱtheirȱindustrialȱapplication.ȱBioȬethanolȱdehydrationȱisȱtheȱmostȱpromisingȱalternativeȱ industrial application. Bio-ethanol dehydration is the most promising alternative for the production forȱtheȱproductionȱofȱ“green”ȱC2H4,ȱasȱnumerousȱstudiesȱhaveȱfocusedȱonȱtheȱselectionȱofȱtheȱmostȱ of “green” C2H4, as numerous studies have focused on the selection of the most suitable catalyst, suitableȱcatalyst,ȱloweringȱtheȱreactionȱtemperature,ȱwhileȱincreasingȱtheȱyieldȱtoȱC2H4.ȱAsȱethanolȱ lowering the reaction temperature, while increasing the yield to C2H4. As ethanol dehydration has dehydrationȱhasȱalreadyȱbeenȱimplemented,ȱtheȱonlyȱsetȬbackȱforȱindustrialȱbioȬethanolȱdehydrationȱ already been implemented, the only set-back for industrial bio-ethanol dehydration to C2H4 is the toȱC2H4ȱisȱtheȱproductionȱandȱavailabilityȱofȱbioȬethanol.ȱMTOȱandȱDMTOȱprocessesȱhaveȱalsoȱbeenȱ production and availability of bio-ethanol. MTO and DMTO processes have also been extensively extensivelyȱ studiedȱ withȱ encouragingȱ resultsȱ regardingȱ theirȱ commercialȱ applications.ȱ Likewise,ȱ studied with encouraging results regarding their commercial applications. Likewise, bio-methanol bioȬmethanolȱandȱbioȬDMEȱproductionȱisȱalsoȱlimited,ȱloweringȱprospectȱproductivity.ȱHowever,ȱ and bio-DME production is also limited, lowering prospect productivity. However, future increases futureȱincreasesȱofȱtheȱproducedȱbioȬfeedstocksȱcouldȱeliminateȱthisȱissue,ȱachievingȱhighȱyields,ȱinȱ of the produced bio-feedstocks could eliminate this issue, achieving high yields, in cost-competitive costȬcompetitiveȱprocesses,ȱasȱsteamȬcrackingȱunits.ȱFutureȱstudiesȱshouldȱfocusȱonȱcostȱreductionȱ processes, as steam-cracking units. Future studies should focus on cost reduction linked with the linkedȱwithȱtheȱimplementationȱofȱtheȱbioȬbasedȱmethods.ȱ implementation of the bio-based methods.

Tableȱ2.ȱComparisonȱofȱC2H4ȱproductionȱprocesses.ȱ Table 2. Comparison of C2H4 production processes. BioȬEthanolȱ Processȱ SteamȱCrackingȱ BioȬSynthesisȱ MTOȱ DMTOȱ DehydrationBio-Ethanol ȱ Process Steam Cracking Bio-Synthesis MTO DMTO Feedstockȱ HCȱ(Naphtha)ȱ ACC/SAMȱ DehydrationBioȬethanolȱ BioȬmethanolȱ BioȬDMEȱ OperatingFeedstockȱ HC675–700 (Naphtha)ȱ°Cȱ Ambient, ACC/SAMȱaerobicȱ Bio-ethanol Bio-methanol300–500ȱ°C,ȱ Bio-DME675–750ȱ°C,ȱ 180–500ȱ°Cȱ ConditionsOperatingȱ 675–700atmospheric◦C atmosphericȱpressureȱ Ambient,conditions aerobicȱ 300–500lowȱpressure◦C, ȱ 675–750lowȱpressure◦C, 180–500 ◦C Conditions Alreadypressureȱinstalledȱ Selectiveconditionsȱsustainableȱ Commercialȱ low pressure low pressure Advantagesȱ Closeȱtoȱcommercialȱapplication Alreadytechnology installedȱ Selectiveproduction sustainableȱ Commercialapplicationȱ Advantages Close to commercial application Energytechnologyȱintense,ȱ production application Limitedȱ environmentalEnergy intense,ȱ Increasedȱcost,ȱlowȱ Limited Limitedȱproductionȱofȱ Disadvantagesȱ Increased cost, low bioȬethanolȱ Limited production of Disadvantages environmentalconcerns,ȱfinite concerns,ȱ productivityȱ bio-ethanol bioȬfeedstocksȱ productivity supplyȱ bio-feedstocks finiteresources resourcesȱ supply YieldȱtoȱC2H4ȱ 31.3%ȱȬȱ99.9%ȱ 41.5%ȱ 45.0%ȱ Yield to C2H4 31.3% - 99.9% 41.5% 45.0%

2.2.ȱPropyleneȱ(C3H6)ȱ 2.2. Propylene (C3H6) AsȱtheȱdemandȱforȱC3H6ȱincreases,ȱresearchȱfocusesȱonȱonȬpurposeȱproductionȱofȱC3H6,ȱbasedȱ As the demand for C3H6 increases, research focuses on on-purpose production of C3H6, based onȱ biomassȱ resources,ȱ aimingȱ atȱ substitutingȱ oil-basedoilȬbasedȱ feedstocks,ȱin ȱmore ȱ environmentallyȱ friendlyȱ processes.ȱ Corn,ȱ vegetableȱoils ȱ andȱ otherȱ biomassȱ productsȱ haveȱ beenȱ effectivelyȱ usedȱ asȱ feedstocksȱ forȱ theȱproduction ȱof ȱbio bio-propylene,Ȭpropylene,ȱvia ȱprocesses ȱsuch ȱ asȱgasification, gasification,ȱmetathesis, metathesis,ȱdehydrogenation, dehydrogenation,ȱ fermentation,ȱandȱcrackingȱ[22].ȱ fermentation, and cracking [22].

ȱ

Catalysts 2018, 8, 2 7 of 18

Scheme 3. Bio‐methanol conversion into olefins (MTO).

An overall comparison of the main C2H4 production processes is shown in Table 2. Despite of the technological advances of the last decades, bio‐ethylene production processes cannot replace those dependent on fossil resources. As bio‐synthesis processes are a recent research subject, further studies on the reduction of the cost and on increasing the productivity are essential prior to considering their industrial application. Bio‐ethanol dehydration is the most promising alternative for the production of “green” C2H4, as numerous studies have focused on the selection of the most suitable catalyst, lowering the reaction temperature, while increasing the yield to C2H4. As ethanol dehydration has already been implemented, the only set‐back for industrial bio‐ethanol dehydration to C2H4 is the production and availability of bio‐ethanol. MTO and DMTO processes have also been extensively studied with encouraging results regarding their commercial applications. Likewise, bio‐methanol and bio‐DME production is also limited, lowering prospect productivity. However, future increases of the produced bio‐feedstocks could eliminate this issue, achieving high yields, in cost‐competitive processes, as steam‐cracking units. Future studies should focus on cost reduction linked with the implementation of the bio‐based methods.

Table 2. Comparison of C2H4 production processes.

Catalysts 2018, 8, 2 Bio‐Ethanol 8 of 19 Process Steam Cracking Bio‐Synthesis MTO DMTO Dehydration Feedstock HC (Naphtha) ACC/SAM Bio‐ethanol Bio‐methanol Bio‐DME OperatingConventionally, 675–700 C H is°C a by-productAmbient, of C aerobicH production via steam cracking300–500 °C, of hydrocarbons675–750 °C, 3 6 2 4 180–500 °C andConditions FCC of gas oil.atmospheric (Figure7 pressure) At high temperatures,conditions various hydrocarbons (e.g.,low pressure naphtha, ethane,low pressure and Already installed Selective sustainable Commercial propane)Advantages are co-fed, under the most suitable operating conditions, to selectivelyClose yieldto commercial C3H6 [application62]. FCC, technology production application also uses hydrocarbonsEnergy to intense, produce C3H6, at moderate pressure and high temperatures, over zeolites, Limited such as, ZSM-5, oftenenvironmental modified with metalsIncreased to increase cost, low selectivity to C3H6 [63]. ItLimited is considered production greener of Disadvantages bio‐ethanol concerns, finite productivity bio‐feedstocks than steam cracking due to lower energy demand and decreasedsupply CO2 emissions. Apart from the resources well-known steam cracking and FCC, olefin metathesis and methanol to C3H6 are also considered Yield to C2H4 31.3% ‐ 99.9% 41.5% 45.0% alternatives for industrial application. Moreover, alkanes can be converted into alkenes via catalytic dehydrogenation; propane can be used as a feedstock, at high temperature and atmospheric pressure. 2.2. Propylene (C3H6) Propane dehydrogenation plants have already been installed worldwide by several companies; Linde/BASFAs the demand (Alabama, for C USA),3H6 increases, Lummus research Technology focuses (New on on Jersey,‐purpose USA), production Snamprogetti/Yarsintez of C3H6, based on(Jubail, biomass Saudi resources, Arabia), aiming UOP (Illinois, at substituting USA), etc.,oil‐based mostly feedstocks, using chromium, in more environmentally and Pt–Sn catalysts friendly [64]. processes.Methanol toCorn, C3H vegetable6 is actually oils includedand other in biomass methanol products to olefins have reactions, been effectively described used in as the feedstocks previous forchapter; the production zeolites (i.e., of bio ZSM-5)‐propylene, are the via most processes active catalysts such as gasification, in order to selectively metathesis, produce dehydrogenation, C3H6 from fermentation,methanol [65]. and cracking [22].

Figure 7. Schematic chart of C3H6 production methods.

As mentioned above, bio-ethanol can be produced through various methods. Apart from constituting a valuable feedstock for C2H4 production, bio-ethanol can also be used for C3H6 production; C2H4 from bio-ethanol can undergo dimerization followed by the well-known metathesis reaction, along with butenes, thus producing C3H6 [66]. For this process, not only bio-ethylene can be derived from biomass (i.e., bio-ethanol), but also bio-butylene from bio-butanol dehydration [67]. In fact, butanol can be produced from bio-ethanol, via the Guerbet process, where higher alcohols can be formed, upon condensation of two primary alcohols [68]. For this purpose, basic oxides, such as MgO and hydrotalcites have been active in ethanol conversion into butanol, reaching 85% selectivity [69]. N-butanol can also be produced through fermentation; several companies (e.g., Versalis -San Donato Milanese, Italy, Global Bioenergies - Evry, France) have already developed biochemical processes for the production of C4 alcohols. Through the ABE process (Acetone-Butanol-Ethanol), C4 alcohols can be produced via carbohydrate fermentation by genetically modified micro-organisms [39]. The metathesis reaction can yield C3H6 using C2H4 and butenes as a feedstock, via two different approaches: (a) dimerization of bio-ethylene and then reaction with remaining bio-ethylene and (b) direct reaction of bio-ethylene and bio-butene. (Scheme4)C 2H4 dimerization processes have already been implemented in the industry (e.g., AlphaButol by Axens- Rueil-Malmaison, France), operating under relatively mild conditions (i.e., 0–100 ◦C) and over metal catalysts, such as Ti and Ni [70]. In direct reaction processes, (e.g., Olefin Conversion Technology-OCT ABB Lummus Global), both homogeneous and heterogeneous catalysts have been employed. Heterogeneous (e.g., tungsten, Catalystsȱ2018,ȱ8,ȱ2ȱ 8ȱofȱ18ȱ

