Growing the Bioeconomy through Catalysis: A Review of Recent

Review Cite This: ACS Catal. 2019, 9, 4145−4172

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Growing the Bioeconomy through Catalysis: A Review of Recent Advancements in the Production of Fuels and Chemicals from Syngas-Derived Oxygenates R. Gary Grim,† Anh T. To,† Carrie A. Farberow, Jesse E. Hensley, Daniel A. Ruddy, and Joshua A. Schaidle*

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National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States ABSTRACT: Synthesis gas (syngas), composed primarily of H2 and CO, can be produced from fossil resources, municipal solid waste, biogas, and terrestrial biomass and can be converted into oxygenated intermediates such as alcohols and aldehydes through both catalytic and biological routes. These oxygenates serve as precursors for the downstream production of fuels and chemicals. However, since these processes all proceed through syngas regardless of the feedstock, renewable resources do not offer any inherent chemical advantage over fossil resources, and the process economics is largely dictated by (i) the spread between feedstock cost and the cost of petroleum (the dominant existing feedstock for fuel and chemical production) and (ii) the conversion efficiency, in terms of both energy and carbon, normalized by capital costs. Thus, lower-cost renewable feedstocks and improved conversion efficiencies combined with policy incentives could enable increased incorporation of biocontent into fuels and chemicals through syngas-derived oxygenates. To that end, this review assesses recent advancements in heterogeneous catalysis for the downstream conversion of syngas-derived oxygenates (i.e., methanol, ethanol/C2+ alcohols, and aldehydes) to fuels and chemicals, specifically seeking to link how these advancements improve the overall conversion efficiency. In the long term, these catalysis advancements can expand the window of market conditions over which these syngas pathways are economically viable, creating an opportunity to “piggyback” on existing and future natural gas to liquids installations by cofeeding renewable feedstocks. KEYWORDS: gasification, indirect liquefaction, catalysis, biomass, gas-to-liquids, synthesis gas

1. INTRODUCTION The past decade has seen significant research activity centered around the use of renewable sources of carbon (e.g., biomass, biogas) for production of fuels and chemicals. However, commercial adoption of biofeedstocks has been slow because of both economic and technological barriers. In recent years, one of the greatest developments for non-petroleum-based fuels and chemicals production, especially in the United States, has been the discovery of shale gas and the associated boom in hydraulic fracturing. The rapid growth of inexpensive natural gas has led to a dramatic shift in the global energy landscape, most notably with respect to trends in power generation, liquefied natural gas (LNG) utilization, and the accelerated expansion of methanol synthesis, methanol-to-olefins (MTO), and methanol-to-gasoline (MTG) processes. As shown in Figure 1, global methanol demand increased by ca. 40% from 2011 to 2016 and is projected to continue to increase through 2021 at a rate of ca. 5% per year.1 Interestingly, most of this growth over the last 5−6 years was in the conversion of methanol into olefins (Figure 1), such that MTO processes consumed 13% of the global methanol produced in 2016, compared with essentially zero in 2011. This trend is projected to continue over the next 5 years, with the majority of this © XXXX American Chemical Society

Figure 1. Global methanol demand by region and global methanol consumption by end use category. The term “Other” refers to South America, Western Europe, Central Europe, Africa, the Middle East, the Commonwealth of Independent States and the Baltics, and the Indian Subcontinent. The term “Traditional Derivatives” refers to chemicals commonly synthesized from methanol, such as formaldehyde, acetic acid, and methyl tert-butyl ether. Adapted from ref 1.

increase in methanol demand being met by gasification of coal feedstocks in Asia and reforming of low-cost natural gas in North America.1 Furthermore, the Energy Information Received: September 30, 2018 Revised: March 12, 2019

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DOI: 10.1021/acscatal.8b03945 ACS Catal. 2019, 9, 4145−4172

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Figure 2. Overview of GTL processes. Reforming reactions were adapted from ref 21.

States and abroad as legislated by programs such as the U.S. Renewable Fuel Standard (RFS) and Europe’s Renewable Energy Directive. While to date RINs and other energy credits (e.g., LCFS) have been vital in spurring the use of biofeedstocks as fuel gases (e.g., biogas from landfills) in the United States, maximizing the generation of renewable fuel credits and continuing to expanding the bioeconomy requires further integration of biofeedstocks into industrial processes through refinement into higher-value advantaged fuels and chemicals. While GTL investments and federal credits help lower the economic barriers for biofeedstock utilization, to measurably increase the market share of bio-GTL processes there must be accompanying technological advancements in the catalytic upgrading of syngas-derived intermediates. For example, wellestablished, technologically viable GTL processes such as MTG and Mobil’s olefins-to-gasoline-and-distillates (MOGD) have yet to capture significant market share compared with the established petrochemical technologies. To advance these GTL processes and other fledgling precommercial technologies to the next stage, catalytic upgrading technologies must be improved to achieve higher product yields under less severe conditions with minimal unit operations to drive down capital costs. In this review, we cover recent advancements (i.e., since 2010) in heterogeneous catalysis research for the conversion of common syngas-derived oxygenates (e.g., methanol, ethanol, dimethyl ether, aldehydes) to fuels and chemicals, specifically seeking to link how these advancements improve the overall conversion efficiency. We also discuss the implications of these catalysis advancements for technology commercialization and the potential impact on the bioeconomy. While there have been advancements in direct syngas conversion such as FT and emerging syngas-to-olefins and syngas-to-aromatics routes during this time period,3−16 these pathways are outside of the scope for this review and thus are not reviewed herein.

Administration (EIA) projects that fuel production from gasto-liquids (GTL) plants (EIA considers GTL to be chemical plants that convert natural gas into liquid hydrocarbon fuels, typically through Fischer−Tropsch (FT), and does not include methanol synthesis) will increase in the future, but only as a result of two large-scale projects in South Africa and Uzbekistan.2 The EIA does not anticipate any other largescale GTL plants to be built and commissioned through 2040; however, there will continue to be growth in output as smallscale facilities (5000 barrels/day or less) come online (e.g., ENVIA Energy’s GTL plant in Oklahoma City, Oklahoma). While over the short term the boom in cheap shale gas is likely to slow the emergence of purely bio-based GTL processes for fuels and chemicals production, there is cause for optimism surrounding the long-term impacts on the bioeconomy. Specifically, although at current commodity prices biomass may be viewed as too risky for the substantial upfront capital investments compared with natural gas or coal, the “bio-GTL” (including methanol synthesis) movement nevertheless stands to greatly benefit from these investments. Since in practice all GTL processes rely on gasification/ reforming of carbonaceous feedstocks and subsequent conversion of synthesis gas (H2 + CO) into higher-value fuels and chemicals, the equipment required for a traditional “fossil-GTL” plant will be nearly identical to that of a bio-GTL plant depending upon the feedstock and synthesis gas (syngas) generation technology. Thus, although modern GTL developments may initially favor utilizing fossil-carbon feedstocks, it is reasonable to expect that these capital investments will also be a boon for biomass, allowing the bioeconomy to effectively “piggyback” on the current wave of infrastructure investments. Furthermore, if commodity prices were to swing in favor of biofeedstocks in the future, only slight modifications would be required to either cofeed with or replace fossil feedstocks within existing GTL facilities. In addition to the advantages associated with increased feedstock diversification, such as allowing operators to leverage the most economically advantaged blend of feedstocks over the lifetime of the plant, incorporation of biofeedstocks also provides immediate tangible economic benefits in the United States in the form of tradable renewable fuel credits (e.g., the U.S. Environmental Protection Agency’s Renewable Identification Numbers (RINs) and California’s Low Carbon Fuel Standard (LCFS)). These credits as well as other state, federal, and international-level incentives help to offset feedstock costs when sold on the open market and contribute toward achieving the mandated renewable levels in fuels in the United

2. GAS-TO-LIQUIDS OVERVIEW The first notable implementation of GTL technology dates to the early 1920s, when Franz Fischer and Hans Tropsch developed a process to convert gasified coal to synthetic fuels.17 By the early 1940s, production of FT fuels had reached over 1 million tons annually, largely as a consequence of supporting the war effort.18,19 However, despite technical viability, FT fuels have been competitive with refined crude oil only when implemented at massive scale. 4146


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Figure 3. Pathways to syngas and biogas from second-generation biomass feedstocks. Adapted from refs 26 and 27.

phase feeds, or (iv) gasification in the case of liquid or solid feeds.33 In the case of gas-phase steam conversion, biogas and natural gas are reacted over nickel catalysts with steam normally at relatively low pressures (14−25 bar) and temperatures in the range of 800−1000 °C.25 During the reaction, H2 and CO are produced in a ratio of 3:1 for each mole of CH4 fed to the system, making steam reforming an excellent technique to produce hydrogen-rich synthesis gas. To further refine the H2/CO ratio, the reversible water-gas shift reaction (WGSR) can be utilized to favor either H2 or CO depending on the final downstream application. For more detailed information on reforming strategies, we direct readers to a recent review by Reimert et al.34 For liquid- or solid-phase feeds including coal, pet coke, heavy oil, or biomass, gasification is the most common method for producing syngas, with an estimated 128 operating plants producing ca. 42 700 MW of syngas worldwide as of 2010.24 Gasification operations rely on high temperatures (>800 °C) and an oxidizing agent, normally steam or separated air, to rapidly break down carbonaceous feeds into syngas, char, ash, and polyaromatic hydrocarbons (tars).24 Gasifiers come in a variety of configurations depending on use and feed properties; the most common commercial ones are downdraft (∼75%) and fluidized bed (∼20%).35 Although gasifiers are capable of utilizing a diversity of feedstocks (e.g., municipal solid waste), the syngas quality, gasifier design, and process economics of gasification are all influenced by feedstock selection.36,37 In comparing common fossil and biomass feedstock properties, notable differences include C/O ratio, volatile content, moisture, heating value, contaminants (e.g., sulfur, mercury, tars), particle shape and size distributions, and delivered cost.38−40 These feedstock properties affect upstream collection and handling as well as gasification chemistry. With regard to upstream collection and handling, the biomass supply chain for industrial energy applications is not well established, largely because of the low energy density of biomass. Further, the flow/melt properties and heterogeneity of biomass feedstocks present challenges for solids feeding (which have often been addressed through slurry systems for coal gasification) and operability (e.g., the contaminant profile can affect slagging). With regard to gasification chemistry, gasified biomass produces syngas with a lower overall carbon footprint, reduced contamination from heavy metals, and a higher H2/CO ratio more conducive to downstream liquid fuel synthesis, with the trade-off of additional upstream preprocessing for drying and homogenization of particle sizes to mitigate challenges with solids feeding.38−40 Further, gas cleanup in biomass-based gasification can be complicated by the formation of excess ash and tars, leading to one of the most expensive technological

More recently, with the abundance of cheap natural gas, rising global energy demands, and the finite nature of easily accessible conventional petroleum reserves, there has been a revival in interest surrounding GTL processes, especially for the production of middle-distillate-range fuels and lowmolecular-weight oxygenates.20 Furthermore, GTL processes are now being considered as a promising pathway to incorporate renewable carbon into fuels and products, providing an overall greener solution to many of the climaterelated issues facing modern energy production. A simplified overview of several common GTL routes is shown in Figure 2. 2.1. Synthesis Gas Production and Utilization. The building block of any GTL process is syngas, which comprises mostly H2 and CO and can be produced from virtually any organic material, fossil- or renewable-based. Coal and natural gas are the most commonly consumed fossil feedstocks for producing syngas,22,23 and as such their conversion to syngas is well understood and reviewed.24,25 In considering possible future directions of GTL and next-generation feedstocks, this discussion will emphasize renewable bio-based feedstocks for syngas production, specifically biomass and biogas. In the context of energy production, biomass broadly falls into two categories: first-generation starch- and sugar-based feeds (e.g., sugar cane and corn) and second-generation lignocellulosic feeds. Second-generation feedstocks do not directly compete with agriculture and contain woods, grasses, algae, and agricultural and paper industry residues as well as municipal wastes and animal byproducts (Figure 3). Utilization of municipal wastes for GTL is interesting in that approximately one-third of all food prepared for human consumption goes to waste, leading annually to ca. 1.3 billion tons of potential feed for GTL that would otherwise be buried or incinerated.28 Some commercial operations have begun to explore the concept of valorizing municipal waste streams, such as Enerkem in Quebec and Fulcrum Bioenergy in California.29,30 Furthermore, waste streams that are not directly converted to fuels can also be utilized for GTL via biogas reforming. As organic matter decays under anaerobic conditions, such as in a landfill, a gaseous mixture known as “biogas” is produced that consists mostly of CH4 and CO2 with trace amounts of H2, H2S, and N2.31 Biogas can also be manufactured directly from waste streams through a series of biochemical processes including hydrolysis, acidogenesis, acetogenesis, and methanogenesis.32 As highlighted in Figure 2, syngas can be produced using a variety of pathways depending on choice of feedstock, but these pathways typically include either (i) steam reforming, (ii) partial oxidation, (iii) autothermal reforming in the case of gas4147


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Figure 4. Direct products from the catalytic and biological conversion of syngas. Adapted from ref 47.

