Renewable Production of Acrylonitrile via 3-Hydroxypropionic Acid

SUSTAINABLE CHEMISTRY Renewable acrylonitrile production Eric M Karp1 Todd R Eaton1 Violeta Sànchez i Nogué1 Vassili Vorotnikov1 Mary J Biddy1 Eric C D Tan1 David G Brandner1 Robin M Cywar1 Rongming Liu2 Lorenz P Manker1 William E Michener1 Michelle Gilhespy3 Zinovia Skoufa3 Michael J Watson3 O Stanley Fruchey4 Derek R Vardon1 Ryan T Gill2 Adam D Bratis2 Gregg T Beckham1 Acrylonitrile ACN is a petroleumderived compound used in resins polymers acrylics and carbon fiber We present a process for renewable ACN production using 3hydroxypropionic acid 3HP which can be produced microbially from sugars The process achieves ACN molar yields exceeding 90 from ethyl 3hydroxypropanoate ethyl 3HP via dehydration and nitrilation with ammonia over an inexpensive titanium dioxide solid acid catalyst We further describe an integrated process modeled at scale that is based on this chemistry and achieves nearquantitative ACN yields 98 2 from ethyl acrylate This endothermic approach eliminates runaway reaction hazards and achieves higher yields than the standard propylene ammoxidation process Avoidance of hydrogen cyanide as a byproduct also improves process safety and mitigates product handling requirements A crylonitrile ACN is one of the most widely used monomers in the chemical industry with more than 14 billion pounds produced annually for use in plastics rubbers resins acrylic fibers and polyacrylonitrile PAN based carbon fibers 1 2 The market outlook for PANderived carbon fibers in particular is projected to grow by 11 to 18 annually driven by interest in reducing the weight of vehicles and aircraft 2 Today industrial ACN production is conducted via the Sohio process which converts propylene to ACN via ammoxidation over a bis muth molybdatebased catalyst Firstgeneration catalysts for the process were developed in the 1950s achieving 55 molar ACN yields from propylene 3 This discovery spurred decades of research to improve ACN yields 4 resulting in stateoftheart materials capable of producing ACN at molar yields of 83 from propylene 5 However fluctuations in the price of the propyl ene feedstock translate directly to ACN price vol atility The carbon fiber industry is especially sensitive to these ACN price fluctuations because roughly 2 lbs of ACN are required to generate 1 lb of fiber 6 Thus concerns about propylene price volatility have motivated the search for alterna tive approaches to propylene ammoxidation to produce ACN Substantial efforts have focused on the ammoxidation of propane which is a cheaper substrate than propylene and has a low er carbon footprint but propane is still derived from fossilbased sources 7 Environmentally sustainable routes to ACN have been described from renewable feedstocks such as glycerol 810 and glutamic acid 11 these efforts are summa rized in 2 The most promising method to date is glycerol dehydration to acrolein followed by ammoxidation to ACN achieving yields of 60 8 Accordingly there is a clear need to develop sustainable costeffective biobased ACN manu facturing routes To that end we present a route from ethyl 3 hydroxypropanoate ethyl 3HP derived from microbially produced 3hydroxypropionic acid 3HP to ACN at molar yields of 90 This approach originates from reports of carboxylic acids being converted to nitriles when passed over solid acids with ammonia 12 However this reaction is made difficult by the relatively low volatility of carboxylic acids and the cor rosiveness of their vapors on equipment Esters are less corrosive more volatile and in general more stable than their acid counterparts using esters as substrates with this chemistry could enable a more viable route to renewable ACN To test the viability of converting ethyl 3HP to ACN we first conducted a steadystate temper ature scan by passing ethyl 3HP over TiO2 with an 81 molar excess of ammonia Fig 1A and fig S1 Ethyl 3HP was consumed in conjunction with the appearance of ethyl acrylate as the tem perature was increased from 150C to 230C In creasing the temperature further from 230C to 320C produced ACN at the expense of ethyl ac rylate From this result we posit that three se quential reactions occur Fig 1B to form ACN First the primary alcohol undergoes dehydration to form ethyl acrylate and water then ethyl ac rylate undergoes aminolysis Fig 1B reaction 2 to form acrylamide and ethanol and finally acry lamide is dehydrated to produce ACN and water Fig 1B reaction 