Figureȱ7.ȱSchematicȱchartȱofȱC3H6ȱproductionȱmethods.ȱ

Conventionally,ȱC3H6ȱisȱaȱbyȬproductȱofȱC2H4ȱproductionȱviaȱsteamȱcrackingȱofȱhydrocarbonsȱ andȱFCCȱofȱgasȱoil.ȱ(Figureȱ7)Atȱhighȱtemperatures,ȱvariousȱhydrocarbonsȱ(e.g.,ȱnaphtha,ȱethane,ȱandȱ propane)ȱareȱcoȬfed,ȱunderȱtheȱmostȱsuitableȱoperatingȱconditions,ȱtoȱselectivelyȱyieldȱC3H6ȱ[62].ȱFCC,ȱ alsoȱusesȱhydrocarbonsȱtoȱproduceȱC3H6,ȱatȱmoderateȱpressureȱandȱhighȱtemperatures,ȱoverȱzeolites,ȱ suchȱ as,ȱ ZSMȬ5,ȱ oftenȱ modifiedȱ withȱ metalsȱ toȱ increaseȱ selectivityȱ toȱ C3H6ȱ [63].ȱ Itȱ isȱ consideredȱ greenerȱthanȱsteamȱcrackingȱdueȱtoȱlowerȱenergyȱdemandȱandȱdecreasedȱCO2ȱemissions.ȱApartȱfromȱ theȱ wellȬknownȱ steamȱ crackingȱ andȱ FCC,ȱ olefinȱ metathesisȱ andȱ methanolȱ toȱ C3H6ȱ areȱ alsoȱ consideredȱalternativesȱforȱindustrialȱapplication.ȱMoreover,ȱalkanesȱcanȱbeȱconvertedȱintoȱalkenesȱ viaȱ catalyticȱ dehydrogenation;ȱ propaneȱ canȱ beȱ usedȱ asȱ aȱ feedstock,ȱ atȱ highȱ temperatureȱ andȱ atmosphericȱpressure.ȱPropaneȱdehydrogenationȱplantsȱhaveȱalreadyȱbeenȱinstalledȱworldwideȱbyȱ severalȱ companies;ȱ Linde/BASFȱ (Alabama,ȱ USA),ȱ Lummusȱ Technologyȱ (Newȱ Jersey,ȱ USA),ȱ Snamprogetti/Yarsintezȱ (Jubail,ȱ Saudiȱ Arabia),ȱ UOPȱ (Illinois,ȱ USA),ȱ etc.,ȱ mostlyȱ usingȱ chromium,ȱ andȱ Pt–Snȱ catalystsȱ [64].ȱ Methanolȱ toȱ C3H6ȱ isȱ actuallyȱ includedȱ inȱ methanolȱ toȱ olefinsȱ reactions,ȱ describedȱ inȱ theȱ previousȱ chapter;ȱ zeolitesȱ (i.e.,ȱ ZSMȬ5)ȱ areȱ theȱ mostȱ activeȱ catalystsȱ inȱ orderȱ toȱ selectivelyȱproduceȱC3H6ȱfromȱmethanolȱ[65].ȱ Asȱ mentionedȱ above,ȱ bioȬethanolȱ canȱ beȱ producedȱ throughȱ variousȱ methods.ȱ Apartȱ fromȱ constitutingȱ aȱ valuableȱ feedstockȱ forȱ C2H4ȱ production,ȱ bioȬethanolȱ canȱ alsoȱ beȱ usedȱ forȱ C3H6ȱ production;ȱ C2H4ȱ fromȱ bioȬethanolȱ canȱ undergoȱ dimerizationȱ followedȱ byȱ theȱ wellȬknownȱ metathesisȱ reaction,ȱ alongȱ withȱ butenes,ȱ thusȱ producingȱ C3H6ȱ [66].ȱ Forȱ thisȱ process,ȱ notȱ onlyȱ bioȬethyleneȱcanȱbeȱderivedȱfromȱbiomassȱ(i.e.,ȱbioȬethanol),ȱbutȱalsoȱbioȬbutyleneȱfromȱbioȬbutanolȱ dehydrationȱ[67].ȱInȱfact,ȱbutanolȱcanȱbeȱproducedȱfromȱbioȬethanol,ȱviaȱtheȱGuerbetȱprocess,ȱwhereȱ higherȱalcoholsȱcanȱbeȱformed,ȱuponȱcondensationȱofȱtwoȱprimaryȱalcoholsȱ[68].ȱForȱthisȱpurpose,ȱ basicȱoxides,ȱsuchȱasȱMgOȱandȱhydrotalcitesȱhaveȱbeenȱactiveȱinȱethanolȱconversionȱintoȱbutanol,ȱ reachingȱ 85%ȱ selectivityȱ [69].ȱ NȬbutanolȱ canȱ alsoȱ beȱ producedȱ throughȱ fermentation;ȱ severalȱ companiesȱ (e.g.,ȱ VersalisȱȬSanȱ Donatoȱ Milanese,ȱ Italy,ȱ Globalȱ BioenergiesȱȬȱEvry,ȱ France)ȱ haveȱ alreadyȱ developedȱ biochemicalȱ processesȱ forȱ theȱ productionȱ ofȱ C4ȱ alcohols.ȱ Throughȱ theȱ ABEȱ processȱ(AcetoneȬButanolȬEthanol),ȱC4ȱalcoholsȱcanȱbeȱproducedȱviaȱcarbohydrateȱfermentationȱbyȱ geneticallyȱmodifiedȱmicroȬorganismsȱ[39].ȱ TheȱmetathesisȱreactionȱcanȱyieldȱC3H6ȱusingȱC2H4ȱandȱbutenesȱasȱaȱfeedstock,ȱviaȱtwoȱdifferentȱ approaches:ȱ(a)ȱdimerizationȱofȱbioȬethyleneȱandȱthenȱreactionȱwithȱremainingȱbioȬethyleneȱandȱ(b)ȱ directȱreactionȱofȱbioȬethyleneȱandȱbioȬbutene.ȱ(Schemeȱ4)ȱC2H4ȱdimerizationȱprocessesȱhaveȱalreadyȱ beenȱimplementedȱinȱtheȱindustryȱ(e.g.,ȱAlphaButolȱbyȱAxensȬȱRueilȬMalmaison,ȱFrance),ȱoperatingȱ underȱrelativelyȱmildȱconditionsȱ(i.e.,ȱ0–100ȱ°C)ȱandȱoverȱmetalȱcatalysts,ȱsuchȱasȱTiȱandȱNiȱ[70].ȱInȱ Catalysts 2018, 8, 2 9 of 19 directȱ reactionȱ processes,ȱ (e.g.,ȱ Olefinȱ Conversionȱ TechnologyȬOCTȱ ̄̅̅ȱ Lummusȱ Global),ȱ bothȱ homogeneousȱ andȱ heterogeneousȱ catalystsȱ haveȱ beenȱ employed.ȱ Heterogeneousȱ (e.g.,ȱ tungsten,ȱ molybdenum,ȱor orȱRhenium Rheniumȱoxides, oxides,ȱon onȱalumina aluminaȱorȱsilica) or silica)ȱareȱ areusually usuallyȱpreferred preferredȱoverȱ overhomogeneous, homogeneous,ȱsuchȱ suchasȱ organometallic as organometallicȱ complexes. complexes.ȱ Tungsten Tungstenȱ oxides, oxides,ȱ supported supportedȱ onȱ onsilica silicaȱ have haveȱ been beenȱ used usedȱ for forȱ the theȱ OCT ȱ ◦ process,ȱat ȱtemperatures temperaturesȱabove aboveȱ260 260ȱ°CCȱand andȱ30–35 30–35ȱbar, bar,ȱreaching reachingȱmore moreȱthan thanȱ90.0% 90.0%ȱselectivity selectivityȱtoȱC to3H C63,ȱHfor6,ȱ for60.0% 60.0%ȱ butene buteneȱ conversion conversionȱ [71].ȱ [71Pre].Ȭreduction Pre-reductionȱ treatment, treatment,ȱ asȱ well asȱ as wellȱ increased as increasedȱ acidity acidityȱ obtained obtainedȱ usingȱ usingmoreȱacidic moreȱ acidicsupports, supports,ȱenhances enhancesȱcatalytic catalyticȱactivity activityȱatȱlower at lowerȱtemperature temperatureȱ[72].ȱRhenium [72]. Rheniumȱcatalysts catalystsȱhaveȱ haveexhibited exhibitedȱ selectivity selectivityȱ ~100%, ~100%,ȱ however howeverȱ fast fastȱ deactivation deactivationȱ requires requiresȱ continuous continuousȱ regeneration regenerationȱ [73]. [73].ȱ Molybdenaȱ catalystsȱ haveȱ alsoȱbeen ȱused ȱin ȱthe ȱShell ȱHigh ȱOlefin OlefinȱProcess ȱ(SHOP), ȱproducing ȱ΅Ȭα-olefinsolefinsȱ throughȱthe ȱ oligomerizationȱof ofȱC C22HH4,4ȱfollowed, followedȱby byȱolefin olefinȱmetathesis metathesisȱ[74]. [74ȱ].

ȱ

Scheme 4. C3H6 formation via metathesis.

Solely 1-butene, produced via bio-butanol dehydration, can also be used as a feedstock for C3H6 formation, as 1-butene is isomerized to 2-butene, and then both react via metathesis to form C3H6 and 2-pentene [75]. Moreover, oligomerization/cracking of C2H4 from bio-ethanol can result in C3H6 production [76–78]. The direct ethanol to C3H6 process (ETP) has recently been explored, over catalysts such as zeolites [79–81] and metal oxide catalysts [82–84] in order to increase yield to C3H6 against the most common by-products (i.e., C2H4, butenes and aromatic hydrocarbons), improve stability and suppress coke formation. Thus, as the yield to C3H6 rarely exceeds 40.0%, the need for novel catalytic materials is imperative [85]. Other biomass-derived oxygenates, such as polyols, aldehydes, and ketones can also be converted into hydrocarbons through oxygen removal catalytic reactions (hydrogenation/ dehydrogenation, and hydro-deoxygenation). In this context, over transition metal oxides, glycerol and other C3 oxygenated compounds can be converted into C3H6. Glycerol is a low cost molecule that can be produced via fermentation, transesterification and hydrogenolysis reactions, from biomass feedstocks. It can be upgraded into valuable compounds through a number of chemical or biological routes [33]. Glycerol to olefins (GTO) methods require the complete removal of oxygen, which is an intricate task. Its catalytic conversion into C3H6 is a novel research subject that has recently attracted attention; Hultenberg and Brandin recently filed a patent introducing the production of lower hydrocarbons (e.g., ethane, propane, and propene) from glycerol, over WO3 on ZrO2 and Pt on CeO2 catalysts. [86] Fadigas et al. initially explored glycerol conversion, in a continuous flow process, over Ni, and Fe–Mo metal catalysts supported on activated carbon, obtaining high selectivity values to C3H6 [87]. Schmidt’s group reported glycerol cracking into propanal, acrolein, C3H6, and C2H4, in three steps (i.e., dehydration, hydrogenation, and upgrading), over HZSM-5 zeolites, Pd/α-Al2O3, and HBEA zeolites, respectively for each step/reaction [88]. Yu et al used Ir/ZrO2 and H-ZSM5 catalysts, in two steps, in order to selectively produce C3H6 via hydro-deoxygenation in a fixed-dual-bed reactor. Over the optimized reaction conditions ◦ (i.e., 250 C and 1 bar hydrogen pressure), selectivity to C3H6 reached 85.0%, for complete glycerol ◦ conversion [89]. (Table3) Sun et al. used WO 3-Cu/Al2O3 catalysts, at 250 C under hydrogen flow (atmospheric pressure) to obtain 47.4% selectivity to C3H6 for 100% glycerol conversion [91]. Combining WO3-Cu/Al2O3 and SiO2-Al2O3 in a dual-bed reactor, selectivity to C3H6 reached 84.8%. Mota et al, used Fe/Mo catalysts supported on activated carbon to obtain 100 glycerol conversion and ◦ 90.0% selectivity to C3H6 at 300 C[92]. In all the above studies, a number of by-products has been detected in the gas phase (e.g., carbon dioxide, propane, and ethylene). Further studies by our group, in a batch reactor, over molybdena-based catalysts supported on carbon, report glycerol production Catalysts 2018, 8, 2 10 of 19