Table 1. Summary of Economics and Market Data for the Syngas-Derived Oxygenates Discussed in This Review47 chemical product

product cost range from biomass-derived syngas

U.S. market value range

U.S. market capacity (MMT/year)

ethanol/C2+ alcohols methanol aldehydes acetic acid butanol 2,3-butanediol

1.4−3.5 USD/gal 0.9−1.5 USD/gal 900−1200 USD/MT 250−500 USD/MT 2.5−4.0 USD/GGE 420−700 USD/MT

1.2−2.2 USD/gal 0.9−2.0 USD/gal 2000−4000 USD/MT 2500−3500 USD/MT 1.0−4.0 USD/GGE 1000−3000 USD/MT

45 7.5 3 2 not available not available

challenges associated with gasification operations. In a typical biomass-to-liquids plant, raw syngas contains on average 180− 350 ppm H2S, 20−40 ppm COS, 2100−3000 ppm NH3/ HCN, 130−250 ppm HCl, and 2−5 g/Nm3 tars in addition to other particulates and ash.41 Although present at relatively low concentrations, these contaminants can have severe consequences within the system such as catalyst poisoning, equipment corrosion, and potential blockages and thus require dedicated mitigation strategies involving cooling, filtering, scrubbing, reforming, WGSR, and acid gas removal.35,41 Coprocessing of biofeedstocks with fossil fuels, e.g., biogas with natural gas for steam reforming or biomass with coal for gasification, is a recent area of interest with potential economic, environmental, and operational benefits compared with sole fossil fuel use. With recent policy incentives such as renewable carbon credits (e.g., RINs, LCFS), operators have new means to partially subsidize associated green costs and lower the barrier to profit from renewable carbon utilization. Furthermore, studies have shown that coprocessing of biomass with coal also contributes to numerous operational synergies, including a reduction in gaseous contaminants (e.g., NOx, SOx, and VOCs),42 a deintensification of downstream cleanup requirements with respect to tar, sulfur, and ash,43,44 a more tunable gas composition (e.g., H2/CO ratio), and an overall increase in conversion to gases.44 However, coprocessing also contributes to new operational challenges, in some cases requiring process modifications in the upstream handling of feedstocks. In the case of gasification, differences in density, shape, and size of feedstocks can contribute to particle segregation during transportation, storage, and operation.45 The breakdown of homogeneous mixtures can create nonuniform rates of gasification within the process, leading to lower carbon conversion and increased frequency of process upsets.45

The addition of upstream processing equipment for pelletizing or briquetting blended feeds as well as torrefaction of biomass are seen as possible pathways to mitigate these challenges.45 In coprocessing of biogas with natural gas, the added CO2 component from biogas can be either removed via upstream separation processes (e.g., pressure swing adsorption, membrane separation) or directly upgraded by catalysts active in both steam reforming and dry reforming reactions such as Ni on CaO/Al2O3.46 Commercial utilization of syngas typically involves either combined heat and power processes or chemical and fuels production. For chemical and fuels production, the major end products from syngas utilization are ammonia (ca. 50%) and bulk H2 (ca. 25%), with methanol, FT fuels, and others constituting the balance.23 Figure 4 shows the chemicals, fuels, and intermediates that can be directly synthesized from syngas by either catalytic or biological conversion, including methanol, mixtures of higher alcohols (mostly C 2 −C 4 alcohols), H2, FT liquids, synthetic natural gas, aldehydes, acetic acid, 2,3-butanediol, and butanol.47 These direct conversion technologies are generally well-established or have been extensively reviewed by others.48−56 With regard to direct conversion of syngas to hydrocarbons (e.g., FT), recent catalyst development efforts have focused on shifting the product slate toward more valuable hydrocarbons such as olefins and aromatics. While direct conversion of syngas to hydrocarbons is outside the scope of this review, we point the reader to references in the literature for more information.3−16 2.2. Chemical and Fuel Production Paths from Syngas-Derived Oxygenates. The following subsections define the scope of this review and provide an overview of the synthetic pathways for producing each oxygenate from syngas and the established and emerging catalytic routes to upgrade 4148


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Figure 5. Reaction pathways for chemicals and fuels production from methanol.

discussed in detail in this review. While butanol and 2,3butanediol hold potential as platform molecules for further upgrading, especially through dehydration to mono- and diolefins, there have been very few recent reports on catalytic upgrading of these compounds to fuels and chemicals, and thus, butanol and 2,3-butanediol are also not discussed in this review. The following subsections introduce the specific pathways for subsequent conversion of the selected oxygenates into fuels and chemicals. 2.2.2. Methanol. Methanol is an important platform chemical for fuel and chemical synthesis, as shown in Figures 1 and 5, and is predominantly produced from syngas in a tubular fixed-bed reactor using a Cu/ZnO/Al2O3 catalyst operating at a temperature range of 250−320 °C.59 Reaction pathways to produce chemicals and fuels from methanol are shown in Figure 5. Among all products, formaldehyde is the dominant chemical produced from methanol, with a global demand of 30 million metric tons/year (MMT/year), including 4.5 MMT/year for the U.S. market.60,61 It is an important basic chemical to produce thermosetting polymers, dyes, resins, adhesives, and explosives. Formaldehyde is produced from methanol via two main industrial processes: oxidation−dehydrogenation over a silver catalyst or, more commonly, selective oxidation over a mixed metal oxide catalyst, usually iron molybdate (i.e., Fe2(MoO4)3 or Fe-Mo-oxide) as in the Formox process.62 The second major use of methanol is for production of methyl tert-butyl ether (MTBE) and tertamyl methyl ether (TAME) via etherification.50,63 However, this pathway is not discussed in this review since the use of MTBE in gasoline has been phased out, especially in the U.S.,50,64 because of environmental concerns such as groundwater contamination.65,66

those oxygenates into fuels and chemicals. Importantly, the various catalytic upgrading routes for these oxygenates share certain reaction steps, and thus, advancements in these specific steps could improve the economic viability of multiple upgrading routes. These common reaction steps will be identified in the subsections with section 2, and the recent catalytic advancements for these steps will be thoroughly reviewed in section 3. 2.2.1. Scope of this Review. This review focuses on the catalytic upgrading of syngas-derived oxygenates, specifically methanol, ethanol/higher alcohol mixtures (C2+ ), and aldehydes, which are among the most commonly exploited oxygenates for further fuel and chemical synthesis.57 These compounds were selected by assessing their (i) biomassderived cost ranges relative to market pricing, (ii) U.S. market capacity, and (iii) extent of recent R&D activities targeted at catalytic upgrading to fuels and chemicals. Table 1 shows the estimated production costs for these oxygenates and their current market capacities in the United States.47 Methanol and ethanol have the potential to be cost-competitive when produced from biomass and have high market demands, thus providing a great opportunity for large-scale utilization of syngas. Although a large fraction of the market demand for ethanol stems from its use as a fuel blendstock, it has the potential to be used as platform chemical for further synthesis of drop-in fuels and chemicals.57 Aldehydes and acetic acid can be cost-advantaged when produced from biomass and have reasonable market capacities. However, acetic acid is mainly used for polymer synthesis via production of monomer esters (e.g., vinyl acetate, cellulose acetate, and acetic anhydride)58 with limited advancements in catalytic upgrading. Therefore, the catalysis advancements for acetic acid upgrading are not 4149


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Figure 6. Reaction pathways for chemical and fuels production from ethanol.

Figure 6. Similar to methanol, dehydration and hydrocarbon pool chemistry of ethanol over acidic catalysts,85 such as alumina or zeolites, are important reactions for production of olefins, aromatics, and fuels.86,87 In addition, selective production of certain olefins and higher alcohols can be achieved with mixed oxide catalysts.88 Condensation of ethanol over acid−base bifunctional catalysts is a promising synthetic route to isobutene and 1,3-butadiene,89 which are important monomers for plastic and rubber production. As of 2012, ca. 10 MMT of 1,3-butadiene was produced annually from naphtha steam cracking units as a byproduct of ethylene manufacturing.90 Higher alcohols (i.e., butanol) can also be produced from ethanol. Compared with ethanol, 1-butanol has higher energy density and better solubility in gasoline, thus making it a better drop-in fuel than ethanol (Table 2). 1-

The third most abundant chemical produced from methanol is acetic acid, mainly via the liquid-phase methanol carbonylation reaction, with current commercial technologies making use of homogeneous catalysts such as Co or Rh complexes.58,67,68 In addition to formaldehyde and acetic acid synthesis, selective oxidation or carbonylation of methanol can also be used to produce methyl formate, a precursor for amides and formic acid production.50 Dimethyl ether (DME) has become an important methanolderived product since it can be used as an alternative fuel for compression ignition engines, as liquefied petroleum gas, and as an intermediate for olefins or fuels production.69 The synthesis of DME from methanol is a simple dehydration reaction over a mildly acidic catalyst such as alumina or zeolites.70 In addition, direct synthesis of DME from syngas has drawn significant attention in the research community.71 Methanol is also used to produce olefins (i.e., the MTO process) or gasoline (i.e., the MTG process) via catalytic conversion over zeolite catalysts.72,73 The reaction goes through dehydration to DME and hydrocarbon pool chemistry for carbon chain growth, including C−C bond formation (e.g., oligomerization, methylation, and homologation), hydrogen transfer, cyclization, aromatization, and cracking reactions.74−79 2.2.3. Ethanol and C2+ Alcohols. In 2016, ethanol production capacity was over 27 billion gallons worldwide, of which the U.S. produces over 15 billion gallons per year.80 In addition to the traditional sugar fermentation route, ethanol and C2+ alcohols can also be produced from syngas via (i) a catalytic thermochemical route81,82 or (ii) a biochemical fermentation route.54,83,84 Ethanol is predominantly used as a gasoline blendstock, accounting for 90% of biofuel demand. With increasing ethanol production capacity and an existing limit on the maximum ethanol blend level in gasoline in the U.S., there is an opportunity to utilize ethanol as a platform chemical for the production of fuels and value-added chemicals.57 Existing reaction pathways to produce building blocks for fuels and chemicals from ethanol are summarized in