3 The primary alcohol of 3HP or ethyl 3HP is known to readily dehydrate to an acrylate 13 Ester aminolysis to an amide has been reported using wet chemistry techniques 14 and amide dehydration to nitriles is also known 15 We refer to these latter reactions together as nitrilation Fig 1B reactions 2 and 3 Little work has been published on nitrilation overall especially in sys tems where ester and ammonia vapors are passed over heterogeneous catalysts to form nitriles 1618 Given sparse mechanistic information for this chemistry we performed periodic density func tional theory DFT calculations to probe the catalytic mechanism over a TiO2101 surface Alcohol dehydration was predicted to proceed via an E2 mechanism fig S2 similar to well known dehydration reactions 19 For the ami nolysis of ethyl acrylate DFT results suggested a stepwise mechanism catalyzed by partial dis sociation of NH3 on TiO2 HNH2 bond scission exhibits the highest barrier 111 kJmol in this mechanism Fig 1C DFT results for the final reaction suggest that acrylamide undergoes de hydration to form ACN via surfacemediated tautomerization to its enol form followed by dehydroxylation Fig 1D The highestbarrier step in the nitrilation reaction was predicted to be the deprotonation of the NH group with a barrier of 101 kJmol See figs S3 to S17 and tables S1 to S5 for details and energetics of these pathways Kinetic measurements performed at low con versions figs S18 and S19 revealed apparent activation energies consistent with the energe tics of the ratelimiting steps from DFT For the ester dehydration an apparent activation energy of 100 4 kJmol was determined from low conversion experiments of ethyl 3HP over TiO2 fig S18 The measured activation energy com pares favorably to the 112 kJmol calculated for dehydration figs S2 and S7 The apparent acti vation energy of nitrilation was measured by performing lowconversion experiments with ethyl acrylate and ammonia over TiO2 fig S19 Here an apparent activation energy of 103 12 kJ mol also compares favorably to the 111 kJmol barrier for the HNH2 bond scission from DFT figs S11 and S12 which is the highest predicted nitrilation barrier Results of totalconversion experiments per formed in a tandem bed reactor are shown in Fig 2 and fig S20 In the first reactor ethyl 3 HP was dehydrated over TiO2 to form ethyl ac rylate and water in quantitative yield at 260C A molar excess of 21 ethanol to ethyl 3HP was used as the feed to the first reactor to suppress acrylic acid formation figs S20 and S21 The product vapors from the first reactor were then mixed with ammonia and passed over a second bed of TiO2 at 315C to form ACN ethanol and water This operation achieved ACN yields of 90 to 92 for 12 hours on stream with minimal deactivation The overall heat of reaction is calculated to be endothermic by 203 kJmol fig S22 At the 12hour time point for each run the reaction was stopped and the catalyst removed and regenerated in air at 550C Images of fresh spent and regenerated catalyst as well as data from pyridine diffuse reflectance infrared Fourier transform spectroscopy BrunauerEmmettTeller RESEARCH Karp et al Science 358 13071310 2017 8 December 2017 1 of 4 1National Bioenergy Center National Renewable Energy Laboratory Golden CO 80401 USA 2Department of Chemical and Biological Engineering University of Colorado Boulder CO 80309 USA 3Johnson Matthey Technology Centre Billingham Cleveland TS23 1LB UK 4MATRIC South Charleston WV 25303 USA These authors contributed equally to this work Corresponding author Email greggbeckhamnrelgov Downloaded from httpsciencesciencemagorg on December 7 2017 isotherms xray diffraction and acid site density measurements figs S23 to S26 and table S6 indicate that the regeneration cycle restores the measured characteristics to those of the fresh sample The regenerated catalyst showed identical performance to the fresh sample Fig 2 Thermogravimetric analysis with Fourier trans form infrared spectroscopy TGAFTIR mea surements of the gas released during catalyst regeneration showed that NOx was not produced fig S27 thereby abating the need for exhaust cleanup during regeneration To ascertain whether this chemistry exhibits different behavior on a biologically derived sub strate we produced 3HP via glucose cultivation using an engineered Escherichia coli strain 20 fig S28 The cultivation used fedbatch dissolve oxygenbased control to feed glucose to the bio reactor and resulted in a titer of 258 gliter sup