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Zeolite materials for biomass conversion to biofuel Carlo Perego, Aldo Bosetti, Marco Ricci, and Roberto Millini Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01057 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017
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Zeolite materials for biomass conversion to biofuel Carlo Perego,*a Aldo Bosettia, Marco Ricci a,b and Roberto Millinia a
Eni S.p.A., Renewable Energy & Environmental R&D – Istituto Eni Donegani, via Fauser 4, I-28100 Novara, Italy. email:
[email protected] b
Current address: versalis S.p.A., Green Chemistry - Novara Research Center, via Fauser 4, I-28100 Novara, Italy.
Abstract The use of zeolite catalysts for the production of biofuels from biomass is reviewed. Zeolites as such or modified by the addition of other active phases are use in several processes for the transformation of the biomass and for the upgrading of the bio-oils deriving from its primary treatment. For each of the different processes, the most relevant results reported in the literature are provided together with some considerations on the effective or potential industrial applicability of the technologies.
Introduction In the past few decades humankind has started to face a major challenge. On one side, indeed, complete awareness has been now gained about the climate change that would occur without a control, and possibly a reduction, of the atmospheric emissions of greenhouse gases, particularly carbon dioxide. On the other side, any further social and economic advancement, particularly in developing countries, will require more and more energy. Since energy consumption mostly involves, at some stage, the combustion of some organic fuels, the request for more energy will almost unavoidably involve further carbon dioxide emissions. One of the possible strategies to face this dilemma is the progressive substitution of traditional fuels, mostly produced from fossil sources, for more sustainable, carbon-neutral biofuels produced from a renewable feedstock such as biomass. Particularly, much attention is currently focused on biofuels for transportation, since this is the sector that accounts for the largest increment in the overall liquid fuels demand. Accordingly, several biofuels have been produced on the industrial scale to be used for transportation. As far as gasoline substitutes are concerned, bio-ethanol was already produced in the 1980s and was used, and is still used, as such or as bio-ETBE, its tert-butyl ether. Few years later, in 1 ACS Paragon Plus Environment
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1991, a plant in Austria paved the way for the commercial production of fatty acids methyl esters (FAME) to be used as diesel fuel (biodiesel). In the last few years, hydrotreated vegetable oils also added to this short list of already available biofuels. In the meanwhile, concern arose about the fact that biofuels were produced from crops traditionally grown for food or for animal feed. Thus, in order to avoid any possible conflict between fuel and food production, today advanced biofuels, are highly desired, to be produced from different, non-food biomass such as ligno-cellulosic crops, agricultural and forest residues, waste of agro-food industry, the organic fraction of municipal waste, and algae. However, industrial production of biofuels, particularly of advanced ones, is still in its early days and most of the technologies for the exploitation of biomass (including the most recalcitrant lignin fraction) must still be developed or proven at industrial scale. With this respect, there is little doubt that catalysis plays a major role in such a development. In the last 50 years, the development of zeolite-based catalysts has been one of the most impressive breakthroughs in the catalysis realm. Once regarded as just a curiosity for their capability to adsorb water and then to release it upon heating, zeolites quickly gained importance when, at the end of the 1940s, researchers of the Linde division of Union Carbide discovered the synthetic zeolites A, B, and X and, in the following years, started their commercialization as adsorbents for gas separation. Soon after, the research groups of both Union Carbide and Mobil realized that a number of shape-selective reactions could be run within the zeolites’ channels, taking advantage from the restrictions imposed by their peculiar microporous structure. The golden era of zeolite catalysis had started. The exploitation of zeolites in the biomass treatment, however, is not straightforward. As a matter of fact, catalytic sites in zeolites are mostly present on the internal surface of the pores or of the channels interconnecting them: structures with typical diameters around half nanometer or so. On the other hand, most of the constituents of the biomass are quite large molecules, either polymers (cellulose, hemicellulose, starch, lignin, proteins, etc.) or not (triglycerides) that, furthermore, are often further packaged in very stable supramolecular arrangements. As the result, mass transfer to the catalytic sites into the small zeolite pores is strongly hindered. This is why, so far, many attempts to use zeolites for the biomass primary attack, in order to transform it into a more versatile and valuable stuff, met only a limited success. On the other hand, it is increasingly realized that zeolites have a tremendous potential in the transformation and up-grading of the primary products of biomass attack and deconstruction.
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Several important papers and reviews already dealt with the use of catalysis and catalysts, including zeolites, in biofuels production. 1-11 Thus, in this review paper, attention will be particularly focused on the processes already at some development stage, which exploit zeolites for the biofuels production.
Zeolites: definition and recent advances in their synthesis Before discussing in detail the applications of zeolites in the conversion of biomass to biofuels, it is useful to briefly introduce these materials and summarize the recent achievements in their synthesis. Zeolites form a family of crystalline microporous materials of primary scientific and technological importance; according to the most recent definition, a zeolite is a “crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of guest species” 12. Known since 1756, zeolites were considered a scientific curiosity for the mineralogists until years 1950s when, thanks to the pioneering work of R. M. Barrer (Imperial College, London) and R. Milton (Union Carbide Corp., USA), the first syntheses and applications of zeolites were reported. In analogy with the natural phases, zeolites were firstly considered as aluminosilicates only. Their physical and chemical properties directly derive from the characteristics of the 3D frameworks in which the organization of the cornersharing [TO4] (T = Si, Al) tetrahedra generates a microporous system, whose architecture varies from a structure to another, being constituted by regular channels and/or cages opened to the exterior of the crystal and with free dimensions in the range 3 – 20 Å. This allows the use of zeolites as molecular sieves, for separating molecules of different dimensions from a mixture. The chemical properties of the frameworks are determined by the presence of [AlO4] tetrahedra, which impart negative charges to the framework compensated by extra-framework alkali or earth-alkali cations located in the micropores. These cations are loosely bound to the framework and can be easily exchanged rendering zeolites ionexchangers for application, e.g., in water softening. When the extra-framework cation is the proton, a zeolite assumes acidic properties and can be applied as heterogeneous catalysts for several acidcatalysed reactions. The use of zeolites as solid acid catalysts has had a strong development in the last few decades, in particular to replace the harmful mineral acids or the unstable and low-performing nonzeolite solid acids in several refining and petrochemical processes. The importance of microporous materials in these industries is demonstrated by the fact that among the 180 solid catalysts employed in 127 different industrial processes 74 are based on zeolites.