Table 2. Fuel Properties of C2−C4 Alcohols Compared with Gasoline and Diesel91,95 fuel

energy density (MJ/L)

research octane number

water solubility (%)

gasoline diesel 1-butanol ethanol

32 35.5 29.2 19.6

81−89 − 78 96

negligible negligible 7 100

Butanol can be produced from ethanol via the direct condensation reaction over alumina supported metals or the Guerbet condensation reaction over basic mixed oxides and hydroxyapatites.91−94 Similar to methanol, ethanol can also be selectively oxidized to its aldehyde, acetaldehyde, which is typically performed over supported mono- and bimetallic catalysts. 2.2.4. Aldehydes. Aldehydes are mostly produced from the oxosynthesis process (i.e., hydroformylation of olefins with syngas).96−98 Among this class of oxygenates, butyraldehyde is the second most important commercial aldehyde (after formaldehyde), with an annual global consumption of over 7 MMT.98 It is mainly used as the precursor for the synthesis of 4150


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Scheme 2. Mechanism of Methanol-Selective Oxidation over Fe-Mo-Oxide Catalystsa

Scheme 1. Synthesis of 2-Ethylhexanol from Butyraldehyde

hydrogenation, with the first step usually using a strong liquid base or hydrotalcite as the catalyst.101−103 Even though the 2ethylhexanol synthesis process is well-developed and commercialized, the aldol condensation reaction of aldehydes has drawn considerable interest recently because the formation of C−C bonds via aldol condensation provides an important pathway to valorize the light oxygenates from either biomass gasification or pyrolysis processes.104 The long-chain aldehydes (C5−C14) produced from the oxosynthesis process are mainly hydrogenated to alcohols, which are used in detergents. The technologies for hydrogenation of aldehydes to alcohols are well-developed97 and thus will not be discussed in this review. 2.2.5. Common Catalytic Reaction Pathways. Interestingly, despite involving different starting materials, the three groups of oxygenates (i.e., methanol, ethanol/C2+ alcohols, and aldehydes) generally share some common reaction pathways to form the final fuel/chemical products. Section 3 of this review addresses advancements in heterogeneous catalyst development since 2010 for the following reaction classes: • Selective oxidation of alcohols • Carbonylation of alcohols • Dehydration and hydrocarbon pool chemistry of alcohols • Condensation of alcohols and aldehydes

a

Adapted from ref 114.

MO site is important for methanol adsorption, and the bridging O site is responsible for oxidation of the methoxy group to form formaldehyde.111 In addition, the presence of Fe2O3 and Fe2(MoO4)3 layers improves the redox activity of the catalyst by enabling subsurface diffusion of anion vacancies when oxygen is consumed for oxidation, enhancing O2 uptake from the gas phase during reoxidation of the surface and facilitating increased surface area of the active MoO x species.61,112 From this fundamental insight, new catalyst preparation methods have been proposed to enhance the surface area of the active MoOx species compared with the conventional coprecipitation method used in industry; these new methods include supporting iron on MoO3 nanorods62 and producing Fe2(MoO4)3 nanoparticles by grinding the precursor metal salts together with oxalic acid.113,117 These methods also mitigate a disadvantage of the coprecipitation method, which produces large amounts of aqueous waste containing unprecipitated iron and molybdenum. Isolated Fe2(MoO4)3 nanoparticles anchored onto MoO3 nanorods exhibited a 15− 40% increase in the formaldehyde formation rate compared with a conventional Fe-Mo-oxide catalyst with similar composition prepared by the coprecipitation method.62 Preparation of Fe2(MoO4)3 nanoparticles by grinding the metal salts with oxalic acid increased the Brunauer−Emmett− Teller (BET) surface area of the active phase by 15% and achieved a slightly higher formaldehyde yield (92.6%) at lower reaction temperature (320 °C) compared with a catalyst of similar composition prepared by the coprecipitation method (90% yield at 350 °C).113,117 In the conventional Fe-Mo-oxide Formox catalyst, vaporization of molybdenum from the catalyst during operation is a major issue leading to catalyst deactivation, and its effect is mitigated industrially by utilizing excess MoO3. However, the catalyst lifetime is still relatively short compared with other industrial processes, i.e., about 1−2 years.107,110 Thus, there is interest in developing methods to stabilize Mo from vaporization and designing alternative Mo-free catalyst systems. Andersson et al. developed stable spinel-type Fe-V-Mo-oxide catalysts, which provide flexibility by allowing the cations to change between different oxidation states within the same basic structure while maintaining the same ratio of the metals.118 Compared with the conventional Fe-Mo-oxide catalyst, this structure significantly improved the catalyst stability toward molybdenum volatilization while maintaining similar activity,

3. RECENT ADVANCEMENTS IN CATALYST DEVELOPMENT 3.1. Selective Oxidation of Alcohols. As noted above, the Formox process is the dominant process for the production of formaldehyde from methanol and employs iron molybdate (i.e., Fe2(MoO4)3 or Fe-Mo-oxide) catalysts.61,105 Although this commercial process is well-developed, there are still ongoing fundamental studies of the Fe-Mo-oxide catalyst system, especially to identify the active phase under operating conditions106−111 and to determine the structure−function relationships between iron and molybdenum oxides and catalyst performance.61,112 These fundamental studies lay the foundation for the development of new catalysts to improve the conversion efficiency62,113 and are based on a long-standing understanding of the inherent reaction mechanism. The adsorption and reaction of methanol on the surface of FeMo-oxide catalysts follow the Mars−Van Krevelen mechanism, which includes three elementary steps (Scheme 2): (i) adsorption of methanol on an active surface site, (ii) oxidation of methanol by surface oxygen followed by desorption of the product, and (iii) reoxidation of the catalyst surface.114−116 By studying MoOx /Fe2 O 3 model materials,106,109,110 Brookes et al. proposed that the octahedral MoOx layer, formed by segregation of Mo oxides at the catalyst surface, is the active site for selective methanol oxidation, specifically a combination of Mo(VI) and Mo(IV) sites.107,108 The terminal 4151


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(up to a 3-fold increase in ethanol conversion) while still maintaining high selectivity for acetaldehyde (>92%). Recent advancements in catalyst development for selective alcohol oxidation have focused on the following areas: • enhancing the active surface area of the Fe-Mo-oxide methanol oxidation catalyst through novel synthetic approaches; • stabilizing the Fe-Mo-oxide catalyst to prevent molybdenum vaporization; • developing Mo-free catalysts based on redox-active materials; • achieving high acetaldehyde yields from selective oxidation of ethanol by understanding the roles of the metallic NP active phase and the underlying support. 3.2. Carbonylation of Alcohols. Carboxylic acids and esters are synthesized commercially via the carbonylation of alcohols using homogeneous transition-metal catalysts, typically Rh, because of their high activity and selectivity.58,67,68 Scheme 3 shows the catalytic cycle for methanol carbonylation

exhibiting negligible molybdenum volatiles after 3 days of operation (compared with 36% loss of molybdenum for a conventional Fe-Mo-oxide catalyst). The same research group also developed a Mo-free spinel Fe-V-oxide catalyst that exhibited better specific activity (1.6 μmol m−2 s−1) than the conventional Fe-Mo-oxide catalyst (0.6 μmol m−2 s−1),119 but further improvements in formaldehyde selectivity are required. Vanadium phosphate (VPO) catalysts have also been demonstrated to be active for selective oxidation reactions because of the redox properties of surface V species and high lattice oxygen mobility. The Bartley group studied various metal-modified VPO catalysts for the methanol oxidation reaction using WO3 or Al2O3.120,121 They showed that modification of VPO with W introduced Brønsted acidity that led to higher activity than a commercial Fe-Mo-oxide catalyst at low reaction temperature (i.e., a formaldehyde yield of 37.5% compared with 8.3% at 200 °C). However, at typical industrial methanol oxidation temperatures (300 °C), the Wmodified VPO catalyst exhibited an only slightly higher formaldehyde yield (95.8%) than the Fe-Mo-oxide catalyst (93.1%). While acetaldehyde is primarily produced via the selective oxidation of ethylene (Wacker process),122 selective oxidation of ethanol to acetaldehyde provides a promising opportunity to replace the use of petroleum-based feedstocks with renewable feedstocks. Cu-containing catalysts are well-known systems for alcohol dehydrogenation, and supported Cu catalysts have been studied for oxidation−dehydrogenation of ethanol to acetaldehyde.123−125 Among all of the investigated supports, carbon has been demonstrated to be a promising material to achieve high acetaldehyde selectivity since its inert and hydrophobic surface inhibits further conversion of acetaldehyde to byproducts.123,124 A Cu catalyst supported on mesoporous carbon developed by Wang et al. exhibited excellent performance for ethanol dehydrogenation to acetaldehyde, with 73% ethanol conversion and 94% acetaldehyde selectivity under severe conditions at high ethanol concentrations (15 vol %) and GHSV (8600 h−1).123 Recently, gold nanoparticles (AuNPs) have shown promise as catalysts for partial oxidation of ethanol to acetaldehyde due to their weak adsorption of the main product,126 low tendency to activate C−H bonds,127 and tunable activity by the use of different support types.128−131 Haruta et al. showed that acidic and basic oxide supports produce high acetaldehyde yields (up to 95%) at temperatures greater than 240 °C.129 Murzin et al. showed that a TiO2-supported AuNP catalyst exhibits high activity for ethanol oxidation at low temperature; 60% ethanol conversion with 80% selectivity for acetaldehyde can be achieved at 120 °C over this catalyst, while negligible oxidation activity is observed with other supported catalysts at this low temperature.128,130 However, further research is needed to address deactivation. Recently, Liu and Hensen developed a AuNP catalyst supported on MgCuCr2O4 spinel that achieved complete ethanol conversion with ca. 95% acetaldehyde selectivity at 250 °C and was stable over 500 h time on stream.131 The high catalyst activity and stability were explained by the synergy between metallic AuNPs and surface Cu+ species, which are stabilized at the surface of MgCuCr2O4 spinel and promote the activation of O2. Guan and Henson proposed adding a second metal (e.g., Ir) to the supported catalyst to enhance O2 activation and therefore the ethanol oxidation activity.132 The Au−Ir bimetallic catalysts exhibited higher activity than their monometallic counterparts at 180 °C

Scheme 3. Catalytic Cycle for the Rh-Complex-Catalyzed Methanol Carbonylationa

a


catalyzed by a Rh complex, and Table 3 summarizes existing commercial technologies for methanol carbonylation. The homogeneous catalysts typically require high water concentration to maintain stability, but that leads to costly separations and competing reactions (i.e., water-gas shift). Thus, recent process advancements have focused on stabilizing the metal complex, promoting methanol activation using iodide salts Table 3. Commercial Processes for Methanol Carbonylation process BASF Monsanto Celanese Cativa (BP) Acetica (Chiyoda/ UOP) 4152

catalyst/cocatalysts Co complex/CH3I Rh complex/CH3I Rh complex/CH3I/LiI Ir complex/CH3I/ Ru(CO)xIy PVP-supported Rh complex/CH3I

temperature (°C)

pressure (MPa)