into C3H6 with 100% selectivity in the gas phase (88.0% glycerol conversion and 76.0% selectivity to ◦ C3H6) at 300 C and under hydrogen atmosphere (80 bar) (Figure8)[ 90]. Our group has also proven that reducible molybdenum oxides selectively drive this reaction into C3H6, most probably via a reverse Mars—van Krevelen mechanism; formation of Mo4+ and Mo5+ species most likely drives the reaction to the formation of the desired product. C3H6 is most probably formed via two consecutive cycles, in a one-step reaction: (a) due to the presence of oxygen vacancies, the adsorption of glycerol proceeds with the two adjacent hydroxyls forming an unstable cyclic intermediate which in turn is released as 2-propenol and (b) the latter after re-adsorption with the remaining hydroxyl is further deoxygenated to C3H6 [93]. Dow Global Technologies have patented a GTO process in a batch reactor, reaching 96% selectivity to C3H6 for 24% glycerol conversion. Hydroiodic acid acts as a catalyst in subsequent reduction-oxidation cycles, over reductive atmosphere [92].

Table 3. Selected catalysts for one-step glycerol conversion into C3H6. Catalystsȱ2018,ȱ8,ȱ2ȱ 10ȱofȱ18ȱ Glycerol Selectivity to Catalyst Temperature (◦C) WHSV (h−1) Reference Conversion (%) C3H6 (%) GTOȱ processȱ inȱ aȱ batchȱ reactor,ȱ reachingȱ 96%ȱ selectivityȱ toȱ C3H6ȱ forȱ 24%ȱ glycerolȱ conversion.ȱ Ir/ZrO2 & HZSM-5-30 100.0 85.0 250 1.0 [89] HydroiodicFe–Mo/Blackȱ acid Carbonȱ actsȱ asȱ aȱ catalyst 88.0ȱ inȱ subsequent 76.0ȱ reduction 300Ȭoxidationȱ cycles, -ȱ overȱ reductive [90] ȱ atmosphereWO3-Cu/Alȱ2[92].O3 &ȱ SiO2-Al2O3 100.0 84.8 250 - [91] Fe/Mo 100.0 90.0 300 5.4 [92]

ȱ

Figure 8. Effect of reaction time on selectivity over the Fe–Mo/BC_A catalyst (H2 pressure: 8.0 MPa, Figureȱ8.ȱEffectȱofȱreactionȱtimeȱonȱselectivityȱoverȱtheȱFe–Mo/BC_Aȱcatalystȱ(H2ȱpressure:ȱ8.0ȱMPa,ȱ temperature: 300 ◦C). Reproduced from [90]. Copyright 2015, Royal Society of Chemistry. temperature:ȱ300ȱ°C).ȱReproducedȱfromȱ[90].ȱCopyrightȱ2015,ȱRoyalȱSocietyȱofȱChemistry.ȱ

Bio-oil,BioȬoil,ȱ producedproduced fromȱ from catalyticȱ catalytic pyrolysisȱ pyrolysis ofȱ of fats,ȱfats, oilsȱ oils andȱand otherȱ other low-valuesȱlowȬvalues compounds,ȱ compounds, can alsoȱ canȱ bealso usedȱbeȱused as aȱ feedstockasȱaȱfeedstock for theȱfor productionȱtheȱproduction of olefins,ȱofȱolefins, throughȱthrough processesȱprocesses thatȱthat areȱ alreadyareȱalready usedȱused in theȱinȱ petrochemicals’theȱ petrochemicals’ industryȱ industry [94]. Syntroleumȱ [94].ȱ Syntroleum Corporationȱ Corporation has alreadyȱ hasȱ implementedalreadyȱ implemented similar processedȱ similarȱ (i.e.,processed Bio-Synfining),ȱ(i.e.,ȱBioȬSynfining), where vegetableȱwhereȱvegetable oils andȱoils fatsȱand canȱfats beȱ convertedcanȱbeȱconverted into fuelsȱintoȱ andfuels propaneȱandȱpropane [95].ȱ Neste[95].ȱNeste Oil isȱOil alsoȱis usingȱalsoȱ theusing NExBTLȱtheȱNExBTL (Next Generationȱ(NextȱGeneration BiomassȱBiomass to Liquid)ȱtoȱ processLiquid)ȱ inprocess orderȱ toinȱ produceorderȱtoȱ liquidproduce fuelsȱliquid andȱfuels olefinsȱand [96ȱolefins]. Throughȱ[96].ȱ steamThrough crackingȱsteamȱ andcracking fluidȱ catalyticandȱfluid cracking,ȱcatalyticȱ liquidcracking, fuelsȱliquid and ȱ Cfuels3H6ȱandcanȱC be3H formed,6ȱcanȱbeȱ whileformed, viaȱwhile theȱ firstviaȱthe routeȱfirst olefinsȱrouteȱolefins are primarilyȱareȱprimarily producedȱproduced [97]. Theȱ[97]. firstȱTheȱ stepfirstȱ ofstep theȱof steamȱtheȱsteam crackingȱcracking processȱprocess is hydro-deoxygenationȱisȱhydroȬdeoxygenation of fattyȱofȱfatty acidsȱacids andȱand triglycerides,ȱtriglycerides, resultingȱresulting in ȱ greeninȱ green hydrocarbonsȱ hydrocarbons and naphtha.ȱ andȱ naphtha. Hydro-deoxygenationȱ HydroȬdeoxygenation proceeds overȱ proceeds conventionalȱ overȱ hydrotreatingconventionalȱ ◦ catalysts,hydrotreating underȱcatalysts, high hydrogenȱunder pressureȱhighȱhydrogen and moderateȱpressure temperaturesȱandȱmoderate (i.e.,ȱtemperatures 280–400 C) [ȱ98(i.e.,]. Nobleȱ280–400 metalȱ°C)ȱ catalysts[98].ȱNoble haveȱmetal alsoȱcatalysts been used,ȱhave supportedȱalsoȱbeen onȱused, carbon,ȱsupported silica,ȱ alumina,onȱcarbon, orȱsilica, zeolitesȱalumina, [99]. Theȱorȱzeolites secondȱ step[99].ȱ ◦ involvesTheȱ second gasolineȱ stepȱ andinvolves C3H6ȱ gasolineproductionȱ and viaȱ C cracking3H6ȱ production at atmosphericȱ viaȱ cracking pressure,ȱ atȱ atmospheric at 800 C. Milderȱ pressure, reactionȱ atȱ conditions800ȱ°C.ȱMilder enhanceȱreaction C3Hȱ6conditionsformation,ȱenhance while suppressingȱC3H6ȱformation, C2H4ȱwhileand aromaticsȱsuppressing production.ȱC2H4ȱandȱaromaticsȱ production.ȱ Tableȱ4ȱsummarizesȱtheȱmainȱcharacteristicsȱofȱtheȱmostȱimportantȱC3H6ȱproductionȱmethods.ȱ Throughȱ steamȱ crackingȱ C3H6ȱ canȱ beȱ formedȱ asȱ aȱ byȬproductȱ ofȱ C2H4ȱ production,ȱ atȱ highȱ temperatures,ȱinȱaȱmarkedlyȱenergyȱintenseȱprocess.ȱLowerȱtemperaturesȱofȱFCCȱmakeȱthisȱprocessȱ moreȱ environmentallyȱ friendly,ȱ comparedȱ toȱ steamȱ cracking.ȱ However,ȱ yieldȱ toȱ C3H6ȱ isȱ highlyȱ dependentȱ onȱ theȱ feedstockȱ andȱ onȱ theȱ selectedȱ condition.ȱ Catalystȱ deactivationȱ andȱ productȱ recoveryȱareȱamongȱtheȱdisadvantagesȱofȱthisȱratherȱpopularȱmethod.ȱPropaneȱdehydrogenationȱisȱ anotherȱ C3H6ȱ productionȱ processȱ fromȱ finiteȱ resourcesȱ thatȱ alsoȱ requiresȱ highȱ temperaturesȱ andȱ atmosphericȱ pressure.ȱ Recentlyȱ installedȱ unitsȱ highlightȱ theȱ potentialȱ ofȱ thisȱ applicationȱ forȱ onȬpurposeȱC3H6ȱproductionȱatȱhighȱyields.ȱViaȱmetathesisȱreactionsȱolefinsȱcanȱbeȱconvertedȱintoȱ C3H6,ȱ atȱ relativelyȱ lowȱ temperaturesȱ andȱ moderateȱ pressure.ȱ BioȬolefinsȱ areȱ theȱ mostȱ suitableȱ feedstockȱbutȱtheirȱavailabilityȱlimitsȱtheȱproductivityȱofȱthisȱmethod.ȱGlycerolȱtoȱolefinsȱisȱaȱnovelȱ researchȱsubjectȱthatȱaimsȱatȱtheȱproductionȱofȱ“green”ȱC3H6ȱthroughȱcatalyticȱreactions,ȱatȱmoderateȱ temperature.ȱNonetheless,ȱmoreȱinȬdepthȱresearchȱisȱessentialȱonȱtheȱmostȱsuitableȱcatalystsȱthatȱwillȱ selectivelyȱ driveȱ theȱ reaction.ȱ Evenȱ thoughȱ C3H6ȱ productionȱ fromȱ glycerolȱ isȱ stillȱ aȱ labȬscaleȱ application,ȱfutureȱstudiesȱfocusingȱonȱscaleȬupȱandȱtechnoȬeconomicȱanalysesȱwillȱenableȱindustrialȱ applicationȱofȱtheseȱbiomassȬvalorizationȱprocesses.ȱ ȱ