6-7
Most of these catalysts are employed in 3
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petrochemical processes that, differently from oil-refining ones, consists of well-defined reactions on pure substrates (e.g. alkylation, transalkylation and disproportionation of aromatics). In these cases, the selectivity towards the desired product(s) is fundamental and zeolites provide significant advantages respect to the homogeneous and to non-zeolite catalysts. In fact, the active sites are located within the pores with dimensions comparable to those of the different species involved in the reaction. Therefore, the zeolite pores may impose a steric control to the reaction, favouring the formation of the target product(s), limiting in the same time the undesired ones. These revolutionary observations were made from the years 1960s when the zeolites started to be defined as shape selective catalysts. In particular, the classical concepts of reactant, product and transition state shape selectivity were proposed for explaining the behaviour of zeolites in different reactions. 13-16 This has had several implications in the research on the microporous solids both with regard to their applications as catalysts and, above all, for the synthesis of new structures able to satisfy the demands of new materials. What emerges from the statistical surveys cited above is that all the zeolite catalysts currently employed are based on a few framework types (actually 13 of the 232 known today, the most widely used being MFI, MOR, FAU and Beta), all belonging to the groups of medium- and large-pore zeolites, with 10R and 12R openings, respectively.17-18 When considering that the corresponding free dimensions of the pore apertures are in the range 5 – 8 Å, it is clear that the applicability of these zeolites is limited to reactions involving small molecules, i.e. those able to diffuse in the medium- and large pores. The problem that the scientific community has faced was therefore to find solutions that can extend the use of zeolites in reactions involving larger molecules. In this regard, the preparation of the M41S family of ordered mesostructured materials, reported in 1992 by researchers at Mobil, opened interesting opportunities for new application in catalysis.
19-20
However, after the initial
enthusiasm, they failed to meet the great expectations because their acidic strength and thermal/hydrothermal stability are much lower than zeolites. In fact, after 25 years from their discovery, there are no industrial applications of these materials. Therefore, the attention moved to the preparation of new structures with pore openings formed by 14 or more tetrahedra, the so-called extralarge pore zeolites. Taking advantage of the increasing knowledge of the phenomena involved in the zeolite nucleation and growth as well as of the influence of the different synthesis parameters, interesting results were achieved. 21-22 Several extra-large pore zeolites were obtained in the presence of elements different from Si and/or Al, in particular Ge, that, stabilizing the D4R (double-four-ring) secondary building unit, favours the formation of low-density zeolite frameworks. 23 Among the extralarge pore zeolites, ITQ-37
24
and ITQ-43
25
are worthy to mention because, unique among the 4
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crystalline porous structures known so far, they are characterized by pore apertures close to 20 Å, i.e. at the border of the lower dimensions of mesopores. In spite of the relative large number of extra-large pore zeolites available today, they did not find any industrial application yet. In some cases, however, the catalytic tests demonstrated the high potential of these zeolites, as in the case of ITQ-33 in the catalytic cracking of vacuum gasoil.
26
In spite of that, their high costs (due to the use of expensive
reagents, including GeO2, and complex non-commercial SDAs) and low thermal/hydrothermal stability prevent their practical use. Parallel to the crystallization of the extra-large pore zeolites, new approaches to prepare materials with increased accessibility of the active sites located within the crystals are under development. One of them is based on the use of the so-called bidimensional (or 2D) zeolites.
27
This term defines layered
phases composed by thin zeolite sheets, which can be intermediates (precursors) in the crystallization pathway of given zeolites or prepared by specifically designed approaches. A well-known example of 2D zeolite is the MWW-type precursor whose characteristics were reported in the mid years 1990s 28. Upon calcination of this precursor, composed by randomly stacked loosely bound MWW-type layers, the ordered 3D zeolite structure forms. On the other hand, 2D zeolites can be prepared also for phases that do not involve layered precursors. An example concerns the preparation of thin (2-3 nm thick) MFI layers using specifically designed bifunctional SDAs with the UTL-type framework type.
30
29
or by post-synthesis treatment of 3D zeolites
Regardless of the synthesis method, the zeolite sheets can be
considered as periodical building blocks used for the preparation of new materials through delamination, intercalation, pillaring, etc. 27 In all cases, the materials thus obtained have very different characteristics from conventional 3D structure, especially with regard to the easy accessibility to active sites. In fact, due to the small thickness of the zeolite layers, virtually all the active sites are located on the external surface of the layers themselves. Consequently, the control of the reaction does not take place through one or more of the classic concepts of shape selectivity described above: the reaction occurs at the pore mouth, under a steric control but without any diffusion limitation a phenomenon known as “nest effect”
31
. It is worth reporting that, in the specific case of 2D zeolites derived from
UTL-type zeolites, the zeolite layers can be reassembled to form new framework topologies by using the so-called ADOR (Assembly-Disassembly-Organize-Reassembly) approach, probably the clearest example of crystal engineering in zeolite science. 32 The elimination (or, at least, reduction) of the diffusion limitations in zeolite crystals is one of the hottest topics in zeolite science and one of the solutions concerns the preparation of zeolites possessing a hierarchical micro-mesoporous system also known as mesoporous zeolites.
33
In this way, the 5
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mesopores present in the zeolite particle facilitate the mass transport to the crystalline nano-domains where the reaction takes place. The reduced length of the micropores assures from one side the easy elution of the products and from the other the full exploitation of the zeolite crystals with clear advantages on the entire catalytic process. Several different approaches for preparing mesoporous zeolites have been proposed, each of them characterized by a different degree of complexity and success. 33 Mesoporous zeolite are already employed in the formulation of industrial catalysts. We can cite, for instance, the zeolite beta nanocrystals, which are the active phase of the PBE-1 catalyst for the cumene and ethylbenzene processes 34, and the mesoporous USY as a component of a commercial FCC catalyst. 35 This brief review of current trends in the synthesis of zeolites is useful to demonstrate how research is moving towards the preparation of materials for applications in areas, such as biomass conversion, where classical zeolites show strong limitations.
Zeolites as hydrolysis catalysts In the last years, increasing attention has been focused on the biorefinery concept. Although a satisfactory definition of it is probably still lacking, a biorefinery is usually thought as an industrial facility where biomass is transformed into fuels and/or other value-added products. Several processes have already, and for a long time, exploited selected biomass fractions, e.g. in the well-established industrial sector of oleochemistry. It is likely, however, that in order to make the biorefinery concept actually viable from the economic point of view, it will be necessary to transform all the biomass, or at least most of it, and not only its most valuable fractions, into useful products. Thus, any transformation chain should reasonably include the deconstruction of the ligno-cellulosic biomass network to make its single components (cellulose, hemicelluloses, and lignin) available for further transformations which, at least for polysaccharides, most often include hydrolysis. This deconstruction is all but simple, due to the tight packaging of cellulose chains and to their intergrowth with the extremely recalcitrant lignin skeleton. Suitable pretreatments are thus needed, quite often by means of harsh hydrolytic conditions. The use of mineral acids such as diluted or concentrated hydrochloric or sulfuric acids, however, has a number of drawbacks including corrosion problems, need of a neutralization with consequent salt formation, and downstream purification. For these reasons, there is a great interest in the development of effective solid acid catalysts for the biomass deconstruction and for the subsequent hydrolysis of cellulose and hemicelluloses to simple monosaccharides. 6 ACS Paragon Plus Environment
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In this frame, significant attention has been paid to zeolites and other micro- or mesoporous materials 4, 36-39
but, as already stated, zeolites met with limited success, probably due to the mentioned limitations
about restricted access to the catalytic sites. Nor other solid catalysts performed much better and, despite few interesting results,
40
no solid catalyst has been so far considered for biomass
deconstruction and hydrolysis on pilot, demonstration, or commercial scale. Only recently Midori Renewables disclosed, in the patent literature, a promising material for polysaccharides hydrolysis. The material is based on sulfonated polystyrene resins further functionalized with cationic units, in most cases containing imidazolium salts. Apparently, some interactions of hydroxyl groups of cellulose with the cationic groups of the resin enhance the catalyst effectiveness in the polysaccharide hydrolysis. 41
Zeolites as esterification and transesterification catalysts The ability of zeolites to catalyse esterification and transesterification reactions makes them appealing catalysts for producing biodiesel from vegetable oils, which are very complex mixtures of mono-, di-, and (mainly) tri-esters of glycerol with fatty acids. Due to their high viscosity and scales formation, oils can be hardly used directly as diesel fuel. Much better fuels can be produced by converting them into biodiesel, a mixture of fatty acid methyl esters (FAME), by transesterification of triglycerides with methanol (reaction 1; see, e.g., ref. 18, R1, R2 or R3 are representing long hydrocarbons chain mainly having from C12 to C22 atoms. Some double bonds can be present in 9, 11, 12 and 15 position).