230 150−200 N/A 190

6−8 3−6 N/A ∼3

160−200

3−6


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product is methyl acetate, which requires further hydrolysis to produce acetic acid. In addition, the catalyst was deactivated as a result of gradual leaching of the anchored iodide species, which can be reversed by treatment in H2 to regenerate the catalytic activity. Acidic zeolites containing eight-membered rings (8-MRs) have also been demonstrated to be active catalysts for selective carbonylation of either methanol or dimethyl ether to acetic acid and methyl acetate, with MOR zeolite being the most active, followed by FER zeolite.144,145 However, because of the contribution of acid sites within the 12-MR channel in MOR zeolite, hydrocarbon pool chemistry is also significant, leading to severe coke formation and catalyst deactivation (as discussed further in section 3.3). To address this issue, Ni et al. recently demonstrated that pyridine preferentially adsorbs on Brønsted acid sites in the 12-MR channels rather than in the 8-MR channels of H-MOR, thus blocking access to the 12MR channels.137,138 The pyridine-modified MOR exhibited excellent performance for methanol carbonylation, with almost complete methanol conversion to acetyls (i.e., acetic acid and methyl acetate) at 240 °C and 95% yield of acetic acid at 270 °C. The catalyst was also stable during 145 h of reaction. Compared with conventional homogeneous and supported metal complex catalysts,134 methanol carbonylation over pyridine-modified MOR requires more severe operation conditions (240−270 °C at 5 MPa compared with ca. 200 °C at 2.4 MPa) and exhibits order-of-magnitude lower acetyl productivity. However, this is still a promising approach to avoid the use of the corrosive CH3I cocatalyst. Li et al. also sought to mitigate MOR deactivation by preparing MOR− FER composite catalysts and demonstrated that these composites exhibit enhanced catalyst activity compared with FER zeolite and are much more stable for up to 40 h of reaction, whereas MOR zeolite deactivates quickly during the first 10 h of reaction.139 In contrast to the metal complex catalyst systems, the presence of water has a negative effect on the carbonylation reaction over zeolites because of its competitive adsorption with CO on Lewis acid sites, thus inhibiting the interaction of CO with adsorbed methyl intermediates. Therefore, zeolites are more effective for carbonylation of DME than methanol.137,138,145 While methanol carbonylation is a commercial technology and is heavily studied, ethanol carbonylation to propionic acid or ethyl propionate has just recently received considerable attention because of the increasing demand for propionates as platform chemicals to produce plastics, pharmaceuticals, fragrances, and pesticides.146 Notestein et al. investigated Rh supported on heteropoly acids and zeolites as catalysts for ethanol carbonylation.147,148 Their studies showed that ethanol dehydration (to ethylene and diethyl ether) significantly competes with carbonylation and that the presence of C2H5I is essential for promoting the carbonylation reaction. Removal of acid sites by exchanging protons in the heteropoly acid or adding alkali to NaX zeolite enhances the selectivity for the carbonylation pathway, with the heteropoly-acid-supported Rh catalyst showing better performance than the NaX-supported Rh catalyst. Recent advancements in catalyst development for methanol carbonylation have focused on the following areas:

(e.g., LiI), and employing alternative metal complex formulations (e.g., Ir−Ru) to achieve better activity at low water concentration.67,68 The development of heterogeneous catalyst systems for alcohol carbonylation, to overcome the inherent disadvantages of the homogeneous reaction systems, is an active research area. Two types of heterogeneous catalyst systems have been studied: immobilized metal complexes133−136 and zeolites.137−139 Recent advancements in immobilized complexes led to the Acetica technology jointly developed by Chiyoda and Honeywell UOP136 (Table 3), which has been licensed by plants in China67 and Brazil.140 Ren et al. incorporated a Rh complex into porous organic ligands (POLs) containing a high content of exposed P groups via impregnation of a Rh2(CO)4Cl2 complex on POL-PPh3.134,141 Strong interaction of the P groups stabilizes the well-dispersed, isolated Rh sites and increases the nucleophilicity of Rh, thus accelerating the rate-limiting oxidative addition of a methyl group to the Rh center. The supported catalyst exhibited only a slightly higher activity than a typical homogeneous Rh complex catalyst for methanol carbonylation (i.e., a turnover frequency of 1558 h−1 compared with 1470 h−1) but was operated at a lower concentration of CH3I cocatalyst (i.e., CH3OH/CH3I = 1.40 instead of 1.08 in the homogeneous catalyst system) and required no additive water or acetic acid. The catalyst also exhibited good stability, maintaining high activity for nearly 180 h of operation. Novel Rh-incorporated graphitic carbon nitride catalysts (Rh−g-C3N4) have also been reported to be excellent catalysts for liquid-phase methanol carbonylation.135,142 Coprecipitation of a Rh salt and melamine (as a carbon nitride source) followed by polycondensation at high temperature was used to prepare a homogeneous distribution of small metallic Rh crystallites (below 1 nm) in a matrix of gC3N4 at low Rh loadings (i.e., 1 and 3 wt %). The resulting catalyst achieved complete methanol conversion with a higher selectivity for acetic acid than comparable supported catalysts based on the Acetica process (i.e., 83.6% compared with 71.3%) and showed good recyclability with negligible Rh leaching. In the homogeneous carbonylation process, as well as many of the heterogeneous catalyst systems, iodide is an essential cocatalyst to activate methanol and generate the methyl substrate that is active for the carbonylation reaction (as shown in Scheme 3). This process generates corrosive HI, which requires exotic reactor materials, resulting in high capital costs for the process. Therefore, considerable emphasis has been placed on developing catalysts that do not require an iodide promoter. Dingwall et al. synthesized bifunctional catalysts containing an organorhodium complex on solid heteropoly acid supports to replace iodide for methanol activation.143 The resulting SiO2-supported RhxHPW was active for methanol carbonylation with a light-off temperature of 150 °C and reached a maximum in activity at 200 °C, with the main product being methyl acetate. However, with this catalyst system, DME from methanol dehydration was also a major product. Simultaneous immobilization of Rh and iodide over a cross-linked copolymer has also been proposed as a promising approach.133 Methyl acetate and anchored iodide groups in the catalyst convert methanol to adsorbed methyl iodide species, which are active for carbonylation catalyzed by the anchored Rh complex. In this system, the catalytic activity is proportional to the iodide loading on the polymer, and high methanol conversions can be achieved (>90%); however, the major

• alleviating difficulties in product and catalyst separation and recovery for homogeneous catalyst systems by 4153


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a C2 and C4 olefin recycle stream enabling high selectivity for the propylene product.152 The historical development of these various methanol-to-hydrocarbons (MTH) processes has been thoroughly reviewed,73,75,76 so the remaining portion of this section focuses on the most recent catalyst research aimed at leveraging our current understanding of zeolite structure− function relationships for MTH reactions to alter the selectivity away from the traditional products and toward more valuable hydrocarbon products and to reduce the energy intensity of MTH processes. 3.3.1.1. Methanol to HydrocarbonsMechanism. Many MTH mechanisms have been postulated, and recently a general consensus has emerged. It is believed that MTH reactions proceed through a “hydrocarbon pool” mechanism involving a dual catalytic cycle in which one cycle produces methylbenzenes and ethylene and a second cycle produces C3+ olefins (Scheme 4).153−156 The reactions occurring within this

developing heterogeneous catalyst systems with comparable or better performance; • designing catalyst systems that do not require the use of methyl iodide as a cocatalyst, e.g., zeolites; • extending knowledge about methanol carbonylation to ethanol carbonylation for the production of propionates. 3.3. Dehydration and Hydrocarbon Pool Chemistry of Alcohols. 3.3.1. Methanol to HydrocarbonsMTG, MOGD, and MTO. Recent catalyst development for methanol conversion on zeolite catalysts builds on decades of research beginning with the MTG process discovered by Mobil Research Laboratories in 1976.73,149 In this process methanol is first converted to an equilibrium mixture of methanol, DME, and water over a mildly acidic alumina-based catalyst. This mixture is fed to a gasoline synthesis reactor, where it is converted into C1−C11 hydrocarbons, typically using a medium-pore (5.1−5.6 Å) ZSM-5 zeolite catalyst (Figure 7)

Scheme 4. Dual Aromatic and Olefin Catalytic Cycle for the Conversion of Methanol to Hydrocarbonsa

a

Figure 7. Illustrations of the framework types, structural characteristics (including pore size, number of T atoms in rings, and channel dimensionality), and primary methanol conversion applications for ZSM-5, SAPO-34, ZSM-22, and BEA zeolites.

Reproduced from ref 158.

dual cycle include six major steps: (i) olefin methylation, (ii) olefin cracking, (iii) hydrogen transfer, (iv) cyclization, (v) aromatic methylation, and (vi) aromatic dealkylation.155 Understanding the mechanisms and rates of each of these steps as a function of zeolite structure and composition is the subject of extensive ongoing research. In the absence of detailed structure−function relationships, the relative propagation of the two overall cycles can provide useful mechanistic insight for catalyst development. This value is often quantified by the ratio of the terminal products, ethylene and 2MB (2methylbutane + 2-methyl-2-butene), which exit the aromatic and olefin cycles, respectively.155,157 3.3.1.2. Aromatic-Free Hydrocarbon Production from Methanol. Mechanistic understanding of the dual cycle led to the discovery that ZSM-22, with noninteracting, onedimensional pores (4.6−5.7 Å), suppresses the formation of aromatic products (∼1%), resulting in high selectivity for branched C5+ alkenes (ca. 70%).159 Additional products include linear and cyclic alkenes. The product distribution at the typical MTG temperature of 400 °C is intermediate to those observed on ZSM-5 and SAPO-34. The elimination of the aromatic products is particularly desirable over traditional MTG, since aromatic-free gasoline provides environmental, health, and performance benefits. Using isotopic switching experiments with 13C-methanol on ZSM-5, SAPO-34, BEA, and ZSM-22 with similar acid site densities (Si/Al 30−50),

at a temperature of ca. 400 °C and pressure of ca. 20 bar. The gasoline-range C5+ fraction is produced with a selectivity of about 80% and consists of paraffins, aromatics, naphthenes, and olefins. The Mobil olefins to gasoline and distillate (MOGD) process developed about a decade later took advantage of the observation that light olefins are formed as intermediates in the MTG reactions.150 The operating conditions for hydrocarbon synthesis were modified to favor olefins by running at higher temperatures and lower pressures, and a subsequent oligomerization reactor was used to coproduce gasoline and distillate products. Both reaction steps in the MOGD process utilized a ZSM-5 catalyst. The MTO process to produce primarily light (C2−C3) olefins from methanol/DME with a selectivity of up to 90% was developed by UOP and Norsk Hydro.151 This process operated at the high-temperature, low-pressure conditions favorable for olefin production on ZSM-5 but used a smallpore (ca. 3.8 Å) SAPO-34 zeolite catalyst. The SAPO-34 catalyst suffered from rapid deactivation by coking relative to the ZSM-5 catalyst, requiring the use of a fluidized bed reactor for continuous catalyst regeneration. The Lurgi process is similar to the UOP MTO process but includes the addition of 4154


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Figure 8. (a, b) Product selectivities for reactions of DME (70−85 kPa) and propene (1.8−3.2 kPa) at 200 °C on mesoporous solid acids and various zeolites. Reproduced from ref 79. (c) Hydrocarbon production rate from DME at 200 °C on BEA and Cu-modified BEA with and without H2 cofeed.77 (d) H2 productivity from isobutane dehydrogenation at 300 °C on catalysts with varying types of Cu and acid sites. Cu(I) sites in the ion-exchanged Cu-modified BEA catalysts were active for isobutane dehydrogenation.164

structure along the chain growth pathway and/or methylation to C8+ results in rapid β-scission reactions. Other zeolites, including FER, MOR, MFI, and FAU, as well as mesoporous solid acids (SiO2−Al2O3, H3PW12O40/SiO2) were explored, and only a large-pore zeolite, FAU, produced a C7 selectivity approaching that of BEA (Figure 8a,b).79 The desirable C7 selectivity obtained on FAU and BEA relative to the other materials was attributed to solvation of the transition states for the hydride transfer and methylation steps within the large pores, providing more favorable energetics relative to the undesired competing isomerization reactions. MTH reactions are hydrogen-deficient, resulting in unsaturated carbon products via the aromatic cycle. In the case of DME homologation to alkanes on BEA, the unsaturated byproduct has been identified as hexamethylbenzene (HMB). The stoichiometric reaction for the formation of triptane and HMB from DME is shown in eq 1:

