Impact of Zeolites on the Petroleum and Petrochemical Industry
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- Vermeiren, W. & Gilson, J. Top Catal (2009) 52: 1131. doi:10.1007/s11244-009-9271-8
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The general features of zeolites that led to their widespread use in oil refining and petrochemistry are highlighted as well as the details of their impact on selected processes. The analysis of the catalyst market and the position of zeolites therein is a good indication of their strategic importance. Zeolites have brought many disruptive changes to these fields (e.g. FCC). They impacted also these industries in an equally important way, although more subtle, by incremental improvement of processes. The new and vast challenges facing oil refining and petrochemistry as well as the managed transition to sustainable environmental benign transport fuel industries and chemical industries will require creative science and technologies. Zeolites offer the basis of many of these technological solutions provided efficient and balanced cooperations between industry and academia are further developed.
KeywordsZeolitesMolecular sievesOil refiningPetrochemistryEmerging technologiesEmerging feedstock
1 Introduction and Scope
Zeolites have ceased to be minerals merely displayed in museums, and have become a commercial and scientific success-story since their large-scale utilization in industry. It all started with the use of synthetic zeolites in oil refining and petrochemicals. The rest is now part of industrial history: it is a case study of breakthrough and incremental innovation [1–3] and a model of interplay between science and technology, from very fundamental academic research and industrial curiosity to efficient large scale production of zeolites and their deployment in catalytic- or adsorbent-based processes.
This contribution is not another updated compilation of zeolite-based processes found in oil refining and petrochemistry; many reviews exist already [4–7] and point to further and more detailed studies. In particular, the two-volume contribution of Marcilly  is a compulsory reading for the serious beginner for the depth and the breadth of its coverage; moreover, it gives a unique perspective by a seasoned practitioner acquainted with both fundamental and applied aspects of zeolites and other acid–base catalytic processes.
This contribution aims to paint a broad picture of the subject, with a few close-ups on selected topics. It is followed by a general reflection on the future use of zeolites in industry and the need for collaboration between industrial and academic researcher. It is intended for newcomers in the world of zeolites science and technology and intends to enable them to deepen and broaden their insights to generate new ideas, processes and products for the benefit of society as a whole. Zeolites have so far been extensively used in the petroleum industry and could still play an important role in the major challenges ahead of us. These are the exploitation of non-conventional petroleum resources (tar sands, heavy crude oil and oil shales), non-petroleum resources like gas and coal and valorization of biomass for our petrochemicals production and fuel supply.
This review focuses mainly on zeolite applications in refining and petrochemistry. For details of zeolite applications in fine chemicals, the reader is referred to the specialized literature.
2 From the Pioneers to Today
Much of the pioneering work on zeolites, particularly the synthesis of new zeolites comes from the former Union Carbide and Mobil companies. Probably one of the most important paradigms is the launch of using quaternary ammonium hydroxides in the synthesis of zeolites . This led to the discovery of zeolite ZSM-5 in 1963 by former Mobil . Often in order to arrive to industrial applications and commercialization, external factors (market push) are required. Chen describes how the US government’s decision to remove lead from gasoline, provided the needed economic incentive to commercialize the expensive ZSM-5 zeolite . Even with such external stimuli, it takes often many years before delivering commercially viable applications. For instance, the MSTDP (Mobil Selective Toluene Disproportionation Process) was commercialized in 1988, seventeen years after the discovery of the ZSM-5 synthesis .