plementary text and figs S29 and S30 After glucose cultivation ethyl 3HP was separated and recovered from the broth figs S31 and S32 yielding 97 purity The ethyl 3HP was catalytically processed Fig 2 and achieved performance identical to that of synthetic ethyl 3HP The high yields of the nitrilation chemistry to produce ACN from microbially derived ethyl 3HP Fig 2 allow us to propose a potential industrialscale process for the hybrid biological and catalytic transformation of lignocellulosic sugars to ACN This process Fig 3 exhibits several notable modifications from the process demonstrated at bench scale First 3HP pro duction ideally would be conducted at low pH below the pKa of 3HP 21 The advantage Karp et al Science 358 13071310 2017 8 December 2017 2 of 4 Fig 2 Catalytic conversion of synthetic and biologically derived ethyl 3HP to ACN Totalconversion reactions of ethyl 3HP in tandem catalytic beds demonstrate 90 to 92 yields of ACN The percent yield of ACN is shown with the carbon balance for the reaction See fig S20 for the complete data set for all observed reaction products and the reaction conditions used Approximately every 12 hours the reaction was stopped and the catalyst regenerated The rightmost graph represents data collected using ethyl 3HP separated from a 3HP cultivation on glucose using an engineered E coli strain showing performance identical to that of the synthetic ethyl 3HP substrate As illustrated in the reactor schematic glass beads were packed in the headspace of the reactors to achieve uniform gas mixing over the catalyst Fig 1 Catalytic scheme for ethyl 3HP dehydration and nitrilation to produce ACN A Steadystate yields of relevant reaction products produced when passing ethyl 3HP over TiO2 as a function of reactor bed temperature Complete reaction conditions and data set are provided in fig S1 B The three reactions that are proposed to yield the results in A C Proposed mechanism from DFT calculations for the aminolysis of ethyl acrylate to form acrylamide and gaseous ethanol reaction 2 in B D Proposed mechanism from DFT calculations for dehydration of adsorbed acrylamide to release gaseous acrylonitrile and water reaction 3 in B RESEARCH REPORT Downloaded from httpsciencesciencemagorg on December 7 2017 gained from lowpH cultivation is that the acidification step is no longer required during separations and neutralization is not required during cultivation thus avoiding generation of waste salt This would improve the process economics and sustainability 22 LowpH strains to produce 3HP at industrially relevant titers rates and yields are under development 21 23 The second difference is that dew atering would occur at scale using a simulated moving bed SMB where 3HP is adsorbed to a resin and eluted off with ethanol Results in fig S33 indicate that polybenzimidazole PBI works well for this allowing 3HP to be com pletely recovered 24 Third ethyl acrylate instead of ethyl 3HP would be separated via reactive distillation Reactive distillation com bines esterification alcohol dehydration Fig 1B reaction 1 and product separation into a single unit operation This is accomplished using the same industrial process previously developed for the production of acrylate esters from bpropiolactone BPL 25 In that pro cess BPL is added to an ethanol solution aci dified with sulfuric acid in a continuous stirred tank reactor where ring opening occurs to form 3HP and then esterifies and dehydrates to form the acrylate ester 25 The solution is then fed to a distillation column where the acrylate ester is recovered and purified in the column overhead 25 26 see figs S34 and S35 for details The process could be stopped here and ethyl acrylate sold as the product at a predicted selling price of 048lb which may be at tractive given that the current market price of ethyl acrylate is 079lb However the market for ACN is approximately 10 times as large and the potential for lowering carbon fiber prices has much greater societal impact to lower the greenhouse gas footprint of auto mobile transportation by reducing vehicle weight 27 28 Recognizing that ethyl acrylate rather than ethyl 3HP would be fed to the nitrilation unit in the conceptual process shown in Fig 3 we performed totalconversion experiments with ethyl acrylate and ammonia over TiO2 in a single catalytic bed Fig 4 and fig S36 These runs resulted in a 98 2 maximum yield of ACN with ethyl acrylate as a sub strate The ACN yield from ethyl acrylate is higher than