Teketel et al. demonstrated that the aromatics present in the channels of ZSM-22, unlike the other topologies, are almost inactive for methanol conversion via the aromatic cycle.160 The narrow channels in ZSM-22 provide insufficient space for the aromatic pool species, such that the alkene cracking and methylation in the alkene cycle control the product selectivity. Teketel et al. also explored other one-dimensional 10membered-ring zeolites, including ZSM-23, EU-1, and ZSM48.161 Analysis of the spent catalysts showed that aromatics are formed within the channels of all four catalysts but can only diffuse out of EU-1 and ZSM-48. Thus, an aromatic-free product was obtained over only ZSM-22 and ZSM-23. 3.3.1.3. Selective Synthesis of Triptane and Isobutane from Methanol. Recent work by Iglesia et al. focused on the production of branched C4 and C7 hydrocarbons from methanol/DME using a large-pore (5.5−7.5 Å) BEA zeolite catalyst under milder conditions (180−220 °C, 0.5−2.5 bar) compared with other MTH processes.162 The branched C7 product is a particularly desirable product because of its high research octane number of 112 and thus potential application as a fuel additive. The C4 and C7 products each accounted for ca. 30% of the carbon selectivity in the products, with ca. 90% isobutyls in the C4 fraction and ca. 80% triptyls in the C7 fraction. Detailed mechanistic studies demonstrated that termination probabilities via hydrogen transfer steps are much higher for triptyl and isobutyl species, whereas methylation is more favorable for C3, linear C4, isopentyl, and 2,3-dimethylbutyl species, resulting in the high selectivity for the branched C4 and C7 products.78,79 Isomerization reactions to produce species that deviate from the C4 backbone

33CH3OCH3 → 6C7H16 + 33H 2O + 2C6(CH3)6

(1)

Simonetti et al. found that the hydrogen deficiency could be addressed through the addition of an alkane (n-butane, isobutane, isopentane, or 2,3-dimethylbutane) along with an adamantane cocatalyst for hydride transfer.163 This strategy increased both the rate of alkane incorporation into the olefin cycle and the ratio of the alkane and HMB formation rates. To address the hydrogen deficiency without the need for a homogeneous cocatalyst, Schaidle et al. explored the addition of Cu to BEA.77 A mixture of ionic and metallic Cu enabled the incorporation of cofed H2 into the products and also resulted in a 2-fold increase in the hydrocarbon productivity 4155


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propylene production and to extend the catalyst lifetime using ZSM-5 catalysts:

compared with the unmodified BEA zeolite (Figure 8c). The authors proposed that hydrogen incorporation was achieved through (i) the addition of H2 activation and olefin hydrogenation activity via metallic Cu nanoparticles present on the external surface of the zeolite and (ii) promotion of hydrogen transfer by the Lewis acidic cationic Cu species present within the BEA pores. In a separate study by the same group, the ionic Cu(I) sites were found to activate C−H bonds in isobutane, which suggests the ability to recycle C4 products to increase the overall selectivity for gasoline-range C5+ products (Figure 8d).164 3.3.1.4. Enhancing the MTH Catalyst Lifetime. Mitigating catalyst deactivation in MTH processes continues to be an important area for catalyst development research, with recent efforts focused on understanding how the size, distributions, and connectivity of meso- and/or macropores impact the catalyst stability. Kim et al. observed that a 5-fold increase in catalyst lifetime during MTH could be achieved by adding mesopores to a variety of MFI-type zeolites and attributed the effect to more facile diffusion of coke precursors from the micropores to the external catalyst surface due to shortened diffusion path lengths.165 Using positron annihilation lifetime spectroscopy, Milina et al. quantified the global pore connectivity in hierarchical MFI zeolites synthesized using the procedures of demetalation, carbon templating, and seed silanization. Globally connected pores were correlated with extended MTH catalyst lifetime while isolated mesopores were not, suggesting that both open and interconnected mesopores are necessary for extended catalyst lifetimes.166,167 3.3.2. Ethanol Conversion to Hydrocarbons. The conversion of ethanol over zeolite catalysts through hydrocarbon pool chemistry is a natural extension of methanol conversion in MTO and MTG. Accordingly, the conversion of ethanol to hydrocarbon products was reported with that of methanol in the 1970s.168 Although the production of ethylene and propylene from ethanol has been the focus of many reports, C4 products including butadiene, C5+ alkanes and alkenes, and aromatics (benzene, toluene, xylene), are consistently observed in the reaction products. Conversion to C4+ products is a potential solution to the ethanol “blend wall” in the U.S., enabling increased renewable ethanol production to serve as a feedstock for chemical or hydrocarbon fuel production. Recent catalyst and process developments for ethanol conversion have focused on directing product selectivity toward the desired product groups of light olefins, C4+ fuel-range hydrocarbons, aromatics, or butadiene. 3.3.2.1. Ethylene and Propylene. The production of light olefins (i.e., ethylene and propylene) using zeolite catalysts has recently been reviewed.86 SAPO-34 (CHA structure) and ZSM-5 (MFI structure) have been studied the most for the transformation of ethanol to ethylene and propylene. Ethylene is highly favored over SAPO-34 because of its small pore size, with reported selectivities of up to 99% at 90% ethanol conversion. However, rapid coking is observed. Therefore, improving the catalyst lifetime is an area for continued research. The larger pore size in ZSM-5 results in a significant shift in selectivity for propylene, with the parent ZSM-5 exhibiting greater than 25% selectivity for propylene at 100% ethanol conversion. Similar to SAPO-34, rapid deactivation is typically observed. Through detailed investigations into the acid site density (i.e., the Si/Al ratio), various metal and bimetallic modifications, porosity and crystal size, and reaction conditions, the following have been identified to favor

• Intermediate SiO2/Al2O3 ratio. Maximum propylene selectivity was observed over catalysts having a SiO2/ Al2O3 ratio in the range of 50−80.169−171 Greater acid site density (lower SiO2/Al2O3) favored longer-chain products, and lower acid site density (higher SiO2/ Al2O3) favored ethylene. • Modification with Sr or La/P. Strontium-modified ZSM5 demonstrated the highest propylene selectivity, achieving 32% selectivity at 100% ethanol conversion.172 Multimetallic modification of Ga-framework-ZSM-5 with P and then La resulted in a propylene selectivity of 29% at 100% ethanol conversion and suppressed carbon deposition, leading to improved lifetimes and regenerability.173 • Hierarchical pore structures and small crystal sizes, including nanocrystals. Lifetimes were extended 2−5 times longer using a hierarchical zeolite, and small crystal sizes also led to longer lifetimes attributed to shorter diffusion path lengths and faster removal of products.174,175 • Reaction temperatures in the range of 350−500 °C with intermediate contact times. Ethylene is favored at lower temperatures of 250−260 °C, and temperatures of 300− 350 °C give a mixture of products, including a higher selectivity for C4+ products.170,171,176,177 3.3.2.2. Fuel-Range Hydrocarbons. Early studies of the conversion of ethanol into C5+ fuel-range products (ethanol-togasoline (ETG)) demonstrated increased C5+ yield by operating at temperatures of 350−450 °C, pressures of 10− 20 bar, and space velocities of 0.5−2.0 h−1, achieving a liquid product yield of 65% at 20 bar and 400 °C using a ZSM-5 catalyst.87 Shortly thereafter, metal modification of ZSM-5 was reported, and a 72% yield was achieved using a Ga- and Znmodified ZSM-5 catalyst at 10 bar and 360 °C.178 Metal modification was recently revisited, exploring Fe-, Ga-, and Nimodified ZSM-5 catalysts.177 It was concluded that these metals do not enable new reaction pathways but rather increase the hydrocarbon yield by increasing the total acid site density. Narula et al. reported an In- and V-modified ZSM-5 catalyst (InV-ZSM-5) that gives a 33% yield of a C5+ liquid product at atmospheric pressure and 350 °C and, importantly, exhibits a low C2 selectivity of 13%.179 Through characterization and performance comparisons with monometallic In-ZSM-5 and VZSM-5 catalysts, the authors demonstrated that the bimetallic InV-ZSM-5 was not simply a physical mixture of these materials. The authors explored the suitability of using fermentation-derived ethanol streams as a feedstock by assessing the water tolerance of the InV-ZSM-5 catalyst and demonstrated no change in product selectivity across a wide range of feedwater concentrations (0−90%). Furthermore, the sensitivity of the catalyst to oxygenate impurities was explored by including 0.1% light oxygenates (acetaldehyde, methanol, ethyl acetate) in the aqueous ethanol feed with extended times on stream. After oxidative regeneration at 450−500 °C, the catalyst regained 94−100% of its original activity. The same research group also explored light gas recycle to convert the gaseous C2−C4 products to C5+ liquid products over the monometallic V-ZSM-5 catalyst, which performed similarly to bimetallic InV-ZSM-5 at 350 °C and atmospheric pressure.180 4156


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Figure 9. (a) BTX yield vs reaction temperature for ZSM-5 and Ga-ZSM-5 catalysts. (b) Relationship between BTX site-time yield and exchanged Ga sites. Reproduced with permission from ref 182. Copyright 2017 Royal Society of Chemistry.

With a simulated recycle of C2−C4 hydrocarbons cofed with ethanol at a ratio of 0.79 molC2−C4/molEtOH, the C5+ yield increased from ca. 40% to 63% at early times on stream. When the C2−C4/EtOH molar ratio was increased to 1.56, the C5+ yield further increased to 80% at early times on stream. It was shown that the olefin and aromatic selectivities in the C5+ product changed with the light gas cofeed. Upon comparison with the ethanol-only feed at 12 h time on stream, the olefin content increased from 15 to 30 wt % and aromatics decreased from 61 to 45 wt % at the 0.79 molar cofeed ratio, and the olefin content further increased to 53 wt % and aromatics decreased to 26 wt % at the 1.56 molar cofeed ratio. In addition to metal modification of ZSM-5 catalysts for fuel production, ZSM-5 nanocrystals have also been explored.181 At 450 °C and atmospheric pressure, nanocrystal catalysts produced a lower yield of noncondensable gases (31.5 wt % vs 37.6 wt % for the traditional, micron-sized analogue with the same Si/Al ratio). Correspondingly, a higher liquid yield was observed over the nanocrystal catalyst (68.5 wt % vs 62.4 wt % over the micron-sized catalyst). The products from both catalysts were predominantly aromatic at 38 wt % of the liquid product. The authors did not report any investigation of the lifetime of the nanocrystal catalyst, but this material may be of interest for continued investigation in metal modification and durability studies. 3.3.2.3. Aromatics. Aromatics represent a significant portion of ETG products. Li et al. recently sought to maximize the conversion of ethanol to benzene, toluene, and xylenes (BTX) over a Ga-modified ZSM-5 catalyst.180,182 Light alkane and alkene products (C2−C4) from ethanol conversion over VZSM-5 were sent to a second reactor using the Ga-ZSM-5 catalyst at 500 °C and atmospheric pressure.180 Conversions greater than 90% and 42−73% were observed for the butanes/ butenes and C2/C3, respectively, after 4 h time on stream. The liquid product was primarily aromatic, with 30 wt % benzene, 39 wt % toluene, and 11 wt % xylenes. The same group explored the conversion of ethanol over a series of Ga-ZSM-5 catalysts having 0.5, 1.7, or 6.2 wt % Ga and over the temperature range of 300−500 °C at atmospheric pressure.182 A maximum BTX yield of 55.3 wt % was achieved at 450 °C with the 6.2 wt % Ga catalyst, which represents a 2-fold increase in yield over the parent ZSM-5 (26.0%) (Figure 9a). The authors highlighted the role of Ga in promoting oligomerization and dehydrocyclization and suppressing the

hydrogen transfer reaction in the zeolite (Figure 9b). Further, experiments with a physical mixture of Ga2O3 and ZSM-5 coupled with detailed catalyst characterization allowed the authors to conclude that (i) extra-zeolitic Ga2O3 particles were not active for increased BTX production and (ii) the amount of exchanged Ga cations is correlated with increased BTX sitetime yields. Therefore, ion-exchanged Ga was postulated to be the active site, which provides a design principle around ionexchanged Lewis acids to further improve the yield of BTX from ethanol. 3.3.2.4. Butadiene. Ethanol could also be used to make 1,3butadiene (BD), which is currently produced as a coproduct of naphtha steam cracking. The increased production of shale gas in the U.S. has led to a shift in the feedstock for steam cracking, where increased ethane in the feed causes a shortage of longerchain products.183 Therefore, there is significant interest in “on-purpose” BD production. The reaction pathway from ethanol to BD involves a series of dehydrogenation, condensation, Meerwein−Ponndorf− Verley−Oppenauer reduction (MPV reaction), and dehydration steps (Scheme 5).184 Mixed metal oxide materials and Scheme 5. Reaction Steps Involved in the Conversion of Ethanol to Butadienea

a


zeolite materials have been investigated as catalysts for this complex cascade reaction, and developments of non-zeolite catalysts will be presented in section 3.4.2. Through a series of reports exploring each individual transformation over oxide and zeolite catalysts, Ivanova et al. designed a multifunctional catalyst consisting of metallic Ag nanoparticles and isolated Zr(IV) sites in the framework of zeolite beta (Ag/ ZrBEA).29,30,185 Comparing the performance of the Ag/ ZrBEA catalyst to non-zeolitic Ag/ZrMCM-41 and Ag/ 4157