Registered zeolite structures by the International Zeolites Association : commercially used zeolites are highlighted
Recently, UOP has introduced a new concept in zeolite synthesis by optimizing the way the typically well-known ingredients are put together, the so-called “Charge Density Mismatch” (CDM) approach [11, 12]. The aluminosilicate reaction mixture is characterized by the mismatch between the charge density on the organoammonium structure directing agent (SDA) and the charge density on the potential aluminosilicate network that is expected to form. These conditions can be accomplished by creating an aluminosilicate reaction mixture (typically a clear gel) using a large SDA (low charge density) and a low Si/Al ratio (high charge density). The crystallization of a zeolite from such a reaction mixture does not proceed spontaneously. Crystallization can be induced by the controlled addition of supplemental SDAs that have charge densities that more suitably match that of the desired low ratio aluminosilicate network. The advantages of this approach are greater control over the crystallization process and efficient cooperation of multiple templates. The approach is demonstrated for a new family of zeolites (UZM stands for UOP Zeolite Material), UZM-4, UZM-5, UZM-8, UZM-9, UZM-15 and UZM-17 [11, 12]. In particular, the UZM-8 with its unique layered structure has a large number of acid sites at or near the external surface of the zeolite crystals. In the alkylation of aromatics with ethylene or propylene with UZM-8, the mono-alkylated product can readily desorbe from the zeolite surface and hence limits the formation of multi-alkylated products and di-aryl-alkanes . It has been announced that this new UZM-8 zeolite has been, after only 6 years of development, offered for sale in late 2006 for the use in the liquid phase alkylation of benzene with ethylene . Although, little scientific research has been published, it seems that this material could exhibit features similar to the MCM-22 and EU-1 zeolites, materials where the acidity and the molecular environment at the external surface of the zeolite crystals are believed to be essential for the improved performance in alkyl-aromatic reactions (alkylation, isomerisation and disproportionation) [15, 16]. These two zeolites possess non-interconnected channels accessible through 10-member ring apertures, but one of the channels exhibit supercages defined by 12-member rings. The outer surface of both zeolites is believed to be covered by open large side-pockets circumvented by 12-member rings.
Recently, other classes of molecular sieves have been synthesized: (i) the mesoporous silica-based materials of the M41S family (MCM-41, MCM-48 and SBA-15 are the best known) and (ii) the crystalline microporous titanosilicates (ETS family developed by former the Engelhard) [17, 18] and the silicotitanates (co-developed by Sandia National laboratories and UOP) . These new materials are claimed to be commercially used: MCM-41 probably as a low acidic support of metallic catalyst for applications like hydro-processing, including hydro-demetallation  and the microporous titanosilicates or silicotitanates are being used commercially in separation and ion exchange. Commercialization of MCM-41 required about 10 years and the major challenge was associated with the identification of an application where the performance incentive was high enough to justify risks and cost of industrialization.
Zeolites not-requiring organic template molecules have reached the largest volume applications: zeolite A, X, Y, high-alumina ZSM-5 and mordenite. Other high-alumina ferrierite, zeolite L, T and F do not require organic template, however their use is not widespread.
Significant performance benefits over existing catalyst must be demonstrated when more exotic reagents, like complex organic templates (often not commercially available) or metal precursors or toxic or hazardous ingredients are employed .
Seldom has the as-synthesized material provided the optimal catalytic or adsorptive performance. An excellent overview of different post-synthesis techniques is provided in volume 3 of Molecular Sieves .
Last but not least, the active and selective molecular sieve material has to be formulated in a shape that can be used in industrial reactors. This requires the design of a composite blend of ingredients that provide mechanical strength to the shaped body and of processing aids required for the shaping process [23–25].
Moreover, the 179 known structures presently harvested are only a tiny fraction of the 2.5 million structures that are theoretically feasible . Such a database could be the basis for “Designer Catalysts”. Powerful computational exploration will be required to screen such large database for the multitude of possible applications . Also high-throughput techniques will contribute to the identification of the most efficient molecular sieves [28, 29].
3 Catalysts and Zeolite Market
Global catalyst manufacturing business , AAGR is Average Annual Grow Rate, constant 2005 $
Fine chemicals, intermediates
Total market [106 $]
However, it should be recalled that these catalysts have a large leverage effect, often more than 2 orders of magnitude and play a key role in the industries they serve (automotive, pollution abatement, refining and petrochemicals). Indeed, if in a refinery or petrochemical complex a major unit shuts down due to a catalyst failure, ripple effects are often felt on the market. Catalysts in general and zeolites in particular are performance chemicals, i.e. their value resides not only in their chemical composition but especially in their ability to promote much desired changes (higher gasoline production, better fuel quality, low cost base chemicals for polymer manufacturing, highly selective processes…).
Zeolite consumption and supply , on anhydrous basis
Zeolite consumption/supply (kta)
2004 share (%)
Synthetic zeolites consumption
Synthetic zeolites capacity
Natural zeolites consumption
Natural zeolites production
Major synthetic zeolite producers, based on 
Major synthetic zeolite producers
Albemarle, including former Akzo
BASF (former Engelhard)
Instituto Mexicano Del Petroleo
Mizusawa Ind. Chem.
Shandong Aluminium Corporation
Süd-Chemie, including Tricat
W. R. Grace & Co
4 Zeolite Use in Industry
During the second half of the former century, the industrial applications of zeolites or in general molecular sieves have emerged. The majority of the base chemicals that constitute our daily consumers goods and energy carriers like transportation fuels have passed through the micro- and mesopores of molecular sieves. Some zeolite structures, mainly MFI and FAU and to a minor extend also MOR are very versatile materials, i.e. their properties can be tuned to the specific requirements of very different industrial applications.