The reaction is usually carried out at 60 °C, in a bi-phase mixture provided by a large excess of methanol (6/1 w/w) and in the presence of a catalyst. Together with FAME, the process also affords glycerol whose amount is roughly 10% of the FAME production (Figure 1). It should be noted that this co-production is mostly regarded as a process drawback, rather than as an opportunity, since FAME and glycerol markets evolve at very different paces. 7 ACS Paragon Plus Environment
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Fig 1 Simplified flow diagram for the traditional biodiesel technology.
Both acids and bases catalyze the transesterification reaction; basic catalysts, however, are preferred due to their higher activity. Usually, commercial biodiesel production is run using homogeneous alkaline catalysts, most often sodium methoxide or potassium or sodium hydroxide.
42
Eventually, the
alkaline catalyst is neutralised thus affording salts which are difficult to be removed from the glycerol. To solve this problem, several heterogeneous catalysts, including zeolites or related micro- and mesoporous materials, have been evaluated.
43
Few of such materials, however, afforded fairly good
results: for instance, in the transesterification of triacetin chosen as a triglycerides model, ETS-10 exchanged with sodium and potassium was almost as active as sulphuric acid, but still less than NaOH. 44
Unfortunately, the presence of free fatty acids quenched the reaction by inhibition of the basic sites.
However, significant amounts of free fatty acids are quite common in oils used as raw materials, especially in cheap feedstock such as used cooking oil. In this case, a single-step, acid catalysed esterification/transesterification process could result more convenient than the alkali-catalysed process, which requires a successive step to convert free acids to methyl esters in order to avoid soap formation. Accordingly, large-pore zeolites (FAU, MOR) have been used in fatty acids esterification. 45 A number of mixed oxides formulations containing Zn, Al, Mg (hydrotalcites or spinels) were patented, usually working at higher temperature (200-250ºC) than homogeneous catalysis.
46
So far, however, the
catalytic performances reported for zeolites, either in their acid or basic form, in the biodiesel production still remain lower than that of conventional catalysts and, in this case too, no zeolite catalysts have been considered for commercial biodiesel production processes.
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Zeolites as isomerization catalysts Since the very beginning of zeolite catalysis, it appeared that zeolites can be extremely efficient isomerization catalysts. Today, this capability of zeolites, and of related mesoporous materials as well, is largely exploited to transform mixtures of linear paraffins produced starting from biomass into more valuable diesel fuels, also containing significant amounts of isoparaffins. Two very different classes of processes have been, indeed, developed to get linear paraffins from biomass.
The core technology of the first of them is the extensive hydrotreating of vegetable oils (reaction 2).
The reaction output is a linear paraffins mixture with a compostion quite similar to that of traditional diesel, but even better chemical-physical properties. Moreover, with respect to traditional biodiesel, the miscibility with conventional refinery streams and fuels is greatly improved. Thus, the process and its products are perfectly integrated with both the existing refinery infrastructures and the fuel distribution system, respectively. In addition, any co-production of glycerol is avoided. The hydrogenation of vegetable oils to diesel fuel has been studied for several years.
47
Only in the
1990s, however, several companies started to develop commercial processes. These companies include Neste Oil (NexBTL process), Petrobras, BP, Conoco-Phillips, Dynamic Fuels, Haldor Topsøe, Axens (Vegan process), and UOP-Eni (Ecofining™ process). Each of them developed its own process, slightly different from any other. Two different plants came on stream in 2007 and 2009 in Porvoo (Finland) with an overall capacity of 380 kton/y (thousands metric ton per year). Two more plants, each one with 800 kton/y capacity, were started up in Singapore (2010) and in Rotterdam (2011). 48 To that point, all the plants were based on the NexBTL process of Neste Oil. Then, in 2013 Diamond Green Diesel, a joint venture of Valero and Darling, started to produce Green Diesel with a capacity of ca. 500 kton/y using the UOP-Eni Ecofining™ process. With nearly two years of successful commercial operation of the plant in Norco, Louisiana, close to its St. Charles refinery site, Diamond Green Diesel 9 ACS Paragon Plus Environment
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proved that the Ecofining™ process meets all expectations.49 Again using its Ecofining™ process, Eni retrofitted existing equipment at its facility in Venice, which had been formerly used to produce diesel from oil, so that it is now producing green diesel from bio-feedstock since 2014. The project will be completed in two phases: the first one is already operative with a capacity of 400 kton/y; the second phase will increase the capacity to 560 kton/y. Eni is also converting the Gela refinery into a new Ecofining™ plant. 49 The many technologies differ for details, and the following discussion mainly refers to the Ecofining process.
50
According to it, the hydrotreating step saturates any carbon-carbon double bond of the
vegetable oil and, at the same time, removes oxygen by three competitive, simultaneous reactions: i) hydrodeoxygenation, removing oxygen as water; ii) decarboxylation, removing it as carbon dioxide; and iii) decarbonylation, that removes oxygen as carbon monoxide (reaction 2). The reaction is usually run at T > 310 °C, using a bimetallic catalysts based on mixtures of Co (or Ni) and Mo oxides supported on alumina. Short contact times suffice for the complete conversion of the feed. The reaction ∆H depends upon the feedstock quality and, particularly, upon the number of its double C–C bonds. The overall reaction, however, is always exothermic and its ∆H is comprised between ca. 400 kJ/kg for the saturated stearic acid and ca. 1750 kJ/kg for for the tri-unsaturated linolenic acid. Apart from water, CO and CO2, the reactions products include linear paraffins and propane. Paraffins are produced, in 99% volume yield, according to three different mechanisms: hydrodeoxygenation of the fatty acids moieties, giving raise to hydrocarbons with the same number of carbon atoms of the parent fatty acids, and decarboxylation and decarbonylation, both affording paraffins with one carbon less. Propane, in its turn, arises from the glycerol moiety and can be recovered and used as a fuel. The Ecofining™ process is very flexible and can be fed with with a number of different feedstock including conventional edible oils (palmm rapeseed, soybean, etc.), inedible ones (e.g., jatropha or camelina), tallow and other animal fats, and even exhausted cooking oils. Thus, it is, to some extent, a bridge between first generation biodiesel and the advanced ones. A simplified flow diagram of the Ecofining™ process is shown in Fig. 2.