that from ethyl 3HP because of decreased carbon deposition on the catalyst which is likely attributable to the reduced presence of water when using ethyl acrylate as the substrate The carbon balance shown in Fig 4 is slightly above 100 because of slightly decreased ethyl acrylate conversion after 12 hours on stream leading to a slight buildup of ethyl acrylate fig S36 A small Karp et al Science 358 13071310 2017 8 December 2017 3 of 4 Fig 3 Conceptual process diagram for renewable ACN production from biomass sugars Process depiction of the unit operations proposed to produce ACN from lignocellulosic sugars via a hybrid biologicalcatalytic upgrading approach Fig 4 Catalytic conversion of ethyl acrylate to ACN Totalconversion reactions of ethyl acrylate in a single catalytic bed demonstrating maximum ACN yields of 98 2 The percent yield of ACN is shown with the carbon balance for the reaction See fig S36 for the complete data set for all observed reaction products and the reaction conditions used Approximately every 12 hours the syringe was refilled as shown and every 18 hours the reaction was stopped and the catalyst removed and regenerated RESEARCH REPORT Downloaded from httpsciencesciencemagorg on December 7 2017 amount of ethyl acrylate present in the re actor outlet leads to measurement uncertainty because of a low signaltonoise ratio The overall heat of ethyl acrylate nitrilation is calculated to be endothermic by 81 kJmol fig S37 In a scaled process Fig 3 it would be eco nomically beneficial to operate the nitrilation reactor in a pure ammonia atmosphere with out the use of N2 as a diluent Our online an alytical system precludes measurements in a pure ammonia atmosphere see supplementary text but catalyst deactivation studies figs S38 to S40 performed using ethyl acrylate as a substrate allowed calculation of a regenera tion cycle time needed under more concen trated conditions fig S41 and supplementary text These calculations estimate that under fullscale conditions 310C 089 atm NH3 011 atm ethyl acrylate the entire catalytic bed must be regenerated every 30 s to main tain ACN yields above 98 Thus the fourth modification in the fullscale process model is that the recovered ethyl acrylate is fed to a riser reactor with continuous catalyst regen eration Fig 3 and fig S42 This continuously regenerates the catalyst with a cycle time of 30 s The liberated alcohol is recovered down stream and recycled figs S43 and S44 A simplified schematic of the modeled process is shown in Fig 3 see figs S34 S35 and S42 to S44 for details of the unit operations On the basis of this proposed process we per formed a technoeconomic analysis predicting the selling price of ACN at 089lb from ligno cellulosic sugars and 076lb from sucrose These target prices are in the range of fossil fuelderived ACN prices between 040 and 100lb over the past decade 29 Additionally the greenhouse gas emissions from this process were estimated to achieve a 141 improvement relative to propylenederived ACN In addition to the sugar platform other lowvalue feedstocks including glycerol 30 31 and waste gases are being pur sued for the production of 3HP If realized these platforms could further lower the ACN selling price when coupled to nitrilation See tables S7 to 15 and figs S34 S35 and S42 to S46 for de tails of these analyses a discussion of the pa rameters used and model sensitivity to these parameters Beyond ACN nitrilation may have broader applications by providing a facile link from carboxylic acid or ester production to nitriles Several biologically derived carboxylates are now being operated at industrial scale eg succinic lactic itaconic and fumaric acid 32 33 and the nitrile derivatives of these acids could likely be readily obtained via ni trilation An economic advantage also may exist in coupling nitrilation to bioprocesses because separation of the ester can often be more economical than separating the free acid 34 35 For ACN production nitrilation provides a number of green chemistry benefits over pro pylene ammoxidation i Nearquantitative yields of ACN can be obtained from this re action whereas stateoftheart ammoxida tion catalysts achieve 80 to 83 ACN yield 5 ii The reaction is endothermic figs S22 and S37 and does not require O2 enabling facile process control By comparison ammox idation is highly exothermic requiring spec ialized reactors to avoid runaway reactions 36 iii Unlike ammoxidation nitrilation does not produce hydrogen cyanide