Tanabe and Hölderich gave an account of acid–base catalyzed processes that have been commercialized or have been proposed for commercialization. A total of 124 acid–base catalyzed processes were available, of which 74 were zeolite-based .
Types of commercial processes using zeolite-based catalysts
Zeolite structure code
LPG or olefin aromatisation
The potential of zeolites is probably best illustrated by their saga in FCC  where they led to a step change in the oil refining industry. Their high activity, increased selectivity towards the gasoline fraction and lower coke make relative to the previous generation of catalysts (amorphous silica–alumina’s) allowed refiners to increase gasoline output while cutting the number of FCC units. This is a dramatic illustration of the leverage of zeolite catalysts (a new catalyst allows to drastically reduce the capital requirement of an industry). Since this breakthrough, many incremental improvements in the FCC catalyst and process have made this zeolite based catalyst ever more efficient … and hard to replace.
Their high acidity or mild basicity can be tuned easily
Their structure resists or adjusts to high temperatures (>400–800 °C) allowing their use in the very harsh conditions (regeneration by air combustion of deposited coke for instance) of refinery and petrochemicals transformations
Their pore system is either 1, 2 or 3 directional allowing an optimum management of the molecular traffic
They are non-corrosive and non-toxic and have a “green” character
Their shaping (extrudates, spheres, microspheres) make them easy to handle and to separate in various catalytic reactor configurations
They have a wide range of composition (Si/Al ratio, presence of other T atoms such as Ti, Ga, Fe, B,…) and their properties can be tuned by ion-exchange
A wide variety of structures (>179) are at the disposal of imaginative application scientists, although only a few have reached the commercial stage
The crystal size of each structure can be tuned, although typical values are in the range of 0.1–5 μm. This allows a precise control of:
The diffusion path length (hence control of zeolite utilization and selectivity based on the matching between the shape/size of the zeolites and the molecules inside the pores)
The ratio of external (typically equivalent to 10–40 m2/g) to internal (typically equivalent to 300–700 m2/g) surface to control shape selectivity, secondary product formation and deactivation
Typically, low-silica zeolites, exhibiting a high Cation Exchange Capacity and a rather low acid strength, are used for adsorption and ion exchange applications. On the other hand, high-silica zeolites, with a low Cation Exchange Capacity and hence a high acid strength, find application as catalysts or adsorbents of apolar molecules.
Examples of shapes and sizes of catalysts
Catalytic reactor type
Cyclar: aromatisation of LPG
Hydro-cracking, aromatics isomerisation
FCC, Beckmann rearrangement, MTO
Exhaust catalysts, ethanol dehydration
Pore opening (Å)
Si/Al = 1, K+
Drying of olefin containing streams (cracked gas in FCC and steamcracker)
Si/Al = 1, Na+
H2O, CO2, C2H6
n-P, n-O > C2
Drying and purification
Si/Al = 1, Ca2+
Si/Al = 1.2–1.5, Na+
P, N and aromatics
Simultaneous drying and purification: H2O, H2S, CO2 and oxygenates
Preference for p-xylene
p-Xylene separation from xylene–ethylbenzene mixtures
Inerts removal from natural gas
5 Some Oil Refining Applications
Increase H/C of crude fractions to make on-spec products
Remove pollutants (metals, S, N,…)
Typical thermal efficiency (based on lower heating value) for a refinery operation is ~90% compared to 67% for methanol synthesis and less than 60% for existing GTL (gas to liquid) processes
Typical carbon efficiency for a refinery is about 93% (>70% transportation fuels, <25% specialties) while the remaining 7% is used for auto-consumption (steam, hydrogen manufacture). Such high carbon efficiency gives oil refining and petrochemistry the best E-Factors (ratio of mass unit by-product per mass unit of desired product) calculated by Sheldon for several industry segments 
Interplay between specifications, molecular characteristics of the products and reaction involved
European 2005 specifications
Preferred hydrocarbon structures
Reactions leading to these hydrocarbon structures
<50 ppm sulphur
>51 cetane number
<50 ppm sulphur
<0.845 gr/ml density
Lay down the stoechiometry
Analyze the thermodynamics (thermicity, equilibrium composition)
Get a grasp for the kinetics and the catalysts involved
Evaluate the deactivation pattern and rate (will determine the reactor configuration)
Integrate at the macro level using the rules of chemical engineering
The following three zeolite-based catalytic processes encountered in oil refining are briefly highlighted within this framework.