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Recycled hydrogen
ACID GAS REMOVAL
Hydrogen Refined vegetable oil
CO2 Propane, light fuels
HYDROTREATING
Naphtha, jet fuel
PHASE SEPARATION
Water Green Diesel
Fig. 2 Flow diagram of the Ecofining™ process.
The second class of processes to produce linear paraffins from biomass includes BtL (Biomass to Liquids) processes, based on technologies similar to those already in use to produce synthetic fuels from coal (coal to liquid, CtL, processes) or natural gas (gas to liquid, GtL, processes). The main steps of a BtL process include: i) biomass gasification, i.e. thermal decomposition of the biomass into synthesis gas (most often referred to as syngas), basically a mixture of hydrogen and carbon monoxide along with several impurities, largely used as feedstock for chemical productions; ii) syngas purification and clean up; and iii) Fischer-Tropsch reaction, affording the synthetic fuel. 51 The overall energy efficiency, defined as fuel heat value/feed heat value, is in the 40-50% range. The Fischer-Tropsch reaction affords a mixture of different hydrocarbons, which range from methane to high molecular weight linear paraffins (waxes). Apart from methane (which forms in higher amounts than expected), the products distribution follows the Anderson-Schulz-Flory model.
52
Thus, it is not
possible to produce selectively a desired products mix. However, a proper choice of the catalyst and the fine tuning of reaction conditions allow to adjust, to some extent, the composition of the final product.
Thus, both the hydrotreating of vegetable oils and the Fischer-Tropsch synthesis afford mixtures of linear paraffins, most appreciated in diesel fuel for their excellent ignition performances. On the other hand, they show poor cold properties, such as cloud point and pour point. A compromise between the 11 ACS Paragon Plus Environment
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amounts of linear and branched hydrocarbons can further improve the possibility to blend them with traditional mineral diesel. To this aim, the mixtures of linear paraffins usually undergo an isomerization step to produce some isoparaffins. The reaction is better performed in the presence of hydrogen and, as a matter of fact, it is a catalytic hydroisomerization. Suitable catalysts are bifunctional ones, most often metals loaded on acidic supports such as amorphous oxides or their mixture (e.g. HF-treated Al2O3, SiO2–Al2O3, ZrO2/SO42−), zeolites (Y, Beta, Mordenite, ZSM-5, ZSM-22), silicoaluminophosphates (SAPO-11, SAPO-31, SAPO-41), or mesostructured materials (MCM-41, Al-MCM-41).
53-57
Some
cracking reactions also occur (Fig. 3),55 the extent of which is much reduced by a suitable combination of support porosity and acidity.
Fig. 3 Simplified reaction scheme of the hydroisomerization step of the Ecofining process.
Best catalytic results were obtained by loading a noble metal (such as palladium, platinum, or nickel) on MSA, an amorphous mesoporous silica–alumina with controlled porosity and with a mild acidity. 58
Zeolites as condensation and oligomerization catalysts An innovative approach to the transformation of renewable feedstock into biofuels was pioneered by James Dumesic.
59
The approach relies upon the selective transformation of carbohydrates, both
pentoses and hexoses. Thus, according to the original scheme, both C5 and C6 sugars undergo dehydration affording, respectively, furfural and hydroxymethylfurfural (HMF) which are further reacted via aldol condensation, including crossed condensation with acetone. C8-C15 oxygenated intermediates are thus obtained and eventually hydrodeoxygenated to C8-C15 liquid alkanes. The formation of alkanes from C6 sugars is shown in Fig. 4. 12 ACS Paragon Plus Environment
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Fig. 4 Proposed scheme for the formation of alkanes from C6 sugars.59 Furfural, in its turn, can undergo analogous reaction affording C8-C13 liquid alkanes. This approach, although resembling more a lab-scale organic synthesis than an industrial process for producing fuels, resulted in the development of the commercial Bioforming process, currently run by Virent Inc. (Fig. 5). The Bioforming process converts aqueous carbohydrate solutions into mixtures of “drop-in” hydrocarbons, i.e. renewable fuels which can be blended with petroleum products (e.g. gasoline) and used in the current infrastructure of pumps, pipelines and other existing equipment. The process has been demonstrated with sugars obtained from conventional sources (corn wet mills, sugarcane mills, etc.) as well as from a wide variety of cellulosic biomass from non-food sources. The process can accommodate a broad range of compounds derived from the deconstruction of the biomass, including C5/C6 sugars, polysaccharides, organic acids, furfurals and other products. The soluble carbohydrate feedstocks are processed in the core of the Virent technology, i.e. the aqueous phase reforming (APR). The APR step utilizes heterogeneous catalysts to reduce the oxygen content of the carbohydrate feedstock. Some of the reactions in the APR step include: (i) reforming to generate hydrogen; (ii) dehydrogenation of alcohols/hydrogenation of carbonyls; (iii) deoxygenation reactions; (iv) hydrogenolysis; and (v) cyclization. The product is a mixture of intermediates including alcohols, 13 ACS Paragon Plus Environment
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ketones, acids, furans, paraffins and other oxygenated hydrocarbons. These intermediates are generally limited to six or fewer C atoms, but their chemistry supports the formation of C–C bond and of hydrocarbons suitable as fuel. By using a modified ZSM-5 catalyst, the Bioforming technology produces a high-octane gasoline with a high aromatic content, similar to an oil-derived reformate stream. Virent has trademarked this product BioFormate™. ZSM-5 promotes a series of acid catalyzed reactions including dehydration of oxygenates to alkenes, alkenes oligomerization, cracking, dehydrociclyzation of alkene oligomers to form aromatics, hydrogen-transfer to form alkanes, and alkanes isomerization.
60
While most of the
hydrocarbons have gasoline-range distillation points, the heavier compounds can be blended into jetfuel. 61 This gasoline fraction is very rich in aromatics and in particular in xylenes. As a matter of fact, more recenly Virent has developed a version of Bioforming technology named BioFormPX™, devoted to the producetion of p-xylene, used for the production of biobased polyethylenterephtalate (PET) bottles.
62
The dehydration of alcohols produced by APR generates alkenes that can be dimerized into
gasoline or oligomerized to kerosene (i.e. jet fuel) and diesel. Oligomerization of alkenes is a quite old catalytic process known since long time. Production of Jet fuel through the oligomerization of small alkenes obtained from alcohols or directly by fermentation is explored by a number of companies, e.g. Gevo (from isobutanol) or Amyris (from isoprene). Also in this case the reaction is catalyzed by acids, including zeolites (e.g. ZSM-5) which were first applied for this reaction by Mobil in the 1970s in the well known MOGD, Mobile Olefins to Gasoline and Distillate.