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demonstrated the highest-ever-reported ethanol-to-BD productivity of 2.33 gBD gcat−1 h−1.191 3.3.3. Summary. Recent advancements in catalyst development for MTH and ETH have focused on the following areas: • developing mechanistic understanding of the dual-cycle hydrocarbon pool chemistry and cascade transformations; • controlling the selectivity through zeolite selection, moving away from the traditional mixture of aliphatics/aromatics and toward specific hydrocarbon products; • promoting specific transformations such as hydrogenation, dehydrogenation, and dehydrocyclization through metal modification of zeolites; • identifying active-site structures to understand the role of metal modification; • combining these concepts to explore multicomponent, multifunctional zeolite catalysts. 3.4. Condensation of Alcohols and Aldehydes. 3.4.1. Guerbet Condensation. Discovered in 1899, the Guerbet reaction, named in honor of its inventor, Marcel Guerbet, involves the coupling of two alcohol molecules, generating a single larger alcohol and water.192 While initially studied for the condensation of 1-butanol to 2-ethylhexan-1-ol, the coupling process can involve any aliphatic alcohols provided that at least one is C2+.94 Over the past century, the reaction has demonstrated excellent versatility in coupling of alcohols across a wide range of chain lengths involving both homogeneous,93,193−198 and heterogeneous catalysts.91,94,192 Recent advancements in heterogeneous catalysis and coupling of syngas-derived alcohols (e.g., C1−C2) are emphasized here; for more information on longer-chain alcohols and reactions involving homogeneous catalysts, we direct readers to recent articles by O’Lenick199 and Aitchison et al.193 3.4.1.1. Ethanol Condensation to n-Butanol. n-Butanol is currently produced using the petrochemical-based oxosynthesis process but could alternately be synthesized from coupling of ethanol. The most commonly accepted coupling pathway for the bimolecular condensation of ethanol proceeds through a multistep process involving dehydrogenation to acetaldehyde, aldol condensation to 3-hydroxybutyraldehyde, dehydration to crotonaldehyde, and hydrogenation to butanol, as shown in Scheme 6. To achieve high conversion, selectivity, and productivity in this complex cascade reaction, Guerbet coupling requires multifunctional catalysts that exhibit balanced acidic and basic properties.200 While a few select solid-

ZrO2/SiO2 catalysts with comparable Ag dispersions, the authors concluded that the superior activity demonstrated by the Ag/ZrBEA catalyst was correlated with the higher Lewis acidity, as characterized by Fourier transform infrared spectroscopy (FTIR) of adsorbed CD3CN.186 A detailed computational−experimental investigation into the structure of the Zr site in ZrBEA using FTIR with CO adsorption at low temperature allowed the authors to determine the relative amounts of “closed” Zr(OSi)4 and “open” HO−Zr(OSi)3 sites across a series of ZrBEA materials with varying Zr content.187 The initial reaction rate for conversion of ethanol to BD was shown to be linearly correlated with the content of open Zr sites across the series of materials, identifying this Zr species as the most efficient site for BD production. The higher activity at the open site was attributed to higher acid strength and better steric accessibility for the reactant. Furthermore, this insight allowed the authors to design an improved synthetic protocol utilizing dealumination of a BEA zeolite and subsequent treatment with ZrOCl2 to generate exclusively open Zr sites, as characterized by FTIR of adsorbed CO.188 After addition of Ag particles, the optimized Ag/ZrBEA catalyst demonstrated an initial BD formation rate that was 2-fold higher than that with hydrothermally prepared Ag/ZrBEA at 320 °C and atmospheric pressure and the highest ethanol-to-BD formation rate of 0.58 gBD gcat−1 h−1 with a BD selectivity near 60%. Utilizing a similar concept, Kyriienko et al. reported that Nband Ta-substituted BEA zeolites are active and selective for ethanol conversion to BD.89,189,190 Characterization of the NbBEA materials identified framework mononuclear Nb(V) sites with 0.7 wt % Nb and a mixture of framework mononuclear and extraframework polynuclear Nb(V) sites with 2.0 wt % Nb.189 The 0.7 wt % NbBEA was more active than the 2.0 wt % NbBEA in the conversion of ethanol and ethanol/acetaldehyde mixtures to BD, which was attributed to the unique acidity of the framework site. However, in the reaction of ethanol to form BD at 325 and 350 °C, only moderate BD selectivities of 27.9 and 22.8 mol %, respectively, were achieved, likely because of the low dehydrogenation activity without the presence of Ag. The initial report of TaBEA without a cocatalyst to affect the dehydrogenation reaction also demonstrated modest ethanol-to-BD selectivity of 16.4 and 28.9 mol % at 325 and 350 °C, respectively.190 However, in a subsequent report, multifunctional catalysts consisting of Cu or Ag with TaBEA resulted in high BD selectivities of 72.6 and 62.6 mol %, respectively, at 325 °C and atmospheric pressure. These high selectivities were achieved with conversions above 80%, but the BD productivity value of 0.19 gBD gcat−1 h−1 did not exceed that of the Ag/ZrBEA catalyst.89 Dai et al. investigated BEA zeolites doped with Zn, Cu, Y, and Ce.191 From previous literature it was known that transition metals (e.g., Zn, Cu) exhibit high activity for the initial dehydrogenation reaction to acetaldehyde but are relatively inactive in the subsequent condensation and dehydration steps. By comparison, rare-earth metals (e.g., Y, Ce) had been shown to enhance the conversion of acetaldehyde to BD. In an attempt to combine the strengths of the two metal types, a one-pot method involving pairs of transition and rare-earth metals on BEA zeolite was studied using a pure ethanol feed at temperatures in the range of 330− 400 °C and WHSVs of 0.3−7.9 h−1. Ultimately the highestperforming material was determined to be a bicomponent 2% Zn/8% Y cluster confined within BEA zeolite, which

Scheme 6. Aldol Intermediate Pathway for the Formation of 1-Butanol from Ethanola

a

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Adapted from ref 91. DOI: 10.1021/acscatal.8b03945 ACS Catal. 2019, 9, 4145−4172

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ACS Catalysis Table 4. Summary of Selected Heterogeneous-Catalyzed Ethanol-to-Butanol Studies (2010−2017) material Pd5MgAlO 20% Ni/Al2O3 Ni/γ-Al2O3 Ir(OAc)3-Phen-AC Co powder Cu10/Ni10-PMO HAP Sr-HAP (Sr:P 1.70) HAP-CO3 MgO Mg3AlOx 8% Ni/γ-Al2O3 Na/ZrO2 Cu/CeO2 (HAS)

EtOH conv. (%) 3.8 25.0 25.3 43.0 4.2 56.5 6.6 11.3 40 23 33.3 35 14 67

butanol selectivity (%)

GHSVc/timed/WHSVe

72.7 80.0 52.4 53.5 69 39.3 75 86.4 56 34 36.3 61.7 − 45

d

5 72d 11d 16d 72d 6d 30000c 0.35e 5000c 7500c 960c 6.4e 0.70e 1.97f

reaction type

productivity (gBu gcat−1 h−1)

temp. (°C)

ref

batch batch batch batch batch batch cont. cont. cont. cont. cont. cont. cont. cont.

0.37a 0.08a 0.11b 0.06b 0.08b 0.70 0.14b 0.03b − 0.07b 0.40a 1.11b 0.01b −

200 250 230 160 200 320 340 300 400 400 350 250 400 260

201 95 202 198 203 204 205 206 207 200 208 209 210 211

Reported in ref 192, calculated at the specified reaction time. bCalculated in this report. cGas hourly space velocity (GHSV) (std cm3 gcat−1 h−1). Batch reaction time (h). eWeight hourly space velocity (WHSV) (h−1). fLiquid hourly space velocity (LHSV) (h−1).

a

d

are otherwise inaccessible in direct condensation reactions, such as isobutanol.225 Following the same four-step reaction pathway as in the case of ethanol self-coupling, C1−C2 alcohols sequentially couple to form n-propanol and then isobutanol (or directly isobutanol in the case of C1 + n-C3), as shown in Scheme 7.226−228 Unlike direct condensation reactions, which

base catalysts such as MgO have been shown to catalyze both aldol condensation and hydrogenation reactions, the addition of a metal hydrogen transfer catalyst (e.g., Ni, Pt, Pd, Cu) enhances the activity for hydrogenation/dehydrogenation steps.94,192 On the basis of reports published since 2010 (Table 4), both batch and continuous flow studies have been performed. The most heavily studied materials for the coupling of C1−C2 alcohols include hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HAP),88,92,205−207,212−216 metal (M)-doped hydrotalcite-derived mixed oxides (M-MgxAlyOz),201,208,217−221 and mixed metal oxides including Cu/CeO2,211 Ni/γ-Al2O3,95,202,209,222 Na/ZrO2,210 and TiO2.223 A significant research focus in this area is on the development of multicomponent catalyst systems that are tuned for each specific reaction step (e.g., dehydrogenation, aldol condensation, dehydration). While these studies have contributed to further understanding of the underlying reaction mechanism and the impact of surface site modifications, in general most have not significantly improved upon the butanol productivity, a key metric for future commercialization.224 Catalysts demonstrating the highest selectivity for butanol (e.g., Sr/HAP) generally do so only at low flow rates and conversions. With increasing conversion, there is a propensity to undergo subsequent condensation to higher-molecularweight alcohols and byproducts.216 A notable exception is the work by Ghaziaskar et al., who in 2013 showed an overall productivity of 1.11 gBuOH gcat−1 h−1 at 35% conversion and 62% selectivity by leveraging high reaction pressures (176 bar) to promote the reaction under supercritical conditions.209 In the future, research is warranted to develop materials that can sustain high selectivity and conversion at high ethanol feed rates. In addition, most of the catalysts evaluated in Table 4 were tested under anhydrous conditions, as water is known to contribute to catalyst deactivation91 and decreased selectivity for higher alcohols.201,222 To better assess commercial viability, catalyst stability in the presence of water needs to be evaluated, considering the positive impact water tolerance has on lessening the burden on upstream purity requirements (i.e., overcoming azeotropes) and separations costs. 3.4.1.2. Mixed C1−C2 Condensation. Although unable to self-condense, methanol can be utilized in cross-coupling reactions with C2+ alcohols, offering pathways to products that