5.1 C5–6 Paraffins Hydro-Isomerization
The goal of this transformation is to increase the octane rating of the light naphtha fraction. It is done by skeletal isomerization of n-paraffin’s as the octane numbers (both the Research Octane Number [RON] and Motor Octane Number [MON]) increase with the degree of branching of the molecules. It is a particularly attractive contribution to the gasoline pool [54, 60, 61, 64] since all other contributing processes have major drawbacks as far as process safety (H2SO4 or HF catalyzed Alkylation) and environmental regulations (Reformate, FCC Naphtha, Oxygenates, Polygasoline, Butanes) are concerned.
Main characteristics of the 3 families of C5–6 hydro-isomerization catalysts
Sensitivity to contaminants
Sulphur, nitrogen removal
Drying needed to prevent chlorine loss
Needs chlorine make up in feed!
Medium to low
Less sensitive to water
Tolerates water and small amounts of sulphur
Since the isomerization of n-paraffin’s is an equilibrium-limited reaction, it would appear that the zeolite catalyst should be the least desirable due to its relatively low activity and associated gain in Octane Numbers. However a closer look at the catalysts and their response to processing conditions, Table 10, shows that the MOR based catalyst is the most “robust” and is relatively easy to regenerate. It is therefore a solid option for a refiner seeking a solution to its octane problem without the need for extensive purification of the feedstock. This illustrates that the selection of an industrial catalyst is a complex process of optimization and that, for instance, the most active catalyst is not automatically the most desired option.
Further improvements are still possible in this refinery operation. For instance the complete integration of the isomerization and the separation sections in a single (in situ adsorption or the use of membranes for in situ separation) reactor  and the co-processing of C5–6 hydrocarbons with C7 (the latter challenge, based on the great activity difference between C5–6 and C7 hydrocarbons is well illustrated in Fig. 4.1 of Ref. ) are highly desirable but very challenging improvements still waiting for a solution. Inherently, once the hydrocarbon contains 7 or more carbons the cracking through β-scission becomes favorable as intermediate secondary or tertiary carbocations crack into tertiary carbocations, which is thermodynamically a favored reaction [67, 68]. Isomerization of light naphtha, especially C5’s, will probably be reduced in the near future because of the blending of bio-ethanol with gasoline. Addition of ethanol does increase the vapor pressure (RVP) of gasoline  and in order to keep the RVP according to specifications, less light isomerate can be blended into the gasoline pool. As far as zeolitic catalysts are concerned, modified MOR is presently the only zeolite in commercial use although another zeolite, MAZ, displays both a higher activity and selectivity towards branched products [70–72]. New and powerful computational tools, based on Monte-Carlo simulations, appear to be very useful  in rationalizing the selectivities of various pore structures; ultimately, they could prove of vital importance in the design of isomerization catalysts by matching the catalyst to the molecular composition of these (relatively) simple feeds.
A final word of caution on the laboratory evaluation of isomerization catalysts: it is important to report catalytic activity/selectivity on feedstocks containing mixtures of hydrocarbons (C5–6 paraffin’s and C6 naphthenes) as adsorption and transport phenomena can play an important role [73, 74]. It has also been observed  that the presence of naphthenes strongly inhibits C5–6 hydro-isomerization on both Pt/MOR and Pt/Sulfated Zirconia catalysts, while some based on Pt/Tungstated Zirconia display a much smaller inhibition. Therefore, catalysts tested with pure feedstocks could wrongly be claimed to have superior performances and their application be restricted to very specific (and rare) cases.
5.2 Cracking and Hydro-Cracking of Heavy Feedstocks
To adjust the feedstock characteristics to the products requirements; in that respect, it can be broadly stated that the role of a refinery is to increase the H/C ratio of the feedstock to match the requirements of the products (whitening of the barrel)
To produce transportations fuels (gasoline, kerosene and diesel) in the right mix while meeting the latest specifications
Main features of FCC (fluid catalytic cracking) and HDC (hydro-cracking) processes
Close to atmospheric pressure
Need for hydrogen
No hydrogen present
A lot of hydrogen present
Nearly no olefins
Naphtha (15–220 °C)
Reformer or steamcracker feed
Kerosine (150–250 °C)
Gasoil (250–370 °C)
5.2.1 Fluid Catalytic Cracking (FCC)
The early oil cracking processes and its modern version, FCC, are landmarks of the industrial history [76, 77]. The modern process is a complex interplay between catalyst, process engineering and the ever-changing needs of the market. General and in-depth reviews are available [78, 79] and the latest advances and challenges have been described recently [80, 81].