63
Depending on the desired
hydrocarbon fraction (gasoline, kerosene, or diesel), different zeolites or mesoporous silica-aluminas can be considered, as reviewed by A. Corma. 64 It is worth noting that the Dumesic’s approach to transform carbohydrates into C5 and C6 aldehydes that then undergo condensation and hydrogenation reactions has inspired other researchers who are exploring and exploiting a surprisingly reach organic chemistry as Corma et al that suggested a hydroxyalkylation reaction between methyl furfural (sylvan) and small ketones such as acetone. It yields an intermediate product with much lower oxygen content than Bioforming process, hence easier to hydrodeoxygenate. Finally, the branched hydrocarbon product targets the diesel fraction and has very good cold flow properties, so that a downstream isomerization step could be not not necessary 6566
.
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Fig. 5 Simplified scheme of the Bioforming process. 61
Zeolites are used in other miscellaneous reactions onto sugar-derived molecules, such as ethanol from fermentation producing pathway. Recently, a new zeolite approach for the transformation of renewable ethanol into biofuels has been proposed by Oak Ridge National Laboratory (USA).
67
A new zeolite
system was developed using non precious-metal catalysts (such as vanadium and indium) on ZSM-5, able to catalytically converting an alcohol to a hydrocarbon fraction that contains a reduced benzene content and lower ethylene production. The conversion can be accomplished on dilute aqueous solutions of an alcohol, as found, for example, in the fermentation stream of a biomass fermentation reactor.
68
Starting from the experimental observations that V-ZSM-5 was a highly effective
oxydehydrogenation catalyst and that In-ZSM-5 was an effective alkane aromatization catalyst, a new catalyst InV-ZSM-5 was developed converting completely aqueous ethanol without added hydrogen at 250-300 °C temperature range. The products are aromatic and aliphatic hydrocarbons having 250-450 °C boiling point range. The metal-metal interaction plays an important role in the catalyst performances and it was established that ethanol dehydration was a main step in the product formation. The Vertimass LLC was recently awarded an exclusive license for this zeolite technology and it is going on to commercialize the process by pilot testing, scale-up, and initial plant design. 69
Zeolites as pyrolysis or liquefaction catalysts 15 ACS Paragon Plus Environment
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Still another completely different approach exists to address fuels production from renewables. This relies on poorly selective transformations of the whole biomass into bio-oils, liquid mixtures of organic compounds with high heat values. 10 Bio-oils attracted much attention due to the apparent simplicity of their production processes. Several technologies can be applied, named in different ways according to the adopted reaction conditions: temperature, pressure, heating rate, and residence time. By far, the most cited ones are pyrolysis and liquefaction. 1
Pyrolysis is the thermal degradation of relatively dry biomass in the absence of oxygen, that produces solid, liquid (the bio-oil) and gaseous products. Preferred conditions for liquids production are high temperatures (400-900 °C) and short contact times (seconds or less). Under these conditions, the bio-oil composition is the result of kinetic rather than thermodynamic control. As a consequence, pyrolysis bio-oils are quite unstable and difficult to store. Furthermore, they still have high oxygen content (3540%) and are poorly miscible with conventional mineral fuels thus being poorly suitable for direct use in diesel or gasoline blending. 70-71 On the other hand, liquefaction involves milder temperatures (250-350 °C) and longer reaction times, from several minutes to few hours. Typical feedstock is a wet biomass, such as domestic organic wastes and sewage sludges. In this case, the energy costs for drying the biomass are avoided, but the system develops significant pressure, up to ca. 180 bar. Liquefaction affords hydrophobic bio-oils in 40-55% yield based on the dry weight of the starting biomass. Liquefaction oils are more stable than pyrolysis ones and with much lower oxygen content and, as a consequence, they have high heat value (75-80% of that of the feedstock) and show good miscibility with mineral fuels. The network of chemical reactions occurring during either pyrolysis or liquefaction is very complex. Likely, dehydration and decarboxylation provide the main pathways to reduce the oxygen content, but hydrolyses, isomerizations and even few redox reactions, such as hydrogen transfer reactions and aromatization, also occur. As a result, both pyrolysis and liquefaction oils are extremely complex mixtures of organic molecules, mostly containing oxygen and many containing nitrogen as well, in which heteroatoms are often part of O- and/or N-containing heterocyclic compounds. Studies on model substrates allow to rationalize products distribution and, to some extent, to get insights on their origin. 72
As far as liquefaction is concerned, a significant pilot experience was run by Shell Research Laboratories since 1982 within the HTU (HydroThermal Upgrading) project.
73
Several different
biomasses with high moisture content were treated at 300-400 °C and pressure up to 180 bar. A tubular 16 ACS Paragon Plus Environment
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pre-heater reactor followed by a larger tubular upflow reactor formed the core of the plant. The research was stopped after some years, and a commercial plant using wet organic fraction of domestic waste was designed but not realized.
74
A continuous hydrothermal liquefaction process, using a plug
flow reactor, is being scaled to commercial size by Licella
75-77
. The biocrude obtained after the
hydrothermal step can be hydrotreated to obtain diesel or other derivatives depending on the hydrotreatment severity. Common hydrotreatment catalyst (supported on alumina or zeolite) can be used and the influence of the amount of biodiesel in a refinery stream on the products formed when using cracking or hydrotreating process are also reported 78. A significant project on the hydrothermal biomass liquefaction in a continuous flow processing system is also currently run by a group of international research centers including the Pacific Northwest National Laboratory (PNNL, Richland, USA) and supported, among others, by the U.S. DoE.
78
Starting from the first pilot plant with a 1 liter continuous reactor, an integrate system has been developed using a plug flow reactor. Feeding and recovery sections, as well as bio-oil upgrading, were also improved along with the reactor development in order to optimize the whole process energy yields. It is likely that this recent and advanced process is, so far, the closest one to a real industrial application. Another significant progress is the fast liquefaction technology. Inspired by the fast pyrolysis approach, with very high heating rates and short residence times to minimize undesirable secondary reactions, fast liquefaction was performed using reaction times of few minutes and very rapid pre-heating phase. The results demonstrated higher yields in bio-oil than those obtained in conventional liquefaction ensuring, at the same time, similar heating values of the products.
79
Furthermore, fast liquefaction
allows a reduced reactor volume and lower capital costs. Due to the very complex reactions network that makes difficult to detect any rate determining step to speed up, and to the dirty reaction mixtures readily fouling and deactivating most possible catalysts, liquefaction is usually a simply thermal, uncatalyzed processes. Nevertheless, few attempts have been done to use catalysts in order to improve the bio-oil yield and quality. Thus, for instance, a recent initiatives is the CatLiq® technology for alkali-catalyzed liquefaction, developed by the Danish company SCF Technologies. The CatLiq® technology adds to the feed some potassium carbonate or sodium hydroxide as catalysts and recycles some products streams to the process, thus enhancing biooil yields.