Scheme 7. Cross-Coupling of Methanol and Ethanol Leading to the Formation of Isobutanola

a


can suffer from low selectivity due to difficulties in controlling the degree of coupling during the aldol condensation step, mixed C1/C2 coupling can reach high selectivity (ca. 98%) to isobutanol at 61% ethanol conversion because steric hindrance and the lack of an available hydrogen α to the methylol group prevent further condensation.226 However, C1/C2 crosscoupling has failed to garner interest partly because of the challenges posed by water formation during condensation. Specifically, during condensation to C4 products, 2 mol of water is formed per mole of alcohol, which necessitates the presence of a scavenging base such as MeONa to prevent hydrolysis and inactivation of the basic aldol condensation 4159


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ACS Catalysis Table 5. Summary of Selected Heterogeneous-Catalyzed Ethanol-to-1,3-Butadiene Studies (2010−2017) catalyst Ta2O5/SiO2 5%Ga/MgO−SiO2 MgO/SiO2 (1:1) Mg−Al MgO−SiO2 (1:1) 1 wt %Ag 10 wt %ZrO2/SiO2 ZrZn/MgO−SiO2 (1:1 MgO/SiO2) 4Ag/4ZrO2/SiO2-646 CuTaSiBEA AgOx/MgO−SiO2 4% ZnO/MgO−SiO2 (1:1) Hf2.5−Zn16/SiO2 1 wt % Ag/MgO−SiO2 Cs2O−1ZnO−5ZrO2/SiO2 2 wt % ZrO2/SiO2 0.5ZnO + ZrO2−SiO2 ZnO/Al2O3 (40:60) 2000 ppm Na Zn1Zr10Oz−H Ag(3.5 wt %)/ZrBEA 1.5 wt % Zr/0.5 wt % Zn/SiO2 2% ZnO 7% La2O3 1% ZrO2−SiO2 talc/Zn talc/Zn MgO/SiO2 (65:35) Cu/MCF + Zr/MCF

ethanol conv. (%) d

33.4 98.8 30.4b 49.0 41.2 29.1 32.0c 89.2 87.9 91.1 56 99.2 84 97.7 45.4a 36.8a 94.4 54.4 14.9 48.0 100 41.6 45.7 95.0 92.0

butadiene selectivity (%) 75.2 53.1 79.9 15.6 57.3 66.1 44.6 73.6 72.6 54 62.1 71.0 50 55.8 69.7 83.5 59.1 26 58.7 47.9 60.2 51.8 48.3 77.0 70.0

WHSV (h−1)

productivity (gBD gcat−1 h−1)

temp. (°C)

ref

e

− 0.02f 0.05 0.08f 0.14f 0.14 0.17 0.17 0.19 0.20 0.20f 0.26 0.29 0.32 − − 0.48f 0.49 0.58g − 0.71 1.06 1.08 1.35 1.40

350 400 350 400 400 320 375 325 325 400 375 360 480 400 320 320 425 350 320 375 400 400 400 450 400

236 229 237 238 239 231 240 183 89 241 242 243 184 230 244 245 246 247 248 232 249 250 251 234 252

1.0 0.075 0.40 1.8 1.0 1.23 0.62 0.47 0.5 − 1.0 0.64 1.2 1.0 1.8 1.8 1.5 6.2 1.2−3.0 1.5e 2.0 8.39 8.39 4.1 3.7

a Cofed with acetaldehyde at an ethanol/acetaldehyde ratio of 3.5:1. bCofed with acetaldehyde and water (22.5:67.5:10 w/w/w acetaldehyde/ ethanol/H2O). cCofed with acetaldehyde at an ethanol/acetaldehyde ratio of 8:2. dCofed with acetaldehyde at an acetaldehyde/ethanol ratio of 2.5:1. eLiquid hourly space velocity (LHSV) (h−1). fNot reported. Calculated productivity (in gBD gcat−1 h−1) = yieldBD(%) × WHSV × 0.587/100. g The authors reported the value as “estimated”.

catalyst.226 This additional requirement increases the process complexity at commercial scale and can contribute to challenges in waste management. 3.4.2. Lebedev Condensation to 1,3-Butadiene. Because of the complexity of the multistep cascade reaction of the Lebedev process (Scheme 5), synthesizing 1,3-butadiene from ethanol with both high selectivity and productivity remains a challenge,229−231 requiring catalysts that carefully balance the relative abundance of acidic, basic, and metallic sites for dehydration, aldol condensation, and hydrogen transfer reactions, respectively. For example, it has been observed that catalysts exhibiting overly strong acid sites tend to directly dehydrate ethanol to ethylene or diethyl ether,232 whereas catalysts that lack sufficient acidity are limiting in the final terminating dehydration steps, leading to the formation of undesirable side products.233−235 In general, mixed metal oxide materials and to some extent zeolite materials (already discussed in section 3.3.2.4) have been investigated as catalysts for this pathway; recent developments in ethanol-to-butadiene catalyst technology for non-zeolite materials are discussed below and summarized in Table 5. Prior to the recent surge in bioethanol production and revival in research into ethanol-to-butadiene processes, one of the catalysts with the highest reported BD productivity was a 40% ZnO−60% Al2O3 mixed metal oxide material studied in 1962 by Bhattacharyya and Ganguly, which demonstrated a one-step ethanol conversion of 94% with 59% selectivity for BD and an overall productivity of ca. 0.48 gBD gcat−1 h−1.246 Subsequent research has emphasized improving the productivity while maintaining the high selectivity for BD using

primarily ZrO2- and MgO-based silicates doped with transition metals.249 However, as shown in Table 5, most of the recent studies (2010−present) have reported that as the selectivity is increased past 60%, there is an accompanying drop in productivity to 95% selectivity for ethylene and ∼66% selectivity for acetone were observed with solely ZrO2 or ZnO, respectively,

surface basicity/acidity ratio between 0.24 and 0.3 in combination with a small fraction of strong acid sites constituting 46−50% of the total acidity.234 However, during extended catalyst testing, the highest-performing materials showed a marked decline in ethanol conversion and BD selectivity of 46% and 36%, respectively, over 42 h, again highlighting the need for improvements not only in productivity but also in catalyst stability. In a departure from the conventional one-pot synthesis technique, in 2016 Cheong et al. reported the use of a dual fixed bed system with catalysts comprising mesoporous siliceous foams (MCF) with ultralarge pores doped with CuO and ZrO (Scheme 8).252 Rather than developing a single Scheme 8. Catalytic Cycle for Acetaldehyde and 1,3Butadiene Production Utilizing Dual Fixed Bed Reactorsa

a


catalyst to drive the entire five-step process, they split the reaction into two parts, one to optimize the dehydrogenation of an ethanol feed to acetaldehyde and the other to facilitate the aldol condensation and dehydration steps. When combined at the optimal reaction temperatures (235 °C for reactor 1 and 400 °C for reactor 2), the Cu/MCF and Zr/MCF dual fixed bed system demonstrated an ethanol conversion of 92% with a BD selectivity of 70% at a relatively high WHSV of 3.7 h−1, Scheme 9. Pathway for the Synthesis of Olefins from Ethanola

a

Adapted from ref 253. 4161


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lower cost of production? Have these advantages been sufficiently quantified? Answers to these questions provide a useful stage-gate for catalyst (and process) development, but unfortunately, many scientists and engineers consider a “cheaper, faster, better” criterion to be sufficient justification for substantial investment in follow-on research and development. A superior approach is to develop a value proposition for the catalyst and/or process. Elements of a good value proposition are shown in Figure 10.

implying that catalytic synergies occur when both acidic and basic sites are available.258 In a 2016 follow-on study, Smith et al. demonstrated the efficacy of the ZnxZryOz mixed metal oxide system in the upgrading of a model syngas-derived mixed oxygenate feedstock comprising ethanol, acetaldehyde, acetic acid, ethyl acetate, methanol, and propanol. Under operating conditions of 400−450 °C and 1 atm, C3−C5 olefins were produced in >50% carbon yield, highlighting the versatility of the system and its relevance in syngas upgrading.253 The formation of propene over acidic zeolites represents an established pathway (discussed in section 3.3.2.1) with a d e m o n s t r a t e d s e l e c t i v i t y o f a p p r o xi m a t e l y 2 0− 30%.169,170,259,260 However, coke formation and rapid catalyst deactivation limit the effectiveness.261 In 2011, Iwamoto et al. reported an alternative pathway to reach propene over nickel on mesoporous silica (Ni-MCM-41), demonstrating yields of 15−20% at 400 °C, thereby opening the door for non-zeolitebased reaction routes that are not controlled through specific shape selectivity.260 In 2012, dramatic improvements in propene yield over Sc/In2O3 catalysts were reported by Mizuno et al., demonstrating a yield of 61.8% at 550 °C with little signs of deactivation over 50 h.254 The high performance was proposed to be a result of the antireductive properties of Sc and cofed water, which reduced coke formation. At the time, such a high and stable yield had not been reported in any other system, zeolite or other mixed metal oxide.254 3.4.4. Summary. Recent advancements in catalyst development for the condensation of alcohols and aldehydes have focused on the following areas: • balancing the active site density (acidic, basic, metallic) over multifunctional mixed metal oxide-based catalysts to achieve high productivity in multistep cascade reactions; • modifying the reactor configuration (two-stage vs onepot) and reactant cofeeds to enhance the selectivity for butadiene at high ethanol conversion; • controlling mass transfer through pore size optimization.

Figure 10. Elements of a strong value proposition. Adapted from ref 263.

In all situations, regardless of whether a commercial process already exists, the value proposition must be based on an identified customer, and it must address a critical need for that customer. For instance, the catalyst researcher may have developed a material that is 20% more selective for methanol carbonylation to acetic acid over currently used technology, but unless the user of that current technology is suffering financially because of a lack of acetic acid selectivity (or will in the future), a customer may not exist. This means that customer discovery is a critical aspect of new catalyst development. Is the would-be user of the catalyst motivated to make more money? Reduce risk? Avoid loss of market share? Be the first to offer a new product? Capture green credits (in whatever form) above and beyond their competition? The technology must provide an incentive for the customer in their terms. Finally, a market must exist for the new catalyst or process. How much might the identified customer purchase? Who else will buy? Can the technology be sold only on the assurance that a single customer will maintain a competitive advantage? These types of questions must be answered to progress from discovery to development to commercialization and should be used at an early stage of development to guide research and development. With regard to fuels and chemicals derived from biomass, the value of those products (and thus the acceptable cost of biomass feedstock and conversion) should be considered in one of three categories: (i) the product competes with existing offerings (no green credit); (ii) the product commands extra value due to bio content (green credit exists); or (iii) the product does not yet exist and can only be produced from biomass (i.e., bioadvantaged). Because syngas can be produced from fossil fuels or biomass, the third category does not apply here. Category 1 is dictated by the free market, and category 2 depends on government policy or customer demand for green content (altruism). Since technology development and the design, construction, and startup of biorefineries take years, the payback on those refineries takes decades, and government

4. CONSIDERATIONS ON THE COMMERCIALIZATION OF CATALYST ADVANCEMENTS AND THE POTENTIAL TO GROW THE BIOECONOMY 4.1. Defining the Value Proposition. A new catalyst is in most cases a new material or new composition. New materials are known to have a long path to full commercial adoption, often 20 or more years from discovery.262 Furthermore, for many of the technologies discussed in this review, the processes are not yet commercial, and thus, development of the catalyst and the process application are occurring concurrently. It is therefore critical to understand the process, the customer, and the value of these new or improved catalysts early in the development cycle. Critical qualitative questions that need to be answered for the catalyst/process include the following: • Will the catalyst enable production of a high-value chemical? • Do the byproducts have value or will they incur disposal/treatment costs? • Do the catalyst and/or process provide a competitive advantage over existing approaches, such as improved selectivity, productivity, resistance to poisoning, increased lifetime, less severe operating conditions, or 4162