The cracking reaction is endothermic, not limited by thermodynamics and increases the number of moles. It is therefore favored at high temperature and low pressure. The catalyst cokes very rapidly (<sec) and needs to be regenerated frequently. This is done commercially by the use of a fast circulating bed (fluidized bed) transporting the deactivated catalyst to the regenerator where the coke is burnt and the catalyst recirculated to the reactor (called a riser). This rapid circulation of the catalyst imposes considerable constraints on the catalyst design and shaping. It is however a blessing since the heat generated during the exothermic regeneration (coke combustion) is returned to the riser section to balance the endothermic cracking reaction. The FCC unit is therefore a two-reactor unit: one, the reactor, operating in a reducing environment, the other, the regenerator, operating in an oxidizing environment. The other important components are the stripping section (where the coked catalyst is steam-stripped from adsorbed hydrocarbons), the cyclone section (where the solid catalyst particles are separated from the hydrocarbon vapors), the slide valves regulating the flow rate of spent/regenerated catalysts and therefore the heat balance of the unit and the injection section (nozzles design).
It is an elegant and efficient way to manage a very unstable operation by allowing it to operate continuously and therefore supplying a constant contribution to the gasoline and light olefins pools.
Olefin: adsorbs on a protonic site as a carbocation
Naphthene: adsorbs on a nearby site
These two molecules being close by in the near spherical cavity of the zeolite (supercage), an hydride can be abstracted from the naphthene, the operation repeated to yield paraffins and aromatics
Octane making FCC catalysts: their low RE (below about 3 wt% RE2O3) content allows the zeolite to dealuminate strongly, thus lowering its acid site density and consequently its hydrogen transfer capability. The olefin rich gasoline has a high octane rating
Gasoline making catalysts: their high RE (up to 15 wt% RE2O3) content prevents the zeolite from dealuminating strongly, thus maintaining a high site density and consequently its hydrogen transfer capability. The paraffin and aromatic rich gasoline is more stable and therefore produced in higher volume
The extra benefit of steaming the FAU zeolite (during the zeolite preparation process or the FCC operation itself) is that mesopores are generated; together with the matrix porosity, they provide an extra accessibility of bulky molecules to the zeolite crystal where the critical hydrogen transfer reaction can take place efficiently. All these features illustrate the exceptional adaptability of the FAU zeolite to the FCC requirements and explain its longevity: so far no other material, zeolitic or not, has been able to displace or even come close to displacing it.
The zeolite crystal, ion-exchanged with the desired level of rare-earth, to achieve the desired gasoline properties
An active matrix designed (composition and porosity) to pre-crack large molecules
A clay (kaolin) acting as a filler and an heath sink
A binder to maintain the integrity of the spray-dried particles
FCC catalysts are now produced in very large-scale equipment with sophisticated process control equipment insuring their manufacturing under some very stringent quality control procedures.
In addition to the cracking catalyst, other particles (additives) of the same size and density can be introduced in the unit [3, 80] to promote some reactions (combustion promoters, SOX transfer, bottoms cracking…). One very important additive is the so-called octane booster: it reduces the quantity of gasoline and increases its olefinicity (hence its name) by promoting its cracking in lower olefins (mainly propylene). It is based on the smaller pore MFI zeolite. It is therefore a new supplier of olefins for the petrochemistry and further increases the link between an oil refining and a petrochemicals complex.
Effect of the FCC catalyst composition (Zeolite and Matrix) on its activity, selectivity and stability 
Dry gas make
Feedstock 340–480 °C
Feedstock >480 °C
The heavier and dirtier crudes, difficult to process due to their metals (Ni affects the coke selectivity and V, Na, Fe,… destroy or severely reduce the zeolite activity), sulfur (SOX emissions, the S content of the FCC gasoline requiring either to process the feed upstream, the products downstream in separate units or to find effective FCC additives), nitrogen (NOX emissions) contents and their higher coke making tendency
The present quality of co-products (the LCO, Light Cycle Oil, a middle distillate high in aromatics and sulfur needs further upgrading to be incorporated in higher quantities in the diesel pool)
New regulations on particulate emissions and disposal of used (and contaminated by heavy metals) FCC catalysts
The market demand, in particular European and Asian, for a lower ratio gasoline/diesel
FCC is still a good route to accomplish high conversion of heavy gasoils, but it is not the preferred route to produce low-aromatics and low sulphur transportation fuels. Subsequent hydro-processing is required to remove sulphur and to reduce aromatics, especially the polyaromatics in the Light Cycle Oil (LCO) fraction, consuming additional hydrogen and destroying some octane-value for the gasoline fraction. A major concern is the LCO, exhibiting a very low cetane value, a high content of polyaromatics and hence increasingly difficult to blend with advanced diesel. The challenge is to develop a new generation of FCC process and catalyst that provide good conversion of heavy fractions to low-aromatics transportation fuels with maximum production of light olefins (mainly propylene).