80
In 2011, the technology and the pilot plant were bought by Altaca Environmental
Technologies & Energy which planned a demonstration plant of 200 ton/day in Gonen, Turkey. 81 17 ACS Paragon Plus Environment
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Zeolites have been scarcely studied as catalysts for the liquefaction reactions since, beside the already mentioned limitations to the use of any heterogeneous catalyst, liquefaction conditons are not so different from hydrothermal ones in which zeolites are usully sinthesized by means of equilibrium reactions, so that some hydrolysis could occur. In the patent literature, however, there are few examples of the use of aluminosilicates as catalysts for liquefaction reactions. Thus, for instance, the addition of 3% bentonite, sepiolite, montmorillonite or of zeolites to wet algal biomass is claimed to increase the bio-oil yield up to 30%. 82-83 Some interest was also payed to the use of zeolites as noble metal supports in liquefaction reactions run in the presence of hydrogen. The latter, indeed, offers the possibility to get products with higher H/C ratio and, thus, higher heat value. Moreover, highly more hydrogenated bio-oils should also be more stable since lower contents of carbon-carbon double bonds and of carbonyl groups (i.e., aldehydes and ketones) decrease the extent of further condensation or polymerization and enhance the quality of the bio-oil.
84
Thus, algae liquefaction was reported, using hydrogen and a Ni/REHY catalyst, prepared
from wet impregnation of nickel nitrate on rare earth loaded HY zeolite, under quite soft liquefaction (200 °C and 20 bar).
85
The authors found a significant increment in bio-oil yield with a higher bio-oil
quality, such as higher heating value, due to decreased final amount of oxygen content, demonstrating that the solid acid catalyst was effective in hydrocracking. The final bio-oil, however, still contained a residual heteroatom content, leaving an open question about the effectiveness of the approach in an industrial scenario. No data about re-use of catalysts were reported.
A completely different use of zeolites and related materials has been envisaged to improve a possible co-feed to a liquefaction process. In a typical liquefaction, 20-30%of the starting biomass results in water soluble by-products, so that the process aqueous effluent may have a very high COD (Chemical Oxigen Demand), difficult to be treated in conventional water treatment plants. On the other hand, this process water can be used to feed oleaginous microorganisms which, in their turn, can be fed to the liquefaction step. In this way, the bio-oil yield is slightly improved and, at the same time, the COD of the effluents is greatly reduced, to the point that they can be disposed in a conventional treatment plant (Fig. 6). 86
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Fig. 6 Simplified flow diagram of bio-oil production by liquefaction of domestic organic waste, also including the re-use of process water to feed a fermentation. 86
Process water, however, shows some toxicity for many microorganisms. Thus, even better results can be obtained by treating with suitable sorbents able to retain small amounts of the more toxic substances. The treatment can be run at ambient pressure and moderate temperatures, usually comprised between room temperature and 80 °C, most usually for 15-120 min. Several natural or synthetic silicates may be used as sorbents, including clays, sepiolite, and zeolites, either natural such as clinoptilolite, or synthetic, e.g. HZSM-5. After the treatment, the sorbent can be recovered and reused, possibly after washing and/or calcination. The carbon content of the process water is only marginally affected, the loss accounting for 2-3%. The beneficial effect detected on the microorganisms’ growth is likely due to the removal of small amounts of toxic, basic nitrogen compounds. This treatment of the aqueous phase allows an improvement of the bio-oil yield up to 10%. 87
Among pyrolysis scenario, fast catalytic pyrolysis appears as a promising technique trying to produce in one step a bio-oil adapted to be industrially used. Fast catalytic pyrolysis is a catalyzed thermal decomposition of biomass under oxygen-depleted conditions to produce as main product a bio-oil. 88 It involves the rapid heating of a dry biomass (up to 104 °C/s) with short vapor phase residence times 19 ACS Paragon Plus Environment
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(less than 5 s). The presence of a catalyst promotes cracking reaction of the starting bio-molecules together with upgrading of the biomass pyrolysis products. Unfortunately, this vision is only schematic because the catalyst can enhance deoxygenation of the biomass (through decarboxylation and dehydration) together with coke formation (due to undesired polymerization catalyzed by acid centers). Some metals naturally present in feedstock can influence the final products and this is one of the reason to utilize pyrolysis bio-char as pyrolysis catalyst. 89 Catalyst type and reactor configuration can change pyrolysis performances too. Indeed, it is sometimes not easy to separate process in which there is only the catalytic synthesis in pyrolysis from those with a secondary catalytic upgrading of the gaseous biooil phase. Several companies are working on different processes of fast pyrolysis and following bio-oil upgrading. The most advanced player with an integrated technology was Kior, who operated a demonstration scale plant in Columbus (Mississippi, US), since early 2013 and planned similar, but larger, facility in Mississippi and Texas. 90 The biomass pyrolysis was performed in a FCC reactor-like and occurred in presence of a FCC or pure zeolite catalysts producing a bio-oil with a lower oxygen content. The Kior’s Columbus facility used 500 ton/day of pine wood chip. Among the claimed catalysts, a phosphorous treated ZSM-5 was reported ensuring higher bio-oil yield and lower coke production.
91
Mordenite, beta and Y zeolites were also used. Kior ensured that the produced bio-oil
could be inserted in standard refinery scheme for final upgrading to diesel and gasoline. Indeed, Kior signed offtake agreements with Hunt Refining and Catchlight Energy (a joint venture between Chevron Corporation and Weyerhaeuser Company). It also patented an integrated catalytic biomass pyrolysis into a conventional refinery asset in which a bio-oil blending with already existing petroleum streams is claimed. 92 Nevertheless, the Kior experience initially reduced, due to results lower than expected, and then ended in 2014 due to bankruptcy. 93-94 In the literature there is a huge amount of patents and paper devoted to catalytic pyrolysis.
95-97
Among them, a patent about the possibility to obtain biogasoline
directly from ligno-cellulosic biomass by fast pyrolysis with ZSM-5 appeared in 2009. 98 Further study used other modified zeolites such as Ga-ZSM-5 taking advantage of Ga-promotion of both decarboxylation reaction and olefin aromatization.