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Figure 11. (a) Predicted demand for gasoline, diesel, and jet fuel in the U.S. (b) Historical and projected demand for premium gasoline. (c) Recent demand-driven changes in premium gasoline price. (d) Volumetric product yields from a barrel of oil.264−267 ULSD stands for ultralow-sulfur diesel, and LPG stands for liquid petroleum gas.

methanol to triptane discussed in section 3.3.1.3. If these processes consume syngas-derived oxygenates produced from biomass, they will also enable biomass utilization without forcing fuel producers to purchase finished biofuels. That is, producers will need to meet demand anyway, and biomass may provide carbon in a form that is identical to syngas-derived oxygenates produced from fossil resources. Similar focus should be placed on chemicals in regard to targeting (i) markets with growing demand, including new markets based on end user demands for improved material performance, and (ii) chemicals that are challenging to produce from petroleum or offer a functional improvement over those accessible from petroleum. However, because chemicals must always meet a purity specification, one does not have the luxury of offering better “grades” as in the case of higher-octane gasoline or higher-cetane diesel. Since syngasderived oxygenates like methanol are used to produce many commodity chemicals, the best approaches may be in the development of catalysts with higher yield or conversion efficiency (e.g., improved butadiene productivity from ethanol, as discussed in section 3.4.2), as is the focus of this review, and the use of biomass or waste resources to produce syngas at a

policy can change with each election cycle (i.e., 2−4 years), the conservative approach is to develop technologies that compete in the free market and to use green credits as a “bonus”, not as justification to move forward with a technology. Fulfilling category 1 implies that focus should be placed on fuels for which there is growing demand (room to compete in the market, higher value if supply cannot be met with fossil carbon) and fuels for which quality specifications will be hard to meet with petroleum (e.g., sulfur content, emissions, and performance metrics like cetane and octane values). As shown in Figure 11a, the EIA predicts that in the U.S., the demand for diesel and jet fuel will grow while gasoline demand will decline.264 At the same time, the demand for premium fuel (higher octane) will increase (Figure 11b), which is already evidenced by an increase in the relative value of premium over regular (Figure 11c). Despite this, refiners are constrained by the typical makeup of a barrel of oil, with gasoline constituting almost half (Figure 11d). That is, petroleum will provide too little distillate and too much gasoline as demand shifts. Therefore, catalysts and processes that can produce more diesel and jet fuel and/or upgrade low-octane gasoline to highoctane gasoline or distillate will be particularly valuable. One example of such a process is the selective conversion of 4163


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Figure 12. (a) Annual averaged WTI petroleum price and Henry Hub natural gas spot price over the last 20 years. The black line corresponds to price parity between petroleum and natural gas on an energy basis. (b) Visualization of the market viability of biofuels and biochemicals.

a potential opportunity for projects that convert biomass and biogas to syngas, as those projects can (i) benefit from integration into large natural gas projects (gain economies of scale), (ii) move from idea to production quickly, and (iii) take advantage of green credits with some certainty in their existence. When both oil and natural gas are expensive (top right quadrant), biomass becomes a competitively priced feedstock both as a cofeed with natural gas and as a standalone feedstock, and the probability of investments in bio-GTL facilities is increased. Importantly, lower-cost renewable feedstocks and catalyst advancements expand the market conditions over which investments in bio-GTL technologies are likely (top right quadrant of Figure 12b). Lower-cost renewable feedstocks (e.g., municipal solid waste and biogas) reduce the natural gas price at which biofeedstocks are cost-competitive (indicated by the vertical black line in Figure 12b). Catalyst advancements can (i) improve the conversion efficiency of the process (i.e., they reduce the $/bbl/day metric for GTL plants) and (ii) provide a product of superior quality compared with that from petroleum, thus increasing margins. Both of these catalystenabled improvements reduce the petroleum price at which GTL technologies can be cost-competitive (indicated by the horizontal black line in Figure 12b). Thus, catalyst advancements associated with downstream conversion of syngasderived oxygenates can enable greater penetration of biofuels and biochemicals, and catalyst development should continue despite currently low oil and natural gas prices so that when the market conditions are appropriate, bioprojects can move quickly with commercial-ready technology. 4.3. Enhancing Process Viability through Catalytic Advancements and Product Selection. As described in section 4.2, the commercial viability of GTL technologies is constrained by feedstock cost, conversion efficiency, and product value. To provide context to the potential cost savings from improvements in conversion efficiency enabled by catalysis, we provide an example based on a published techno-economic analysis performed for the methanol-totriptane pathway described in section 3.3.1.3.268 In that study, an increase in product yield from 65 to 70 gal of high-octane gasoline per dry ton of biomass resulted in a 7% decrease in the minimum fuel selling price (akin to the cost of production), from $3.41 to $3.16 on a gasoline gallon equivalent (GGE) basis.268 This example illustrates the direct connection between conversion efficiency and cost of production and

lower cost, either via stand-alone biomass gasification or integration with gasification/reforming of fossil carbon. 4.2. Improving the Value Proposition. Ultimately, the potential for commercial production of fuels and chemicals from biomass-derived syngas will be constrained by the same parameters as existing fossil-based GTL technologies: feedstock cost, conversion efficiency (productivity and energy/ carbon efficiency normalized by capital costs), and product value. For these technologies, the feedstock cost corresponds to the cost for natural gas, the conversion efficiency is defined as the total installed capital costs divided by the fuel production rate (i.e., $/bbl/day), and product value is mainly driven by oil price as the direct competitor (unless products with sufficiently higher quality are produced from the GTL technology and customers are willing to pay more for the improved quality). Thus, fossil-based GTL process economics is largely dictated by (i) the spread between the prices of natural gas and petroleum and (ii) the conversion efficiency. Figure 12a shows the annual average West Texas Intermediate (WTI) petroleum price compared with the Henry Hub natural gas spot price over the last 20 years in the U.S.. The black line corresponds to price parity between petroleum and natural gas on an energy basis. Over the last two decades, it is apparent that there have been years when the price of petroleum was much higher than that of natural gas (2010−2014) and years when petroleum and natural gas reached near price parity on an energy basis (1997−2005). As plants operate for 20−30 years or more, this market dynamic is the primary reason why investment in GTL technologies has been limited in the U.S. However, low-cost renewable feedstocks and improved conversion efficiencies through catalysis advancements, combined with policy incentives, could adjust these constraining market parameters and enable increased incorporation of biocontent into fuels and chemicals through syngas-derived oxygenates. Figure 12b provides a conceptual visualization of the plot shown in Figure 12a, illustrating the market conditions that may render production of fuels and chemicals from biomass-derived syngas economical. When oil and natural gas are cheap and abundant (bottom left quadrant) or when oil is cheap and natural gas is expensive (bottom right quadrant), biomass cannot compete without green incentives, and because of the long period from idea to profitability in which incentives could disappear, investments are not likely. When oil is expensive and natural gas is cheap (top left quadrant), GTL projects are viable, as syngas can be produced inexpensively from methane and natural gas liquids. This market condition is 4164


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by expanding the market conditions over which bio-GTL can be cost-competitive with petroleum-derived products. These advancements fall into two major categories: improving the conversion efficiency and accessing products with performance advantages and/or superior quality (e.g., octane rating). While this review has specifically focused on the upgrading of methanol, ethanol/C2+ alcohols, and aldehydes, several underlying reaction pathways are common across these catalytic upgrading approaches, specifically (i) carbonylation, (ii) selective oxidation, (iii) hydrocarbon pool chemistry of alcohols, and (iv) condensation of alcohols and aldehydes. Thus, catalyst advancements in any one of these reaction pathways could be applicable to a wide variety of oxygenates, not just the ones discussed here. In general, catalysis research and development around the upgrading of methanol, ethanol/ C2+ alcohols, and aldehydes focuses on understanding reaction mechanisms and identifying the active catalytic sites, using that knowledge to develop novel catalyst formulations and preparation methods to enhance the yield and stability, and ultimately achieving productivities and lifetimes under industrially relevant process conditions that either (i) improve upon existing catalysts in commercial processes or (ii) enhance the value proposition of precommercial processes. Of particular interest are (i) shifting away from homogeneous catalysts to heterogeneous materials, such as for methanol carbonylation, (ii) designing multicomponent catalysts with tailored active site densities (acidic, basic, metallic) to control product selectivity during multistep reaction pathways, such as for ethanol condensation, and (iii) extending the zeolite lifetime in alcohol-to-hydrocarbons processes. Moving forward, to enable these catalyst advancements to intercalate into commercial applications and expand market penetration of biofeedstocks, greater attention needs to be paid to defining the value proposition of these new/modified catalyst materials. Addressing the questions outlined in section 4.1 at an early stage of development will help guide R&D toward the most critical research challenges and industrial “pain points”. It will be particularly helpful to the broader catalysis and bioenergy communities if the value propositions of new/improved catalysts are discussed openly in peerreviewed literature in terms of both the cost of manufacturing these new catalytic materials and the industrial “pain points” being addressed. Through this approach, catalyst development will be accelerated, and new opportunities will be identified, such as applying the catalyst advancements covered in this review to other oxygenates such as butanol and butanediol.

the potential for catalysis advancements to shift the horizontal black line in Figure 12b downward. With regard to product value, incumbent chemical industries are fueled predominantly by petroleum-based feedstocks comprising almost entirely carbon- and hydrogen-containing species. By contrast, a typical syngas (ca. H2/CO = 2:1) contains roughly 50% oxygen by weight. Thus, for product selection it is important to consider both the theoretical mass yield of the end product and the market value of that product. Oxygenated end products will inherently have higher theoretical mass yields from syngas than hydrocarbons. For example, in a comparison of the Guerbet and Lebedev ethanol condensation routes discussed in section 3.4, the stochiometric mass yields of butanol and 1,3-butadiene are widely different simply on the basis of the ability to retain oxygen. As shown in Figure 13, the reactants, two ethanol molecules, have a mass of

Figure 13. Effect of water formation on mass yield in the condensation of ethanol to n-butanol and 1,3-butadiene.

ca. 92 g, and after they undergo condensation, 80% of that mass is retained in butanol versus less than 60% in 1,3butadiene. Since butanol and butadiene have similar per-mass market prices, there is a clear economic advantage for generating butanol assuming that similar conversion metrics are attainable. Thus, it is critical to consider both market value and theoretical mass yield when selecting a product for GTL technologies, especially regarding oxygen retention. Key considerations for driving growth in the bioeconomy through commercialization of catalyst advancements for upgrading of syngas-derived oxygenates include the following: • customer discovery is critical to commercialization of new catalyst technologies; • rigorous development should not proceed until a customer has been identified and a clear and concise value proposition has been developed; • catalyst advancements can enable greater market penetration of biofuels and biochemicals, and thus, promising catalysts should continue to be explored even in an era of cheap oil and natural gas so that biofuel and biochemical processes can be deployed when market conditions are suitable; • product selection for upgrading of syngas-derived oxygenates should consider both market value and the theoretical mass yield of the end product in the context of oxygen retention.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daniel A. Ruddy: 0000-0003-2654-3778 Joshua A. Schaidle: 0000-0003-2189-5678 Author Contributions †

G.G. and A.T. contributed equally.

Notes

5. CONCLUSIONS AND FUTURE PERSPECTIVES Catalyst advancements for the downstream upgrading of syngas-derived oxygenates to fuels and chemicals present an opportunity for increased market penetration of biofeedstocks

The views expressed in this review do not necessarily represent the views of the U.S. Department of Energy or the United States Government. The authors declare no competing financial interest. 4165


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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies Office (BETO), under Contract DE-AC36-08-GO28308 at the National Renewable Energy Laboratory and in collaboration with the Chemical Catalysis for Bioenergy Consortium (ChemCatBio), a member of the Energy Materials Network (EMN). The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work or to allow others to do so for U.S. Government purposes.

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