Because of the limited flexibility of processing heavier and dirtier feedstock and some lower quality features of the gasoline and diesel fractions, FCC faces tough competition from hydro-cracking and thermal cracking (visbreaking, coking) processes.
However, history tells us that researchers and engineers have so far always been up to the numerous challenges encountered in FCC.
5.2.2 Hydro-Cracking (HDC)
Hydrogenation of (poly)aromatics, olefins
Hydro-dealkylation of alkyl aromatics
Hydro-decyclization of naphtheno-aromatics and ring opening
Isomerization of paraffins and naphthenes
These reactions are all exothermic and only the aromatics saturation is limited by thermodynamics (i.e. at high temperature and low hydrogen partial pressures, aromatics concentration will increase spontaneously thus defeating the transformation purpose). The relative high stability of the catalysts thanks to the high hydrogen partial pressure, preventing formation of coke precursors, allows the use of fixed bed reactors with the possibility to stage catalyst beds with different compositions. The presence of multiple catalyst beds per reactor is a consequence of the exothermicity of the reaction [87–89]: interstage cooling of the reactants/products mixture is necessary in order to maintain the temperature gradient within manageable values (about 20 °C).
Single-Stage: all catalysts are in the same (single) reactor and all the products (including H2S and NH3) emerging from one catalyst bed move to the following bed of the unique reactor
Series Flow: the catalysts are distributed in two reactors, one dedicated to hydro-treating, the other specifically to hydro-cracking. As in the Single-Stage configuration, all products (including H2S and NH3) emerging from the first reactor move to the second one
Two-Stage: the catalysts are distributed in two reactors, one dedicated to hydro-treating, the other specifically to hydro-cracking. The difference with the Series-Flow configuration resides in the removal of H2S and NH3 between the two reactors. Such a configuration sends an almost H2S and NH3 free feed to the second reactor where a so-called “well-balanced” hydro-cracking catalyst (i.e. the hydro-/dehydrogenating function is a noble metal) can operate near the “ideal” conditions (acid function being the rate determining step) and maximize the yields of the desired products [82, 90, 91]
Acidic function: supplied by amorphous silica–aluminas (ASA), fluorided aluminas, zeolites or combinations of those
Metallic function: supplied either by sulfided NiMo and/or NiW or a noble metal (Pt, Pd)
As in FCC, zeolites (Y type FAU) are present in HDC and play an important role alongside amorphous silica–aluminas (ASA). The zeolite Y used in HDC is dealuminated; it brings high activity (high acid site strength), stability (low coke make and resistance to nitrogen poisons compared to pure ASAs) together with a good accessibility due to the presence of mesopores generated during the steam dealumination process . As in FCC, the zeolite Y brings a high degree of flexibility: the Si/Al ratio can be finely tuned by dealumination (steaming, leaching…) and regulates the selectivity (naphtha versus middle distillates) of the zeolite containing catalyst. As the Al content decreases (measurable by a shrinking of the unit cell size of the zeolite crystal), the kerosene (and diesel) selectivity increases as successive cracking reactions (towards naphtha and gases) are minimized. The operation in fixed bed reactors prevents however to achieve a degree of flexibility such as in FCC where the catalyst could enter the (fluidized) bed on very short notice.