99
On this basis, a startup called Anellotech was
launched in the following period. More recently, its goal switched from bio-fuels to bio-based chemicals. Using its proprietary catalyst, Anellotech’s single step catalytic fast pyrolysis enables biomass to be converted in a fluidized-bed reactor into commercially viable aromatics, principally BTX, i.e. benzene, toluene and xylenes. 100 In 2014 Anellotech announced to open a small commercial unit with production of kilogram-scale of green BTX. The technology is able to work with a variety of renewable feedstocks including palm wastes, bagasse, corn stover, and wood feedstocks. In 2015 20 ACS Paragon Plus Environment
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Anellotech Inc., IFP Energies Nouvelles and its subsidiary Axens announced a strategic alliance to develop and commercialize a new technology for the low cost production of bio-based BTX from nonfood biomass. It is expected that the technology will be ready for industrial deployment in 2019. 101 Conversely, non catalyzed fast pyrolysis is already in a commercial progress. 102 Starting from the first application at University of Waterloo (Canada) in the 1980’s, numerous plants with different technologies and feedstock capacity, have been constructed and tested. Most of these were demonstration plants, may be operating intermittently. Several universities and companies world-wide play a significant role in the development of fast pyrolysis and catalytic fast pyrolysis, such as NREL, PNNL, Battelle Memorial Institute, Empyro, Dynamotive Technologies Corporation, Fortum and VTT. This list is by no means exhaustive. Among them, Envergent is a joint creation between UOP LLC, a Honeywell company based out of the United States, and Ensyn, a Canadian based company that has developed and patented a Rapid Thermal Processing (RTPTM) technology. RTPTM is a fast thermal process in which hot sand vaporizes the biomass, which is then rapidly quenched into a bio-oil.
103
Several Envergent plants are currently operational. Even if Envergent’s technique is a no zeolite catalyzed approach, UOP studied zeolite catalytic pyrolysis and some new zeolite materials were recently claimed. 104-105 UOP prepared a new family of materials, named UZM-44 and UZM-39, having structures similar to IMF and TUN with intersecting 10-ring channels. 106 UZM-39 and UZM-44 were used in pyrolysis tests producing bio-oil with a reduced oxygen content and with high selectivities to aromatics and hydrocarbons.
Zeolites in the up-grading of bio-oils As already stated, whereas both pyrolysis and liquefaction are usually uncatalyzed processes, catalysis may play a significant role in bio-oil up-grading. Indeed, it may be that few bio-oils could be used as such as fuels in robust engine (likely for electric energy production, rather than for transportation) but, in order to get high quality fuels, bio-oils must undergo some post-synthesis treatment to improve their quality and properties. Many pathways can be depicted for the bio-oil up-grading, including cracking (e.g. FCC), decarboxylation, and hydrotreating/hydrodeoxygenation. It is likely that many of these pathways will include the use of zeolites or related materials, but this is a field in which further research is definitely needed. Among
the
on-going
strategies,
hydrodeoxygenation
(HDO)
is
relatively
similar
to
hydrodesulphurization (HDS) process in the refinery industry to deeply reduce sulphur from oil streams.
107
As in HDS, sulfide metal catalysts and transition metal-based catalysts are bio-oil-HDO 21 ACS Paragon Plus Environment
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effective catalysts with Co-MoS2 and Ni-MoS2 as the most frequently used catalysts and, when present, the carrier materials are normally γ-alumina, silica or black carbon. Studies by Elliot et al. showed that the degree of deoxygenation of bio-oil was dependent on process severity, with higher temperatures and higher H2 pressures capable of almost complete deoxygenation. 108 However, the above mentioned metal-catalysts could only promote the insertion of hydrogen in the molecules of bio-oil but no other reactions are normally catalyzed. Indeed, as already stated, bio-fuels have to match car-engine requirements in terms of octane number and cold start performances.
109
Isomerization of alkanes to
isoalkanes are recommended and therefore multifunctional catalysts should be studied and developed. In these areas, metal center could act as HDO promoter while the zeolite can act in alkane isomerization together with other pathways including cracking.
110
As process trend, the HDO
upgrading process could be performed in two steps, the first one at relatively mild conditions in order to partially upgrade bio-oil components stabilizing the oil-product before performing a heavier final hydrotreatment step.
111,112
. Actually, the HDO two-step process was generally developed using non-
zeolite catalyst, but zeolites could be introduced in the upgrading scheme, enhancing upgrading performances. A good support for bio-oil HDO should provide high affinity for the oxygen-containing molecules while presenting moderate acidity in order to minimize the formation of coke. In this scenario, a Ni/HZSM-5 was used in upgrading hexane-extracted pyrolysis oil, where the zeolite supports 20 wt% of Ni. 113 The transformation was performed in presence of water as solvent at 250 °C and 50 bar of H2 affording almost quantitatively a C5-C9 hydrocarbons cut. This was composed by cycloalkanes, alkanes and aromatics and could be used in gasoline blending. The study suggested that the Ni/HZSM-5 was effective in promoting HDO reactions with concurrent zeolite-catalyzed dehydration and dehydro-aromatization reactions, e.g. phenol derivatives transformed into cycloalkanes and then to aromatics. On the contrary, cracking by zeolites is a valid alternative pathway since it does not require hydrogen nor pressurized process conditions. Under cracking conditions, oxygenate bio-oil components are mostly converted to aromatic molecules with coke as by-product. The cracking mechanism is based on a series of reactions, the dehydration being the main route.
114
Among others, H-mordenite, HY,
HZSM-5, SAPO-11 and SAPO-5 were used in batch or continuous upgrading of bio-oil via zeolite cracking with different results. HZSM-5 is the mostly frequently tested due to its higher activity dependent on the availability of acid sites. 115-117 It was found that the activity of the catalysts followed the order HZSM-5>H-mordenite>HY>silica-alumina>silicalite-1, demonstrating that zeolite cracking is highly dependent on the availability of acid sites. Furthermore, HZSM-5 and mordenite produced 22 ACS Paragon Plus Environment
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more aromatic components together with less coke with respect to other zeolites. In fixed bed reactors, high-silica zeolites (e.g. HY, ZSM-5 and beta zeolites having Si/Al molar ratio (SAR) of 100, 1500 and 100, respcetively) were tested for selective catalytic conversion of bio-oils.
118
It was found that beta
zeolite showed the highest selectivity to hydrocarbons product slate with respect to other products, while ZSM-5 was more selective to aromatics. Furthermore, re-use of the catalysts was successfully tested even if using higher temperature conditions. The conversion of pine pyrolysis vapors over beta zeolite catalysts with SAR varying from 21 to 250 was recently carried out in a flow microreactor to investigate the effect of number of acid sites on product speciation and deactivation of the catalyst. 119 Beta zeolites effectively upgrade oxygenated products to form olefins and aromatic hydrocarbons. The yields of aromatics per measured acid site increase as SAR decreases, indicating that a greater separation of the acid sites makes them more efficient at converting the pyrolysis vapors. Furthermore, all catalyst had a similar hydrocarbon/coke ratio. Beta zeolites with more acid sites (low SAR) deactivated at a slower rate. Chemical modification of zeolites could also improve the bio-oil cracking performances. A recent example is the desilication of HY zeolites by means of alkaline treatments introducing mesopores in the materials.
120
The formation of a hierarchical micro-mesoporous system could enhance the mass
transfer of the bulky reactants within the zeolite crystals and the resulting modified HY-zeolites were found more active in bio-oil cracking and more selective toward olefins than aromatics with respect to the parent zeolite. The cracking of bio-oil could be performed together with petroleum stream enhancing the industrial feasibility of the process. Recently a joint research study devoted to directly thermally upgrade a bio-oil from liquefaction of sorted organic urban waste using FCC catalysts was performed by Eni research center and ITQ-Valencia.