Catalysts 2018, 8, 2 11 of 19

Table4 summarizes the main characteristics of the most important C 3H6 production methods. Through steam cracking C3H6 can be formed as a by-product of C2H4 production, at high temperatures, in a markedly energy intense process. Lower temperatures of FCC make this process more environmentally friendly, compared to steam cracking. However, yield to C3H6 is highly dependent on the feedstock and on the selected condition. Catalyst deactivation and product recovery are among the disadvantages of this rather popular method. Propane dehydrogenation is another C3H6 production process from finite resources that also requires high temperatures and atmospheric pressure. Recently installed units highlight the potential of this application for on-purpose C3H6 production at high yields. Via metathesis reactions olefins can be converted into C3H6, at relatively low temperatures and moderate pressure. Bio-olefins are the most suitable feedstock but their availability limits the productivity of this method. Glycerol to olefins is a novel research subject that aims at the production of “green” C3H6 through catalytic reactions, at moderate temperature. Nonetheless, more in-depth research is essential on the most suitable catalysts that will selectively drive the reaction. Even though C3H6 production from glycerol is still a lab-scale application, future studies focusing on scale-up and techno-economicCatalystsȱ2018,ȱ8,ȱ2ȱ analyses will enable industrial application of these biomass-valorization processes.11ȱofȱ18ȱ

Tableȱ4.ȱComparisonȱofȱC3H6ȱproductionȱprocesses.ȱ Table 4. Comparison of C3H6 production processes. Processȱ SteamȱCrackingȱ FCC Dehydrogenation Metathesisȱ GTO FeedstockProcessȱ Steam CrackingHCȱ FCCHCȱ DehydrogenationPropaneȱ MetathesisBioȬolefinsȱ Glycerol GTO ȱ Feedstock HC HC Propane500–700ȱ°C,ȱ Bio-olefins0–260ȱ°C,ȱ Glycerol Operatingȱ 750–900ȱ°C,ȱ 500–550ȱ°C,ȱ 250–400ȱ°C,ȱ ◦ ◦ atmospheric◦ ȱ moderate◦ ȱ ◦ ConditionsOperating ȱ moderate750–900ȱpressureC, ȱ moderate500–550ȱpressureC, 500–700 C, 0–260 C, hydrogen250–400ȱpressureC, Conditions moderate pressure moderate pressure atmosphericpressure pressureȱ moderatepressure pressureȱ hydrogen pressure GreenerGreener thanȱthanȱ AlreadyAlready installedȱinstalledȱ SteamSteam Cracking,ȱCracking,ȱ AlreadyAlready installedȱinstalledȱ AlreadyAlready installedȱ SustainableSustainableȱ AdvantagesAdvantagesȱ technologytechnologyȱ FlexibilityFlexibility ofȱofȱ unitsunitsȱ installedunitsȱunitsȱ productionproductionȱ operationoperationȱ EnergyEnergy intense,ȱintense,ȱ YieldYield dependsȱdepends onȱonȱ Catalystȱ Require hydrogen environmental feedstock, catalyst Catalyst deactivation, LimitedLimited productionȱ Requireȱhydrogenȱ Disadvantages environmentalȱ feedstock,ȱcatalystȱ deactivation,ȱ atmosphere, Disadvantagesȱ concerns, finite deactivation, endothermic reaction ofproduction bio-feedstocksȱofȱ atmosphere,ȱ concerns,ȱfiniteȱ deactivation,ȱ endothermicȱ lab-scale resources product recovery bioȬfeedstocksȱ labȬscaleȱ resourcesȱ productȱrecoveryȱ reactionȱ YieldYieldȱto ȱ C3H6ȱ 18.0%18.0%ȱ 25.0%25.0%ȱ 85.0%85.0%ȱ 90.0%90.0%ȱ 90.0%90.0%ȱ

2.3.ȱButadiene ȱ(C 4HH66))ȱ Alternativeȱfeedstocks feedstocksȱhave haveȱbeen beenȱproposed proposedȱinȱorder in orderȱtoȱeffectively to effectivelyȱproduce produceȱbioȬbutadiene bio-butadieneȱfromȱ fromrenewable renewableȱ resources, resources,ȱ thusȱ thussubstituting substitutingȱ theȱ energy the energyȱ consuming consumingȱ processes processesȱ ofȱ naphtha of naphthaȱ cracking. cracking.ȱ Inȱ InFigure Figureȱ9,ȱ9the, theȱvarious variousȱproduction productionȱprocesses processesȱfor forȱC4 CH46ȱHare6 areȱsummarized. summarized.ȱ

ȱ

FigureFigureȱ 9.9.ȱSchematicȱ chartchartȱ ofofȱ CC44H6ȱproductionproductionȱmethods. ȱ

C4H6ȱ isȱ alsoȱ aȱ byȬproductȱ ofȱ C2H4ȱ productionȱ throughȱ crackingȱ ofȱ hydrocarbons.ȱ C4H6ȱ productionȱfromȱbioȬethanolȱisȱaȱpromisingȱalternativeȱthatȱhasȱbeenȱindustriallyȱimplementedȱpriorȱ toȱtheȱinstallationȱofȱnaphthaȱcrackingȱtechnologies;ȱethanolȱtoȱC4H6ȱprocessesȱhaveȱbeenȱusedȱsinceȱ 1920,ȱconstitutingȱtheȱmainȱproductionȱpracticeȱuntilȱtheȱendȱofȱWorldȱWarȱIIȱ[22,100].ȱMainȱroutesȱ ofȱ thisȱ processȱ includeȱ dehydrogenation,ȱ dehydration,ȱ andȱ condensationȱ overȱ suitableȱ catalysts,ȱ eitherȱinȱoneȬstepȱ(i.e.,ȱLebedevȱapproach)ȱ(Schemeȱ5)ȱorȱtwoȬstepȱprocessesȱ(i.e.,ȱOstromisslenskiȱ approach)ȱ(Schemeȱ6)ȱ[100].ȱInȱtheȱfirstȱcase,ȱmultifunctionalȱcatalystsȱhaveȱbeenȱemployed,ȱmainlyȱ aluminaȱandȱmagnesiaȬsilicaȱcatalysts,ȱviaȱC2H4ȱdimerizationȱandȱsubsequentȱmetathesis,ȱwhileȱatȱ theȱlatter,ȱethanolȱwasȱinitiallyȱdehydrogenatedȱintoȱacetaldehyde,ȱoverȱcopperȬbasedȱcatalysts,ȱatȱ moderateȱ temperatures,ȱ andȱ thenȱ ethanolȱ reactedȱ withȱ acetaldehydeȱ toȱ formȱ C4H6,ȱ overȱ Ta2O5ȱ catalyst,ȱasȱwellȱasȱotherȱoxides,ȱsupportedȱonȱsilicaȱ[101,102].ȱSeveralȱresearchȱprojectsȱsuggestȱthatȱ aldolȱcondensationȱofȱacetaldehydeȱisȱaȱkeyȱstepȱinȱethanolȱconversionȱintoȱC4H6ȱ[103].ȱOnȱtheȱotherȱ hand,ȱlatestȱworks,ȱincludingȱDRIFTSȱ(DiffuseȱReflectionȱInfraredȱSpectroscopy)ȱanalysesȱandȱDFTȱ (DensityȱFunctionalȱTheory)ȱcalculations,ȱproposeȱthatȱethanolȱcarbanionicȱintermediatesȱareȱofȱkeyȱ importanceȱforȱtheȱreactionȱmechanismȱ[104].ȱ

ȱ

Schemeȱ5.ȱC4H6ȱproductionȱthroughȱtheȱLebedevȱprocess.ȱ

Catalystsȱ2018,ȱ8,ȱ2ȱ 11ȱofȱ18ȱ

Tableȱ4.ȱComparisonȱofȱC3H6ȱproductionȱprocesses.ȱ

Processȱ SteamȱCrackingȱ FCC Dehydrogenation Metathesisȱ GTO Feedstockȱ HCȱ HCȱ Propaneȱ BioȬolefinsȱ Glycerolȱ 500–700ȱ°C,ȱ 0–260ȱ°C,ȱ Operatingȱ 750–900ȱ°C,ȱ 500–550ȱ°C,ȱ 250–400ȱ°C,ȱ atmosphericȱ moderateȱ Conditionsȱ moderateȱpressureȱ moderateȱpressure hydrogenȱpressure pressureȱ pressureȱ Greenerȱthanȱ Alreadyȱinstalledȱ SteamȱCracking,ȱ Alreadyȱinstalledȱ Alreadyȱ Sustainableȱ Advantagesȱ technologyȱ Flexibilityȱofȱ unitsȱ installedȱunitsȱ productionȱ operationȱ Energyȱintense,ȱ Yieldȱdependsȱonȱ Catalystȱ Limitedȱ Requireȱhydrogenȱ environmentalȱ feedstock,ȱcatalystȱ deactivation,ȱ Disadvantagesȱ productionȱofȱ atmosphere,ȱ concerns,ȱfiniteȱ deactivation,ȱ endothermicȱ bioȬfeedstocksȱ labȬscaleȱ resourcesȱ productȱrecoveryȱ reactionȱ YieldȱtoȱC3H6ȱ 18.0%ȱ 25.0%ȱ 85.0%ȱ 90.0%ȱ 90.0%ȱ

2.3.ȱButadieneȱ(C4H6)ȱ AlternativeȱfeedstocksȱhaveȱbeenȱproposedȱinȱorderȱtoȱeffectivelyȱproduceȱbioȬbutadieneȱfromȱ renewableȱ resources,ȱ thusȱ substitutingȱ theȱ energyȱ consumingȱ processesȱ ofȱ naphthaȱ cracking.ȱ Inȱ Figureȱ9,ȱtheȱvariousȱproductionȱprocessesȱforȱC4H6ȱareȱsummarized.ȱ

ȱ Catalysts 2018, 8, 2 12 of 19 Figureȱ9.ȱSchematicȱchartȱofȱC4H6ȱproductionȱmethods.ȱ