Diesel: low in S, N and aromatics yielding high cetane and on spec fuel. HDC diesel is the best contributor to the diesel pool
Kerosene: the same features apply and the smoke point (linked to soot formation during combustion and hence the aromatic content) is excellent
Max middle distillate
Less NH3 sensitive → one stage applications
Higher activity and hence favours aromatics hydrogenation equilibrium and superior coke precursor hydrogenation
More active and hence lower temperature, less coke production
Shape selectivity excludes large coke precursor to enter: less coke
Microporosity results in consecutive cracking of primary products and hence lighter products
Many challenges are facing HDC: the widespread use of heavier conventional feedstocks and the emerging extra-heavy unconventional crudes (tar sands, shale oils…). The intrinsic low coking tendency of zeolites makes them prime candidates, but their microporosity could be a potential drawback. While a solution may come from new mesoporous materials, it is likely that hybrid catalysts combining well controlled, interconnected and homogeneous micro- and meso-porosity could prove an interesting research avenue [92–94]. In addition, these new (nano) engineered materials could well have to operate in moving bed reactors (ebullating, slurry…), so that their shaping will require close collaboration between catalyst and process engineering. The field is thus gaining in interdisciplinarity and attractiveness for new researchers; it could prove very fertile in the coming years.
Another application of zeolite catalysts, closely related to HDC, is the production and upgrading of lube fractions (base oils) to design modern lubricants. While the volume of lubricants is dwarfed by the transportation fuel market, they have per volume a higher value and zeolites technologies have already contributed to very significant breakthroughs. The zeolite-based processes (production of lube bases by HDC and further upgrading on shape selective zeolites) are increasingly displacing the older extraction and crystallization processes  along the whole lube chain. The high value and tight specifications of lubricants provided refiners and process licensors the opportunity to introduce more expensive zeolites or molecular sieves (zeolites β, TON, MTT, AEL…). The chemistry involved is a combination of hydro-isomerization, hydro-cracking modulated by the shape-selectivity of the zeolites involved; its fundamentals have been extensively studied especially by the Belgian (P.A. Jacobs/J.A. Martens and co-workers, see for instance ) and German (J. Weitkamp and co-workers, see for instance ) schools. A recent review illustrates the beneficial effect of mono-dimensional zeolites or molecular sieves (ZSM-48, ZSM-22/23 and SAPO-11) for isomerisation whereas large-pore zeolites (Beta, USY, and ZSM-12) allow the formation of multibranched isoparaffins, which are susceptible to cracking . The field of zeolites applied to lube oil processing has been introduced and reviewed quite recently in excellent papers [99–101]. Most of these lube-producing catalysts perform dewaxing by isomerization and are bifunctional catalysts. Beside this hydro-isomerisation of long-chain molecules to improve their fluidity properties, also monofunctional dewaxing catalyst have been commercialized (so-called Cold Flow Improvement or CFI) . These catalysts are typically placed at the backend of hydro-desulphurization catalysts in order to improve the cold flow properties of heavy gasoils.
A related new application of selective hydro-isomerization is the isomerization of the paraffinic hydrocarbons produced during the hydro-deoxygention of natural oils [103–106]. In this new process of transforming natural triglycerides into diesel components, linear intermediate paraffin’s are produced that have to be isomerized slightly to improve the cold flow properties. Hence highly selective isomerization catalysts, precluding cracking, are required in order to maximize the bio-diesel yield. SAPO-11 has been proposed in this respect .
Further advances in lube oils upgrading will probably be strongly influenced by the application of new experimental techniques (High Throughput Experimentation in the synthesis, characterization and testing of zeolite materials) and computer-based (in-silico) screening of potential (even yet to be synthesized) molecular sieves where the tailoring of the catalyst to the product specification will play an ever increasing role . The feedstock supplied by the newly installed or announced GTL (gas to liquid) plants in countries with advanced natural gas feedstock, will be free of almost any impurity (olefins, aromatics, sulfur, nitrogen…) and the upgrading catalysts will be able to operate under quasi ideal conditions, thus maximizing the yields of high value products .