mitigating toxicity and handling requirements iv The cost of TiO2 is approximately 30 that of ammox idation catalysts 4 37 v The process provides a costcomparable sustainable route to ACN with potential greenhouse gas emission offsets from a renewable feedstock Because the Sohio process depends on pro pylene ACN prices have historically been tied to petroleum prices and therefore volatile In dustrial deployment of an ACN production pro cess using an alternative feedstock such as described in this work could stabilize the ACN price by unhinging it from sole dependence on fossil resources Combined with further process development the use of nitrilation could lead to a sustainable process for biobased ACN and ultimately to products such as renewable car bon fiber REFERENCES AND NOTES 1 J M Thomas W J Thomas Principles and Practice of Heterogeneous Catalysis Wiley 2015 2 R K Grasselli F Trifirò Top Catal 59 16511658 2016 3 The Sohio Acrylonitrile Process American Chemical Society 1996 wwwacsorgcontentdamacsorg 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Science and Engineering Discovery Environment XSEDE allocation MCB090159 at the Texas Advanced Computing Center and by the National Renewable Energy Laboratory Computational Sciences Center supported by the DOE Office of Energy Efficiency and Renewable Energy under contract DEAC36 08GO28308 We thank L Berstis S Kim and P Kostestkyy for helpful discussions regarding the catalytic mechanism D Salvachúa and X Wang regarding bioreactor cultivation B Black K Ramirez and M Reed for analytical assistance and members of the Renewable Carbon Fiber Consortium for helpful discussions EMK TRE DRV and GTB are inventors on patent application WO 2017 143124 A1 US 2017018272 submitted by the Alliance for Sustainable Energy that covers nitrile production from biobased feedstocks RTG is a coinventor on intellectual property related to biological 3HPA production which is now owned by Cargill All data generated in this study are in the supplementary materials The US Government retains and the publisher by accepting the article for publication acknowledges that the US Government retains a nonexclusive paid up irrevocable worldwide license to publish or reproduce the published form of this work or allow others to do so for US Government purposes SUPPLEMENTARY MATERIALS wwwsciencemagorgcontent35863681307supplDC1 Materials and Methods Supplementary Text Figs S1 to S48 Tables S1 to S15 References 38100 5 March 2017 accepted 2 November 2017 101126scienceaan1059 Karp et al Science 358 13071310 2017 8 December 2017 4 of 4 RESEARCH REPORT Downloaded from httpsciencesciencemagorg on December 7 2017 Renewable acrylonitrile production Watson O Stanley Fruchey Derek R Vardon Ryan T Gill Adam D Bratis and Gregg T Beckham Robin M Cywar Rongming Liu Lorenz P Manker William E Michener Michelle Gilhespy Zinovia Skoufa Michael J Eric M Karp Todd R Eaton Violeta Sànchez i Nogué Vassili Vorotnikov Mary J Biddy Eric C D Tan David G Brandner DOI 101126scienceaan1059 358 6368 13071310 Science Science this issue p 1307 oxidation process relies on inexpensive titania as a catalyst and avoids the side production of cyanide that accompanies propylene manufactured this compound from an ester ethyl 3hydroxypropanoate that can be sourced renewably from sugars The precursor to a wide variety of plastics and fibers that is currently derived from propylene Karp et al efficiently However there are also numerous opportunities in commodity chemical production One such candidate is acrylonitrile a Much of the attention directed toward displacing petroleum feedstocks with biomass has focused on fuels A sweet source to make acrylonitrile ARTICLE TOOLS httpsciencesciencemagorgcontent35863681307 MATERIALS SUPPLEMENTARY httpsciencesciencemagorgcontentsuppl2017120635863681307DC1 REFERENCES httpsciencesciencemagorgcontent35863681307BIBL This article cites 69 articles 3 of which you can access for free PERMISSIONS httpwwwsciencemagorghelpreprintsandpermissions Terms of Service Use of this article is subject to the Science is a registered trademark of AAAS licensee American Association for the Advancement of Science No claim to original US Government Works The title Science 1200 New York Avenue NW Washington DC 20005 2017 The Authors some rights reserved exclusive print ISSN 00368075 online ISSN 10959203 is published by the American Association for the Advancement of Science Downloaded from httpsciencesciencemagorg on December 7 2017