CC44HH66ȱ isisȱ also alsoȱ a ȱ by-productbyȬproductȱ ofofȱ CC22HH44ȱ productionproductionȱ throughthroughȱ crackingcrackingȱ ofofȱ hydrocarbons.hydrocarbons.ȱ CC44H6ȱ productionproductionȱfrom fromȱbio bio-ethanolȬethanolȱis isȱa aȱpromising promisingȱalternative alternativeȱthat thatȱhas hasȱbeen beenȱindustrially industriallyȱimplemented implementedȱprior priorȱ totoȱthe theȱinstallation installationȱ ofȱnaphtha ȱcracking ȱtechnologies; ȱethanol ȱto ȱ C4HH66ȱprocessesprocessesȱhave haveȱbeen beenȱused usedȱsince sinceȱ 1920,ȱconstituting ȱ theȱ mainmainȱ productionproductionȱ practicepractice untilȱuntil theȱthe endȱend ofȱof WorldȱWorld WarȱWar IIȱII [22ȱ[22,100].,100]. MainȱMain routesȱroutes ofȱ ofthisȱ this processȱ process includeȱ include dehydrogenation,ȱ dehydrogenation, dehydration,ȱ dehydration, and condensationȱ andȱ condensation over suitableȱ overȱ suitable catalysts,ȱ catalysts, either inȱ eitherone-stepȱinȱone (i.e.,Ȭstep Lebedevȱ(i.e.,ȱ approach)Lebedevȱapproach) (Scheme5ȱ(Scheme) or two-stepȱ5)ȱor processesȱtwoȬstepȱ (i.e.,processes Ostromisslenskiȱ(i.e.,ȱOstromisslenski approach)ȱ approach)(Scheme6)[ȱ(Scheme100]. Inȱ6) theȱ[100]. firstȱIn case,ȱtheȱfirst multifunctionalȱcase,ȱmultifunctional catalystsȱ havecatalysts beenȱhave employed,ȱbeenȱemployed, mainly aluminaȱmainlyȱ aluminaand magnesia-silicaȱandȱmagnesia catalysts,Ȭsilicaȱcatalysts, via C2H4ȱviadimerizationȱC2H4ȱdimerization and subsequentȱandȱsubsequent metathesis,ȱmetathesis, while atȱ thewhile latter,ȱatȱ theethanolȱlatter, wasȱethanol initiallyȱwas dehydrogenatedȱinitiallyȱdehydrogenated into acetaldehyde,ȱintoȱacetaldehyde, over copper-basedȱoverȱcopper catalysts,Ȭbasedȱ atcatalysts, moderateȱatȱ moderatetemperatures,ȱ temperatures, and then ethanolȱ andȱ then reactedȱ ethanol withȱ reacted acetaldehydeȱ withȱ toacetaldehyde form C4H6,ȱ overtoȱ form Ta2ȱOC54Hcatalyst,6,ȱ overȱ asTa well2O5ȱ catalyst,as otherȱ oxides,asȱwellȱ supportedasȱotherȱoxides, on silicaȱsupported [101,102ȱ].on Severalȱsilicaȱ[101,102]. research projectsȱSeveral suggestȱresearch thatȱprojects aldolȱ condensationsuggestȱthatȱ aldolof acetaldehydeȱcondensation is aȱof keyȱacetaldehyde step in ethanolȱisȱaȱkey conversionȱstepȱinȱethanol into C4Hȱconversion6 [103]. Onȱinto theȱ otherC4H6ȱ[103]. hand,ȱOn latestȱthe works,ȱotherȱ hand,includingȱlatest DRIFTSȱworks, (Diffuseȱincluding ReflectionȱDRIFTSȱ Infrared(Diffuseȱ Spectroscopy)ReflectionȱInfrared analysesȱSpectroscopy) and DFT (Densityȱanalyses FunctionalȱandȱDFTȱ (DensityTheory)ȱ calculations,FunctionalȱTheory) proposeȱcalculations, that ethanolȱpropose carbanionicȱthatȱethanol intermediatesȱcarbanionic are ofȱintermediates key importanceȱareȱ forofȱkey theȱ importancereaction mechanismȱforȱtheȱreaction [104]. ȱmechanismȱ[104].ȱ

ȱ

Catalystsȱ2018,ȱ8,ȱ2ȱ Schemeȱ5. ȱC44HH66ȱproductionproductionȱthrough throughȱthe theȱLebedev Lebedevȱprocess. process.ȱ 12ȱofȱ18ȱ

ȱ

Schemeȱ 6.ȱC4HH66ȱproductionproductionȱthrough throughȱthe theȱOstromisslenski Ostromisslenskiȱprocess. process.ȱ

ApartȱfromȱbioȬethanol,ȱC4H6ȱcanȱbeȱformedȱfromȱotherȱbiomassȱderivedȱoxygenatesȱsuchȱasȱ Apart from bio-ethanol, C4H6 can be formed from other biomass derived oxygenates such as butanolsȱandȱbutanediols,ȱproducedȱthroughȱbiomassȱfermentationȱorȱgasificationȱ[105,106].ȱInȱfact,ȱ butanols and butanediols, produced through biomass fermentation or gasification [105,106]. In fact, bioȬ1,4Ȭbutanediolȱ isȱ currentlyȱ producedȱ byȱ Genomatica,ȱ atȱ aȱ smallȱ scaleȱ plant,ȱ planningȱ onȱ bio-1,4-butanediol is currently produced by Genomatica, at a small scale plant, planning on installing installingȱanȱindustrialȱscaleȱoneȱwithȱNovamontȱ[107].ȱAcidȱcatalyzedȱdehydrationȱofȱbioȬbutanolsȱ an industrial scale one with Novamont [107]. Acid catalyzed dehydration of bio-butanols yields a yieldsȱ aȱ mixtureȱ ofȱ nȬbutenes.ȱ Subsequentȱ dehydrogenationȱ resultsȱ inȱ theȱ productionȱ ofȱ C4H6.ȱ mixture of n-butenes. Subsequent dehydrogenation results in the production of C4H6. N-butanol NȬbutanolȱcanȱbeȱconvertedȱintoȱ1ȬbuteneȱorȱnonlinearȱC4ȱolefins,ȱviaȱdehydration,ȱoverȱcatalystsȱ can be converted into 1-butene or nonlinear C4 olefins, via dehydration, over catalysts with mild withȱmildȱorȱhighȱacidity,ȱrespectively;ȱoverȱzeolitesȱaȱ~60%ȱyieldȱtoȱisobuteneȱhasȱbeenȱobtainedȱ or high acidity, respectively; over zeolites a ~60% yield to isobutene has been obtained [108]. [108].ȱ Theȱ overwhelmingȱ costȱ ofȱ theȱ buteneȱ dehydrogenationȱ stepȱ hasȱ ledȱ toȱ researchȱ onȱ directȱ The overwhelming cost of the butene dehydrogenation step has led to research on direct dehydration dehydrationȱ ofȱ butanediolsȱ toȱ C4H6,ȱ overȱ suitableȱ catalysts;ȱ overȱ sodiumȱ phosphateȱ catalysts,ȱ of butanediols to C4H6, over suitable catalysts; over sodium phosphate catalysts, 1,4-butanediol can be 1,4Ȭbutanediolȱ canȱ beȱ converted◦ ȱ intoȱ C4H6ȱ atȱ 280ȱ °C,ȱ whileȱ conversionȱ ofȱ 1,3Ȭbutanediolȱ requiresȱ converted into C4H6 at 280 C, while conversion of 1,3-butanediol requires higher temperatures, over higherȱtemperatures,ȱoverȱtheȱsameȱcatalystsȱ[109].ȱC4H6ȱyieldȱupȱtoȱ95%ȱhasȱbeenȱreportedȱfromȱ the same catalysts [109]. C4H6 yield up to 95% has been reported from 1,4-butanediol [110] and 90% 1,4Ȭbutanediolȱ [110]ȱ andȱ 90%ȱ fromȱ 1,3Ȭbutanediolȱ [103].ȱ Inȱ bothȱ cases,ȱ increasedȱ byȬproductȱ from 1,3-butanediol [103]. In both cases, increased by-product formation is a critical issue that further formationȱisȱaȱcriticalȱissueȱthatȱfurtherȱstudiesȱonȱcatalyticȱapproachesȱcouldȱaddress.ȱDehydrationȱ studies on catalytic approaches could address. Dehydration of other C4 diols (e.g., 2,3-butanediol) is ofȱ otherȱ C4ȱ diolsȱ (e.g.,ȱ 2,3Ȭbutanediol)ȱ isȱ muchȱ moreȱ challenging,ȱ requiringȱ multipleȱ andȱ moreȱ much more challenging, requiring multiple and more complex reaction steps [111]. 2,3-butanediol, complexȱreactionȱstepsȱ[111].ȱ2,3Ȭbutanediol,ȱproducedȱviaȱglucoseȱfermentation,ȱcanȱbeȱconvertedȱ produced via glucose fermentation, can be converted into C4H6, over scandium oxide catalysts, at intoȱ C4H6,ȱ overȱ scandiumȱoxideȱ catalysts,ȱ atȱ highȱ temperaturesȱ (i.e.,ȱ411ȱ °C),ȱ reachingȱ88%ȱyield.ȱ high temperatures (i.e., 411 ◦C), reaching 88% yield. Using two catalytic beds, with scandium oxide Usingȱtwoȱcatalyticȱbeds,ȱwithȱscandiumȱoxideȱandȱalumina,ȱselectivityȱtoȱC4H6ȱwasȱ94%,ȱprovingȱ and alumina, selectivity to C4H6 was 94%, proving the feasibility of direct double bond dehydration theȱ feasibilityȱ ofȱ directȱ doubleȱ bondȱ dehydrationȱ ofȱ 2,3Ȭbutanediolȱ [91].ȱ Alternatively,ȱ twoȬstepȱ of 2,3-butanediol [91]. Alternatively, two-step processes, using two different catalysts (e.g., silica processes,ȱusingȱtwoȱdifferentȱcatalystsȱ(e.g.,ȱsilicaȱorȱaluminaȬbased)ȱforȱsubsequentȱdehydrations,ȱ or alumina-based) for subsequent dehydrations, have also been proposed, yielding C4H6 via the haveȱ alsoȱ beenȱ proposed,ȱ yieldingȱ C4H6ȱ viaȱ theȱ formationȱ ofȱ unsaturatedȱ alcoholsȱ [112].ȱ Novelȱ formation of unsaturated alcohols [112]. Novel chemical and biochemical technologies enable the chemicalȱandȱbiochemicalȱtechnologiesȱenableȱtheȱproductionȱofȱC4H6ȱfromȱsyngasȱoriginatingȱfromȱ production of C4H6 from syngas originating from the gasification of biomass or waste gases from theȱgasificationȱofȱbiomassȱorȱwasteȱgasesȱfromȱtheȱsteelȱindustry;ȱbutanediolsȱcanȱbeȱproducedȱfromȱ the steel industry; butanediols can be produced from syngas via fermentation [113]. As syngas can syngasȱviaȱfermentationȱ[113].ȱAsȱsyngasȱcanȱbeȱproducedȱfromȱvariousȱorganicȱmaterials,ȱsuchȱasȱ be produced from various organic materials, such as biomass, it is rather inexpensive. Thus, it is biomass,ȱ itȱ isȱ ratherȱ inexpensive.ȱ Thus,ȱ itȱ isȱ anȱ excellentȱ resourceȱ forȱ theȱ productionȱ ofȱ valuableȱ an excellent resource for the production of valuable bio-chemicals and bio-fuels [114]. Bio-catalytic bioȬchemicalsȱandȱbioȬfuelsȱ[114].ȱBioȬcatalyticȱprocessesȱenableȱsyngasȱfermentationȱinȱmoderateȱ conditions,ȱincreasingȱenergyȱsaving,ȱimprovingȱproductȱyieldȱandȱinvolvingȱlessȱtoxicȱcompoundsȱ orȱproductsȱ[115,116].ȱDespiteȱtheȱfactȱthatȱsyngasȱfermentationȱisȱstillȱanȱimmatureȱapproach,ȱitsȱ futureȱpotentialȱcannotȱbeȱignored.ȱ