121
The direct catalytic cracking of the pure liquefaction bio-oil under
classical industrial FCC process conditions has been tested in a Micro-Activity Test (MAT) unit, using an industrial FCC zeolite catalyst. Cracking performances of the bio-oil has been initially compared with the ones of standard refinery FCC Vacuum Gas-Oils (VGOs), i.e. a light FCC VGO, and a heavy aromatic FCC VGO. MAT tests were performed at 530 °C with 30 second of time on stream and with a catalyst to oil ratio of 2-3 g/g. The results of 60 % conversions are reported in Fig. 7. In all tests, liquefaction bio-oils having a heteroatom (O, S and N) content of about 15 wt%
was used in
comparison with pure hydrocarbon refinery streams.
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Fig. 7 Product distribution of MAT zeolite cracking upgrading of bio-oil (blue) versus same tests using refinery streams (red and green). 121
Data with pure liquefaction bio-oil show different results with respet to pure VGO feeds. The bio-oil was sucessfully transformed producing more light cycle oil (LCO, hydrocarbons with boiling temperature in the range 216 < bp < 359 ºC) with respects to heavy cycle oil (HCO, hydrocarbons with bp > 359 ºC) and gasoline (hydrocarbons with bp < 216 °C). Gas fraction are uncondensable light hydrocarbons. A blend of heavy VGO with liquefaction bio-oil (90/10 wt/wt) was also tested by MAT, using the same procedure but with 10 wt% of ZSM-5 as additive of the above FCC catalyst. The results of 40 % of conversion in MAT experiments, in comparison with pure heavy VGO tests, are shown in Fig. 8.
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Fig. 8 Product weight percentage distribution of MAT tests using a blend of heavy VGO/liquefaction bio-oil 90/10 (blue) with respect to pure heavy VGO (green). 121
The blend with bio-oil showed a moderate product change of selectivities having a final results to increase the production of HCO. Therefore demonstrating that liquefaction bio-oil without any pretreatments, is suitable for co-procesessing with standard FCC feeds in order to produce useful refinery streams.
Outlook of zeolite and mesoporous materials in biofuel productions from biomass At present fossil fuels are widely being used in all over the world, but the increased environmental awareness has forced to look for alternatives for replacing them in automotive sector. While waiting a dawn of electric engines or other new renewable and suistainable discoveries in the automotive field, the role of biomass transformation remains a challenging key-step in the world fuel scenario. Indeed, the specific characters of zeolite and mesoporus materials, such as large surface area, controlled porosity, tunable accessibility of reactive molecules into reactive cages, possibility to introduce different and selective chemical functionalities on the solid surface or in the inner pores, metal exchanges, etc., offers unique key tools to unlock the potential of biomass in the energy and fuels sectors. However, many problems still face the development of zeolite in industrial biomass valorization to biofuels. To become an innovative technology in biofuel production, zeolite- and mesoporous material-catalysts should be able to: 25 ACS Paragon Plus Environment
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(i)
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interact with biomass bulky macromolecules such as polysaccharides, lipids and proteins and to convert them into valuable products. As example, we can consider wood and its derivatives, e.g. lignocellulose, which is the most abundant source of biomass essentially constituted by cellulose (38-50%), hemicelluloses (23-32%) and lignin (15-25%)10. All these components are polymeric molecule insoluble in most of conventional organic solvents. To be transformed the biomass requires a dispersant as reaction medium and the so obtained sluggish medium is itself heterogeneous. The recovery and reuse of the solid catalyst at the end of the reaction is another critical issue. Moreover, biomass composition is not often homogeneus and constant, varying from batch to batch. Biomass elemental composition includes quite high amount of oxygen (close to 40-45 % in weight) and relevant presence of nitrogen and sulphur and these facts renders the biomass different with respect to the petroleum scenario with its deep know-how of catalytic systems facing hydrocarbons molecules to convert;
(ii)
to be tolerant and robust to all the natural impurities that are characteristic peculiarities in the biomass and organic waste slates such as, in a not exhaustive list, associated water or moisture, inorganic compounds having acid or basic properties, organic acid byproducts (such as free fatty acids in natural oil), gums, organic molecules containing nitrogen (as residuals protein or other related products coming from their degradation) or phosporous (such as polyphosphate present in different manners in cell walls) and so on. These compounds are diffferent in amount according to the source and kind of biomass and they show different chemical influences due to their chemical nature and abundance. Therefore, all of them could affect solid catalyst performances in different way, even if normally they decrease the desired catalyst activity expecially in continuous mode. Of course, you can purifiy biomass substrate to a well defined extent in order to minimize or inhibit impurities and contaminant problems in the use of solid catalysts. Nevertherless, a high-pure macromolecules to be converted by zeolite still remains only a dream in an industrial scenario due not, may be, to technical limitations but to the related high cost of clean up treatments trying to have as much as possible pure streams from raw biomass. This fact renders the whole renewable catalytic process not industrially economical feasible even if you have an high active and selctive catalyst system. The use of model compounds in biomass transformations sometimes hides the real problems that you could face trying to scale up a process for industrial purposes and the present review tried to present as much as 26 ACS Paragon Plus Environment
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possible the state of the art selecting industrial approaches. All in all, the present critical limitations of zeolite applications could represent both a challenge and a demanding chance for chemical researchers in the near future. At the beginning of petrochemical industries, the petroleum streams were difficult to be transformed into all the compounds that we have today in a petrochemical plants and refining schemes. Academic and industrial organisations studied and developed high efficient solid catalysts overcoming the problem of poor-selectivity and robustness to contaminants in long-stand operativity offering valuable processes that are the base of our daily technolgy and life. The same developmnet is needed in the growing up biorefineries, where the main efforts in research should be devoted to prepare and use high robust zeolites.
Conclusions Humankind needs to reduce GHG emissions in the atmosphere. The use of carbon-neutral biofuels still stands up as one of the most immediate and effective approaches to the problem. Several technologies are being developed to make biofuels, especially advanced ones, really available. In few cases, these technologies may even be competing among them. Eventually, the success of one or few of them will depend on several issues, such as availability and quality of the feedstock, complexity of the processes, and quality of the fuel. Catalysis provides a superb tool to solve the problems arising from many of such issues: for instance, it can help to simplify processes, or to improve the fuels quality. So far, merging zeolites features with biomass properties proved to be difficult but, nevertheless, several applications of zeolites are already at the commercial or demonstration scale, e.g. in the Bioforming process, or in the hydrotreating of vegetable oils. Nevertheless, tremendous effort is required to get technology breakthroughs, the weapons that will enable us to face many problems still unsolved. We do believe that zeolites and mesoporous materials will offer an important contribution to this effort. In this regard, the important developments in the preparation of zeolites with increased pores accessibility summarized at the beginning of this review may pave the way for a wider and more efficient use of crystalline-porous materials in biomass conversion processes.
Acknowledgements The authors wish to thank Mr. Gabriele Bianchi for the preparation of Figures 1, 2, 5, and 6.
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