Tableȱ5.ȱComparisonȱofȱC4H6ȱproductionȱprocesses.ȱ

Lebedev/Ostromisslens Processȱ SteamȱCrackingȱ Dehydrogenationȱ Dehydrationȱ kiȱ Feedstockȱ Naphthaȱ Butane/Butenesȱ BioȬethanolȱ BioȬbutanediolsȱ Operatingȱ 750–900ȱ°C,ȱmoderateȱ 600–700/400–500ȱ°C 400–650ȱ°Cȱ 250–350ȱ°Cȱ Conditionsȱ pressureȱ WellȬestablishedȱ BioȬbased,ȱ technology,ȱ BioȬbased,ȱonȬpurposeȱ Advantagesȱ Installedȱtechnologyȱ onȬpurposeȱ onȬpurposeȱ productionȱ productionȱ productionȱ Limitedȱ Energyȱdemanding,ȱ Highȱ productionȱofȱ environmentalȱconcerns,ȱ Catalystȱdeactivation,ȱ Disadvantagesȱ endothermicity,ȱ bioȬfeedstock,ȱ finiteȱresources,ȱlimitedȱ variousȱbyȬproductsȱ catalystȱdeactivation variousȱ productionȱ byȬproductsȱ YieldȱtoȱC4H6ȱ 4.5%ȱ 70.0%/71.8%ȱ 72.0%/56.5%ȱ upȱtoȱ95.0%ȱ

Asȱlighterȱolefinsȱareȱmostȱpreferablyȱproducedȱviaȱsteamȱcracking,ȱC4H6ȱproductionȱisȱratherȱ limitedȱ throughȱ thisȱ process.ȱ Viaȱ dehydrogenation,ȱ butane—orȱ butenesȱ viaȱ oxidativeȱ

Catalysts 2018, 8, 2 13 of 19 processes enable syngas fermentation in moderate conditions, increasing energy saving, improving product yield and involving less toxic compounds or products [115,116]. Despite the fact that syngas fermentation is still an immature approach, its future potential cannot be ignored. As lighter olefins are most preferably produced via steam cracking, C4H6 production is rather limited through this process. Via dehydrogenation, butane—or butenes via oxidative dehydrogenation, can be selectively converted into C4H6 but the high temperatures and catalyst deactivation are key disadvantages impeding commercialization. Bio-ethanol conversion into C4H6 is the most promising alternative; numerous studies have focused on the selection of the most suitable catalyst. However, catalytic deactivation and increased by-product formation are still critical issues that further research could resolve. Via dehydration/hydrogenation steps, bio-butanol can be effectively converted into C4H6, but the limited availability of the feedstock, along with the significantly high cost due to high temperature (up to 700 ◦C) and to the hydrogenation step, hinder industrial application. (Table5) On the other hand, relatively lower temperatures of highly selective bio-butanediol dehydration could enable its applicability, even though production of the bio-feedstock is quite limited and by-product formation is not negligible. To conclude, all bio-based methods for C4H6 production are still in lab-scale. Nonetheless, in the future, novel catalysts can be synthesized for the selective C4H6 production. Moreover, further studies on production of the biomass-derived feedstocks could reduce the cost and increase their availability in order to facilitate the implementation of these processes.

Table 5. Comparison of C4H6 production processes.

Process Steam Cracking Dehydrogenation Lebedev/Ostromisslenski Dehydration Feedstock Naphtha Butane/Butenes Bio-ethanol Bio-butanediols Operating 750–900 ◦C, 600–700/400–500 ◦C 400–650 ◦C 250–350 ◦C Conditions moderate pressure Well-established Bio-based, on-purpose Bio-based, on-purpose Advantages Installed technology technology, production production on-purpose production Energy demanding, Limited production of environmental concerns, High endothermicity, Catalyst deactivation, Disadvantages bio-feedstock, various finite resources, catalyst deactivation various by-products by-products limited production

Yield to C4H6 4.5% 70.0%/71.8% 72.0%/56.5% up to 95.0%

3. Concluding Remarks Production of bio-olefins is a broad research field that is continuously expanding, as the demand is incessantly increasing, worldwide. Biomass derived intermediates offer numerous opportunities for alternative reaction pathways, yielding ethylene, propylene, or butadiene, in less energy demanding and cost effective processes that do not exploit the finite fossil resources. Bio-olefins can be produced via numerous processes, some of which have already been implemented in industrial applications. Ethanol dehydration is the most promising bio-based process for bio-ethylene production that has already been installed, operating at relatively moderate temperatures. MTO and DMTO are close to commercial application, using bio-methanol and bio-DME as feedstocks, for selective ethylene production. In all cases, limited bio-ethanol supply is a key drawback that affects possibility of industrial applications. Bio-synthesis is an interesting research subject on selective sustainable production, but more in-depth studies are essential in order to increase productivity and significantly lower the cost. Bio-propylene can be effectively produced through bio-olefin metathesis and glycerol to olefins methods, operating at relatively moderate conditions with increased bio-propylene yields. In fact, olefin metathesis technology has already been implemented; however, limited bio-feedstock availability strongly affects productivity and viability of this process. GTO methods include a wide range of processes that could yield bio-propylene in lab-scale applications. Nevertheless, studies on the most Catalysts 2018, 8, 2 14 of 19 efficient and stable catalyst that will lower hydrogen demand are expected to make these processes applicable in the near future. Bio-butadiene can be primarily produced from bio-ethanol via the Lebedev/Ostromisslenski methods that have been extensively studied in the past decades. Dehydration of bio-butanediols is the most promising approach, reaching 95% yield to butadiene, at lower temperature than the other alternatives. Bio-butanol can also be used as a feedstock for bio-butadiene production, but the high operation cost due to hydrogenation step and high temperature limits feasibility of implementation. Additionally, C4 bio-feedstock production is also limited in order to ensure future viable industrial applications. Overall, several bio-based processes have been proposed with high potential in bio-olefin yields. Even though the majority of them are still in laboratory scale, a few have already been implemented around the world. The main drawback in the scale-up of these processes is the availability of the bio-feedstocks which can be produced from various biomass derivatives via fermentation, bio-synthesis, cracking, and deoxygenation among others. Future studies should mainly focus on increasing the productivity of these methods, along with reducing the cost, in order to facilitate their implementation in bio-olefin production units. In most catalytic approaches, novel low-cost catalytic systems with improved properties regarding selectivity and reaction conditions should also be researched to advance future applications.

Acknowledgments: This research has been financed by the State Scholarships Foundation (IKY) through the program “RESEARCH PROJECTS FOR EXCELLENCE IKY/SIEMENS”. Conflicts of Interest: The authors declare no conflict of interest.

References

1. BP. Statistical Review of World Energy; BP: London, UK, 2017. 2. Amghizar, I.; Vandewalle, L.A.; Van Geem, K.M.; Marin, G.B. New Trends in Olefin Production. Engineering 2017, 3, 171–178. [CrossRef] 3. Bender, M. An Overview of Industrial Processes for the Production of Olefins—C4 Hydrocarbons. ChemBioEng Rev. 2014, 1, 136–147. [CrossRef] 4. Torres Galvis, H.M.; de Jong, K.P. Catalysts for Production of Lower Olefins from Synthesis Gas: A Review. ACS Catal. 2013, 3, 2130–2149. [CrossRef] 5. Matar, S.; Hatch, L.F. Chemistry of Petrochemical Processes; Gulf Professional Publishing: Houston, TX, USA, 2009. 6. Weissermel, K.; Arpe, H. Oxidation Products Ethylene. In Industrial Organic Chemistry; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1978; pp. 145–192; ISBN 9783527619191. 7. Zimmermann, H.; Walzl, R. Ethylene. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2009; ISBN 9783527306732. 8. Mohsenzadeh, A.; Zamani, A.; Taherzadeh, M.J. Bioethylene Production from Ethanol: A Review and Techno-economical Evaluation. ChemBioEng Rev. 2017, 4, 75–91. [CrossRef] 9. Chang, C.D. Methanol Conversion to Light Olefins. Catal. Rev. 1984, 26, 323–345. [CrossRef] 10. Haro, P.; Trippe, F.; Stahl, R.; Henrich, E. Bio-syngas to gasoline and olefins via DME—A comprehensive techno-economic assessment. Appl. Energy 2013, 108, 54–65. [CrossRef] 11. McManus, M.T. The Plant Hormone Ethylene; Wiley-Blackwell: Hoboken, NJ, USA, 2012; ISBN 1444330039. 12. Ding, J.; Hua, W. Game Changers of the C3 Value Chain: Gas, Coal, and Biotechnologies. Chem. Eng. Technol. 2013, 36, 83–90. [CrossRef] 13. Eisele, P.; Killpack, R. Propene. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; ISBN 9783527306732.

14. Sahebdelfar, S.; Zangeneh, F.T. Dehydrogenation of Propane to Propylene Over Pt-Sn/Al2O3 Catalysts: The influence of operating conditions on product selectivity. Iran. J. Chem. Eng. 2010, 7, 51–57. 15. Lwin, S.; Wachs, I.E. Olefin Metathesis by Supported Metal Oxide Catalysts. ACS Catal. 2014, 4, 2505–2520. [CrossRef] Catalysts 2018, 8, 2 15 of 19

16. Chen, D.; Moljord, K.; Holmen, A. A methanol to olefins review: Diffusion, coke formation and deactivation on SAPO type catalysts. Microporous Mesoporous Mater. 2012, 164, 239–250. [CrossRef] 17. Tian, P.; Wei, Y.; Ye, M.; Liu, Z. Methanol to Olefins (MTO): From Fundamentals to Commercialization. ACS Catal. 2015, 5, 1922–1938. [CrossRef] 18. Werpy, T.; Petersen, G.; Aden, A.; Bozell, J. Top Value Added Chemicals from Biomass. Volume 1-Results of Screening for Potential Candidates from Sugars and Synthesis Gas; U.S. Department of Energy: Washington, DC, USA, 2004. 19. Bruijnincx, P.C.A.; Weckhuysen, B.M. Shale Gas Revolution: An Opportunity for the Production of Biobased Chemicals? Angew. Chem. Int. Ed. 2013, 52, 11980–11987. [CrossRef][PubMed] 20. Siirola, J.J. The impact of shale gas in the chemical industry. AIChE J. 2014, 60, 810–819. [CrossRef] 21. Fiorentino, G.; Ripa, M.; Ulgiati, S. Chemicals from biomass: Technological versus environmental feasibility. A review. Biofuels Bioprod. Biorefin. 2017, 11, 195–214. [CrossRef] 22. Chieregato, A.; Ochoa, J.V.; Cavani, F. Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks; Cavani, F., Albonetti, S., Basile, F., Gandini, A., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2016. [CrossRef] 23. Yang, S.F.; Hoffman, N.E. Ethylene Biosynthesis and its Regulation in Higher Plants. Annu. Rev. Plant Physiol. 1984, 35, 155–189. [CrossRef] 24. Lynch, S.; Eckert, C.; Yu, J.; Gill, R.; Maness, P.-C. Overcoming substrate limitations for improved production of ethylene in E. coli. Biotechnol. Biofuels 2016, 9, 3. [CrossRef][PubMed] 25. Nickerson, W.J. Ethylene as a metabolic product of the pathogenic fungus, Blastomyces dermatitidis. Arch Biochem. 1948, 17, 225–233. [PubMed] 26. Eckert, C.; Xu, W.; Xiong, W.; Lynch, S.; Ungerer, J.; Tao, L.; Gill, R.; Maness, P.-C.; Yu, J. Ethylene-forming enzyme and bioethylene production. Biotechnol. Biofuels 2014, 7, 33. [CrossRef][PubMed] 27. Wang, J.P.; Wu, L.X.; Xu, F.; Lv, J.; Jin, H.J.; Chen, S.F. Metabolic engineering for ethylene production by inserting the ethylene-forming enzyme gene (efe) at the 16S rDNA sites of Pseudomonas putida KT2440. Bioresour. Technol. 2010, 101, 6404–6409. [CrossRef][PubMed] 28. Markham, J.N.; Tao, L.; Davis, R.; Voulis, N.; Angenent, L.T.; Ungerer, J.; Yu, J. Techno-economic analysis of a conceptual biofuel production process from bioethylene produced by photosynthetic recombinant cyanobacteria. Green Chem. 2016, 18, 6266–6281. [CrossRef] 29. Zhang, M.; Yu, Y. Dehydration of ethanol to ethylene. Ind. Eng. Chem. Res. 2013, 52, 9505–9514. [CrossRef] 30. Fan, D.; Dai, D.J.; Wu, H.S. Ethylene formation by catalytic dehydration of ethanol with industrial considerations. Materials 2013, 6, 101–115. [CrossRef][PubMed] 31. Kosaric, N.; Duvnjak, Z.; Farkas, A.; Sahm, H.; Bringer-Meyer, S.; Goebel, O.; Mayer, D.; Kosaric, N.; Duvnjak, Z.; Farkas, A.; et al. Ethanol. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011; pp. 1–72; ISBN 9783527306732. 32. Althoff, J.; Biesheuvel, K.; De Kok, A.; Pelt, H.; Ruitenbeek, M.; Spork, G.; Tange, J.; Wevers, R. Economic Feasibility of the Sugar Beet-to-Ethylene Value Chain. ChemSusChem 2013, 6, 1625–1630. [CrossRef][PubMed] 33. Sheldon, R.A. Green and sustainable manufacture of chemicals from biomass: State of the art. Green Chem. 2014, 16, 950–963. [CrossRef] 34. Fukumoto, M.; Kimura, A. Process for the Manufacture of Ethylene by Dehydration of Ethanol. EP 2594546 A1, 17 November 2011. 35. Chen, Y.; Wu, Y.; Tao, L.; Dai, B.; Yang, M.; Chen, Z.; Zhu, X. Dehydration reaction of bio-ethanol to ethylene over modified SAPO catalysts. J. Ind. Eng. Chem. 2010, 16, 717–722. [CrossRef] 36. Wu, L.; Shi, X.; Cui, Q.; Wang, H.; Huang, H. Effects of the SAPO-11 synthetic process on dehydration Of ethanol to ethylene. Front. Chem. Sci. Eng. 2011, 5, 60–66. [CrossRef] 37. Zhang, D.; Wang, R.; Yang, X. Effect of P content on the catalytic performance of P-modified HZSM-5 catalysts in dehydration of ethanol to ethylene. Catal. Lett. 2008, 124, 384–391. [CrossRef] 38. Patrick, G.B.; Partington, S.R. Process for Preparing Ethene. U.S. Patent 8822748 B2, 8 October 2008. 39. Lanzafame, P.; Centi, G.; Perathoner, S. Evolving scenarios for biorefineries and the impact on catalysis. Catal. Today 2014, 234, 2–12. [CrossRef]

40. Kang, M.; DeWilde, J.F.; Bhan, A. Kinetics and Mechanism of Alcohol Dehydration on γ-Al2O3: Effects of Carbon Chain Length and Substitution. ACS Catal. 2015, 5, 602–612. [CrossRef] Catalysts 2018, 8, 2 16 of 19

41. Christiansen, M.A.; Mpourmpakis, G.; Vlachos, D.G. DFT-driven multi-site microkinetic modeling of ethanol

conversion to ethylene and diethyl ether on γ-Al2O3 (1 1 1). J. Catal. 2015, 323, 121–131. [CrossRef] 42. Potter, M.E.; Cholerton, M.E.; Kezina, J.; Bounds, R.; Carravetta, M.; Manzoli, M.; Gianotti, E.; Lefenfeld, M.; Raja, R. Role of Isolated Acid Sites and Influence of Pore Diameter in the Low-Temperature Dehydration of Ethanol. ACS Catal. 2014, 4, 4161–4169. [CrossRef]

43. DeWilde, J.F.; Czopinski, C.J.; Bhan, A. Ethanol Dehydration and Dehydrogenation on γ-Al2O3: Mechanism of Acetaldehyde Formation. ACS Catal. 2014, 4, 4425–4433. [CrossRef] 44. Kochar, N.K.; Merims, R.; Padia, A.S. Ethylene from Ethanol. Chem. Eng. Prog. 1981, 6, 66–70. 45. Ondrey, G. The Launch of a New Bioethylene-Production Process; Chemical Engineering: New York, NY, USA, 2014.

46. Chen, G.; Li, S.; Jiao, F.; Yuan, Q. Catalytic dehydration of bioethanol to ethylene over TiO2/γ-Al2O3 catalysts in microchannel reactors. Catal. Today 2007, 125, 111–119. [CrossRef] 47. Zhan, N.; Hu, Y.; Li, H.; Yu, D.; Han, Y.; Huang, H. Lanthanum-phosphorous modified HZSM-5 catalysts in dehydration of ethanol to ethylene: A comparative analysis. Catal. Commun. 2010, 11, 633–637. [CrossRef] 48. Ciftci, A.; Varisli, D.; Tokay, K.C.; Sezgi, N.A.; Dogu, T. Dimethyl ether, diethyl ether & ethylene from alcohols over tungstophosphoric acid based mesoporous catalysts. Chem. Eng. J. 2012, 207–208, 85–93. 49. Varisli, D.; Dogu, T.; Dogu, G. Silicotungstic acid impregnated MCM-41-like mesoporous solid acid catalysts for dehydration of ethanol. Ind. Eng. Chem. Res. 2008, 47, 4071–4076. [CrossRef] 50. Chang, C.D.; Silvestri, A.J. The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J. Catal. 1977, 47, 249–259. [CrossRef] 51. Olsbye, U.; Svelle, S.; Bjorgen, M.; Beato, P.; Janssens, T.V.W.; Joensen, F.; Bordiga, S.; Lillerud, K.P. Conversion of methanol to hydrocarbons: How zeolite cavity and pore size controls product selectivity. Angew. Chem. Int. Ed. 2012, 51, 5810–5831. [CrossRef][PubMed] 52. Güllü, D.; Demirbas, A. Biomass to methanol via pyrolysis process. Energy Convers. Manag. 2001, 42, 1349–1356. [CrossRef] 53. Liu, Z.; Liang, J. Methanol to olefins conversion catalysts Curr. Opin. Solid State Mater. Sci. 1999, 4, 80–84. [CrossRef] 54. Galindo, C.P.; Badr, O. Renewable hydrogen utilisation for the production of methanol. Energy Convers. Manag. 2007, 48, 519–527. [CrossRef] 55. Taylor, C.E.; Noceti, R.P.; Joseph, R.D. New developments in the photocatalytic conversion of methane to methanol. Catal. Today 2000, 55, 259–267. [CrossRef] 56. Shamsul, N.S.; Kamarudin, S.K.; Rahman, N.A.; Kofli, N.T. An overview on the production of bio-methanol as potential renewable energy. Renew. Sustain. Energy Rev. 2014, 33, 578–588. [CrossRef] 57. Liang, J.; Li, H.; Zhao, S.; Guo, W.; Wang, R.; Ying, M. Characteristics and performance of SAPO-34 catalyst for methanol-to-olefin conversion. Appl. Catal. 1990, 64, 31–40. [CrossRef] 58. Raffelt, K.; Henrich, E.; Koegel, A.; Stahl, R.; Steinhardt, J.; Weirich, F. The BTL2 Process of Biomass Utilization Entrained-Flow Gasification of Pyrolyzed Biomass Slurries. Appl. Biochem. Biotechnol. 2006, 129, 153–164. [CrossRef] 59. Henrich, E.; Dahmen, N.; Dinjus, E. Cost estimate for biosynfuel production via biosyncrude gasification. Biofuels Bioprod. Biorefin. 2009, 3, 28–41. [CrossRef] 60. Wright, M.M.; Brown, R.C.; Boateng, A.A. Distributed processing of biomass to bio-oil for subsequent production of Fischer-Tropsch liquids. Biofuels Bioprod. Biorefin. 2008, 2, 229–238. [CrossRef] 61. Kvisle, S.; Fuglerud, T.; Kolboe, S.; Olsbye, U.; Lillerud, K.P.; Vora, B.V.; Kvisle, S.; Fuglerud, T.; Kolboe, S.; Olsbye, U.; et al. Methanol-to-Hydrocarbons. In Handbook of Heterogeneous Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008; ISBN 9783527610044. 62. Ren, T.; Patel, M.; Blok, K. Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes. Energy 2006, 31, 425–451. [CrossRef] 63. Akah, A.; Al-Ghrami, M. Maximizing propylene production via FCC technology. Appl. Petrochem. Res. 2015, 5, 377–392. [CrossRef] 64. Sattler, J.J.H.B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B.M. Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114, 10613–10653. [CrossRef][PubMed] 65. Zhang, S.; Gong, Y.; Zhang, L.; Liu, Y.; Dou, T.; Xu, J.; Deng, F. Hydrothermal treatment on ZSM-5 extrudates catalyst for methanol to propylene reaction: Finely tuning the acidic property. Fuel Process. Technol. 2015, 129, 130–138. [CrossRef] Catalysts 2018, 8, 2 17 of 19

66. Banks, R.L.; Kukes, S.G. New developments and concepts in enhancing activities of heterogeneous metathesis catalysts. J. Mol. Catal. 1985, 28, 117–131. [CrossRef] 67. Knifton, J.F.; Sanderson, J.R.; Stockton, M.E. Tert-butanol dehydration to isobutylene via reactive distillation. Catal. Lett. 2001, 73, 55–57. [CrossRef] 68. Kozlowski, J.T.; Davis, R.J. Heterogeneous catalysts for the guerbet coupling of alcohols. ACS Catal. 2013, 3, 1588–1600. [CrossRef] 69. Dowson, G.R.M.; Haddow, M.F.; Lee, J.; Wingad, R.L.; Wass, D.F. Catalytic conversion of ethanol into an advanced biofuel: Unprecedented selectivity for n-butanol. Angew. Chem. Int. Ed. 2013, 52, 9005–9008. [CrossRef][PubMed] 70. HOOD, A.D., Jr.; Bridges, R.S. Staged Propylene Production Process. WO 2014110125 A1, 17 July 2014. 71. Mol, J.C. Industrial applications of olefin metathesis. J. Mol. Catal. A Chem. 2004, 213, 39–45. [CrossRef] 72. Huang, S.; Liu, S.; Xin, W.; Bai, J.; Xie, S.; Wang, Q.; Xu, L. Metathesis of ethene and 2-butene to propene on

W/Al2O3-HY catalysts with different HY contents. J. Mol. Catal. A Chem. 2005, 226, 61–68. [CrossRef] 73. Bouchmella, K.; Hubert Mutin, P.; Stoyanova, M.; Poleunis, C.; Eloy, P.; Rodemerck, U.; Gaigneaux, E.M.; Debecker, D.P. Olefin metathesis with mesoporous rhenium-silicium-aluminum mixed oxides obtained via a one-step non-hydrolytic sol-gel route. J. Catal. 2013, 301, 233–241. [CrossRef] 74. Busca, G. Heterogeneous Catalytic Materials: Solid State Chemistry, Surface Chemistry and Catalytic Behaviour; Elsevier: Warsaw, Poland, 2014. 75. Popoff, N.; Mazoyer, E.; Pelletier, J.; Gauvin, R.M.; Taoufik, M. Expanding the scope of metathesis: A survey of polyfunctional, single-site supported tungsten systems for hydrocarbon valorization. Chem. Soc. Rev. 2013, 42, 9035–9054. [CrossRef][PubMed]

76. Inoue, K.; Inaba, M.; Takahara, I.; Murata, K. Conversion of ethanol to propylene by H-ZSM-5 with Si/Al2 ratio of 280. Catal. Lett. 2010, 136, 14–19. [CrossRef] 77. Kazuhisa, M.; Takahara, I.; Inaba, M. Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts. React. Kinet. Catal. Lett. 2008, 93, 59–66. 78. Lin, B.; Zhang, Q.; Wang, Y. Catalytic conversion of ethylene to propylene and butenes over H-ZSM-5. Ind. Eng. Chem. Res. 2009, 48, 10788–10795. [CrossRef] 79. Huangfu, J.; Mao, D.; Zhai, X.; Guo, Q. Remarkably enhanced stability of HZSM-5 zeolite co-modified with alkaline and phosphorous for the selective conversion of bio-ethanol to propylene. Appl. Catal. A Gen. 2016, 520, 99–104. [CrossRef] 80. Zhang, N.; Mao, D.; Zhai, X. Selective conversion of bio-ethanol to propene over nano-HZSM-5 zeolite: Remarkably enhanced catalytic performance by fluorine modification. Fuel Process. Technol. 2017, 167, 50–60. [CrossRef] 81. Hayashi, F.; Iwamoto, M. Yttrium-modified ceria as a highly durable catalyst for the selective conversion of ethanol to propene and ethene. ACS Catal. 2013, 3, 14–17. [CrossRef] 82. Hayashi, F.; Tanaka, M.; Lin, D.; Iwamoto, M. Surface structure of yttrium-modified ceria catalysts and reaction pathways from ethanol to propene. J. Catal. 2014, 316, 112–120. [CrossRef] 83. Iwamoto, M.; Mizuno, S.; Tanaka, M. Direct and selective production of propene from bio-ethanol on

Sc-loaded In2O3 catalysts. Chem. A Eur. J. 2013, 19, 7214–7220. [CrossRef][PubMed] 84. Xia, W.; Wang, F.; Mu, X.; Chen, K.; Takahashi, A.; Nakamura, I.; Fujitani, T. Catalytic performance of H-ZSM-5 zeolites for conversion of ethanol or ethylene to propylene: Effect of reaction pressure and

SiO2/Al2O3 ratio. Catal. Commun. 2017, 91, 62–66. [CrossRef] 85. Xue, F.; Miao, C.; Yue, Y.; Hua, W.; Gao, Z. Direct conversion of bio-ethanol to propylene in high yield over

the composite of In2O3 and zeolite beta. Green Chem. 2017.[CrossRef] 86. Hulteberg, C.; Brandin, J. Process for Preparing Lower Hydrocarbons from Glycerol. U.S. Patent 20110224470 A1, 15 September 2011. 87. Souza, F.J.C.; Gambetta, R.; de Araujo Mota, C.J.; da Conceicao Goncalves, V.L. Preparation of Heterogeneous Catalysts Used in Selective Hydrogenation of Glycerin to Propene, and a Process for the Selective Hydrogenation of Glycerin to Propene. U.S. Patent 8841497 B2, 24 June 2009. 88. Blass, S.D.; Hermann, R.J.; Persson, N.E.; Bhan, A.; Schmidt, L.D. Conversion of glycerol to light olefins and gasoline precursors. Appl. Catal. A Gen. 2014, 475, 10–15. [CrossRef] 89. Yu, L.; Yuan, J.; Zhang, Q.; Liu, Y.M.; He, H.Y.; Fan, K.N.; Cao, Y. Propylene from renewable resources: Catalytic conversion of glycerol into propylene. ChemSusChem 2014, 7, 743–747. [CrossRef][PubMed] Catalysts 2018, 8, 2 18 of 19

90. Zacharopoulou, V.; Vasiliadou, E.S.; Lemonidou, A.A. One-step propylene formation from bio-glycerol over molybdena-based catalysts. Green Chem. 2015, 17, 903–912. [CrossRef] 91. Sun, D.; Yamada, Y.; Sato, S. Efficient production of propylene in the catalytic conversion of glycerol. Appl. Catal. B Environ. 2015, 174, 13–20. [CrossRef] 92. Deshpande, R.; Davis, P.; Pandey, V.; Kore, N. Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst. WO 2013090076 A1, 20 June 2013. 93. Zacharopoulou, V.; Vasiliadou, E.; Lemonidou, A.A. Exploring the reaction pathways of bio-glycerol hydro-deoxygenation to propene over Molybdena-based catalysts. ChemSusChem 2017.[CrossRef][PubMed] 94. Pyl, S.P.; Schietekat, C.M.; Reyniers, M.F.; Abhari, R.; Marin, G.B.; Van Geem, K.M. Biomass to olefins: Cracking of renewable naphtha. Chem. Eng. J. 2011, 176–177, 178–187. [CrossRef] 95. Ramin, A.H.; Lynn Tomlinson, G.R. Biorenewable Naphtha Composition and Methods of Making Same. U.S. Patent 8581013 B2, 12 November 2013. 96. Vermeiren, W.; Van Gyseghem, N. A process for the Production of Bio-Naphtha from Complex Mixtures of Natural Occurring Fats & Oils. WO 2011012439 A1, 3 February 2011. 97. Bielansky, P.; Weinert, A.; Schönberger, C.; Reichhold, A. Catalytic conversion of vegetable oils in a continuous FCC pilot plant. Fuel Process. Technol. 2011, 92, 2305–2311. [CrossRef] 98. Mortensen, P.M.; Grunwaldt, J.D.; Jensen, P.A.; Knudsen, K.G.; Jensen, A.D. A review of catalytic upgrading of bio-oil to engine fuels. Appl. Catal. A Gen. 2011, 407, 1–19. [CrossRef] 99. Ruddy, D.A.; Schaidle, J.A.; Ferrell, J.R., III; Wang, J.; Moens, L.; Hensley, J.E. Recent advances in heterogeneous catalysts for bio-oil upgrading via “ex situ catalytic fast pyrolysis”: Catalyst development through the study of model compounds. Green Chem. 2014, 16, 454–490. [CrossRef] 100. Angelici, C.; Weckhuysen, B.M.; Bruijnincx, P.C.A. Chemocatalytic conversion of ethanol into butadiene and other bulk chemicals. ChemSusChem 2013, 6, 1595–1614. [CrossRef][PubMed] 101. Quattlebaum, W.M., Jr.; Toussaint, W.J. Process of Making Olefins. U.S. Patent 2407291 A, 10 September 1946. 102. Toussant, W.J.; Dunn, J.T.; Jackson, D.R. Production of Butadiene from Alcohol. Ind. Eng. Chem. 1947, 39, 120–125. [CrossRef] 103. Makshina, E.V.; Dusselier, M.; Janssens, W.; Degrève, J.; Jacobs, P.A.; Sels, B.F. Review of old chemistry and new catalytic advances in the on-purpose synthesis of butadiene. Chem. Soc. Rev. 2014, 43, 7917–7953. [CrossRef][PubMed] 104. Chieregato, A.; Ochoa, J.V.; Bandinelli, C.; Fornasari, G.; Cavani, F.; Mella, M. On the chemistry of ethanol on basic oxides: Revising mechanisms and intermediates in the lebedev and guerbet reactions. ChemSusChem 2015, 8, 377–388. [CrossRef][PubMed] 105. Xiu, Z.L.; Zeng, A.P. Present state and perspective of downstream processing of biologically produced 1,3-propanediol and 2,3-butanediol. Appl. Microbiol. Biotechnol. 2008, 78, 917–926. [CrossRef][PubMed] 106. Celi´nska,E.; Grajek, W. Biotechnological production of 2,3-butanediol-Current state and prospects. Biotechnol. Adv. 2009, 27, 715–725. [CrossRef][PubMed] 107. Mark, J.B.; Anthony, P.B.; Robin, E.O.; Jun, S.P.P. Microorganisms for Producing Butadiene and Methods Related Thereto. WO 2012177710 A1, 27 December 2012. 108. Zhang, D.; Barri, S.A.I.; Chadwick, D. N-Butanol to iso-butene in one-step over zeolite catalysts. Appl. Catal. A Gen. 2011, 403, 1–11. [CrossRef] 109. Arpe, H.-J.; Weissermel, K. Industrial Organic Chemistry; Wiley-VCH: Weinheim, Germany, 2010; ISBN 3527320024. 110. Reppe, W.; Steinhofer, A.; Daumiller, G. Verfahren zur Herstellung von Diolefinen. DE 899350, 10 August 1953. 111. Kopke, M.; Mihalcea, C.; Liew, F.; Tizard, J.H.; Ali, M.S.; Conolly, J.J.; Al-Sinawi, B.; Simpson, S.D. 2,3-Butanediol Production by Acetogenic Bacteria, an Alternative Route to Chemical Synthesis, Using Industrial Waste Gas. Appl. Environ. Microbiol. 2011, 77, 5467–5475. [CrossRef][PubMed]

112. Duan, H.; Yamada, Y.; Sato, S. Selective dehydration of 2,3-butanediol to 3-buten-2-ol over ZrO2 modified with CaO. Appl. Catal. A Gen. 2014, 487, 226–233. [CrossRef] 113. Mohammadi, M.; Najafpour, G.D.; Younesi, H.; Lahijani, P.; Uzir, M.H.; Mohamed, A.R. Bioconversion of synthesis gas to second generation biofuels: A review. Renew. Sustain. Energy Rev. 2011, 15, 4255–4273. [CrossRef] Catalysts 2018, 8, 2 19 of 19

114. Choi, D.W.; Chipman, D.C.; Bents, S.C.; Brown, R.C. A techno-economic analysis of polyhydroxyalkanoate and hydrogen production from syngas fermentation of gasified biomass. Appl. Biochem. Biotechnol. 2010, 160, 1032–1046. [CrossRef][PubMed] 115. Bredwell, M.D.; Srivastava, P.; Worden, R.M. Reactor Design Issues for Synthesis-Gas Fermentations. Biotechnol. Prog. 1999, 15, 834–844. [CrossRef][PubMed] 116. Madhukar, G.R.; Elmore, B.B.; Huckabay, H.K. Microbial conversion of synthesis gas components to useful fuels and chemicals, Microbial Conversion of Synthesis Gas Components to Useful Fuels and Chemicals Symposium. In Seventeenth Symposium on Biotechnology for Fuels and Chemicals; Wyman, C.E., Davison, B.H., Eds.; ABAB Symposium, Volume 57/58; Humana Press: Totowa, NJ, USA, 1996.

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