6 Petrochemical Applications
6.1 Aromatics Production
Existing and proposed processes producing aromatics from various feedstock
Light olefins, FCC gasoline or naphtha
Former Mobil 
Pygas, FCC gasoline
CP Chem 
Pt(Re, Sn)/Cl · Al2O3
UOP, IFP 
6.2 Aromatics Processing
Supply of various aromatics and relevant properties of the C8 cut
HT coke oven light oil + others
Equilibrium @450 °C
Contribution to BTX
1. Isomerisation of C8 aro
With EB isomerisation into xylenes
Very active catalyst
UOP and IFP developed non-mordenite catalysts
With EB hydro-dealkylation
Dealkylation and ethylene hydrogenation
2. Disproportionation of toluene
Limited amount of C9+ aro can be treated due to pore size
Can handle C9+ aro
ZSM-5 precoked or silica-treated
3. Transalkylation Tol/C9+ aromatics
Can treat > 50% C9+ aro
4. p-Xylene isolation
Adsorption on Ba(K)X
Eluxyl (IFP)–Parex (UOP)
ZSM-5 in gasphase >400 °C
High B/E ratio > 5
Small make of xylenes
MCM-22/49, Beta in liquid phase
Low B/E ratio <5
Low xylene make
Y in catalytic distillation
Can use diluted ethylene
Y in catalytic distillation
Can use diluted propylene
Highly active → low T and hence very small amounts of n-PB
Transalkylation of multi-alkylbenzenes
Same zeolites as for liquid phase alkylation
Requires separate reactor
7 Some Emerging Applications
They are far less documented than the well-established processes described above but some have been reviewed recently by Degnan . It is often difficult to know the exact nature of the zeolite used, albeit the various post-synthesis modifications. In addition, the meaning of “commercialization” is also often unclear (pilot plant results versus demonstration unit or captive use of a proprietary technology). In the following, an attempt is made to list the emerging zeolite applications in the field of refining and petrochemistry, the reader is referred to specialized literature for applications in fine chemical production. These emerging/forthcoming applications fall into the following categories:
7.1 Emerging Technologies Providing Improvements Over Existing Technologies
Balancing the gasoline/diesel ratio: Oligomerization of light olefins (EmoGas from ExxonMobil , COD from Süd-Chemie ). The EMOGAS catalyst demonstrated equivalent or improved performance than the existing SPA (SiO2 Supported Phosphoric Acid) catalyst in terms of feed processed, product qualities, pressure drop and ease of handling. Patent filing indicates that the used catalyst is probably of the ZSM-57 or ZSM-22 family, which prevents the formation of higher oligomers . The COD catalyst is a ZSM-5, presently used at Mossgas for the conversion of Fischer–Tropsch light α-olefins into diesel with a cetane number higher than 50  and is now part of the MtSynfuel® process of Lurgi. In MtSynfuel®, methanol is converted into light olefins and further oligomerized into middle distillates . This is an alternative to the Fischer–Tropsch synthesis step in XTL (Coal, gas or biomass-to-liquids) processes.
Solid acid alkylation over zeolites of butenes with isobutane (Alkyclean® from ABB Lummus-Albemarle ; Eurofuel® from Lurgi-Süd Chemie  and ExSact® from Exelus ) have been proposed. The zeolites claimed are large-pore zeolites like USY and Beta. The main issue with solid acid alkylation is still a relatively fast deactivation of the catalyst. This feature requires that the zeolite catalysts contain minute quantities of noble metal (Pt, Pd) because in the regeneration step, the catalyst is flushed with hydrogen to saturate the olefinic oligomers that tend to stick on the catalyst surface and deactivate the catalyst. Use of a supercritical solvent has also been proposed to regenerate USY zeolite .
Benzene reduction in reformate can be done by alkylation of benzene with ethylene or propylene into a heavier aromatic (Benzout® from ExxonMobil)  with stringent benzene specification for commercial gasoline.
Hydro-cracking of heavy aromatics into light paraffin’s that can be applied to produce high value feedstock for steamcrackers (ARINO® form Süd Chemie/Veba Oil/Linde . Heavy pyrolysis gasoline, very rich in aromatics becomes harder to blend as a gasoline component. This process consists first in a hydrogenation step to convert the aromatics into naphthenes over a nickel catalyst and subsequently the latter are converted by ring opening over a Pd/ZSM-5 into light paraffin’s (80% yield of ethane and propane). Although, ethane is an excellent feedstock for steam cracking (~80% ultimate ethylene yield), the high hydrogen requirement for the hydro-cracking step (>10 wt% on feed basis) makes the process difficult to justify economically.
Separation of C5–C11naphtha into normal-paraffin’s and into iso-paraffin’s, naphthenes and aromatics is being proposed for the optimization of feedstock for steam crackers and naphtha reformers (MaxEne® form UOP) . Normal paraffin’s are the ideal feedstock for steam crackers as they yield >20% more ethylene whereas naphthenes are the ideal feedstock for the aromatization activity in a reformer. The process is based on a simulated moving bed adsorptive separation (Sorbex® family of processes) using molecular sieves.
Separation of water from ethanol can now been done by means of LTA-zeolite containing membranes developed by Mitsui & Co [150, 151]. Conventionally, the last 5% of water in bio-ethanol at the azeotrope is removed by PSA (Pressure Swing Adsorption) over LTA zeolite beds. The use of a similar zeolite in a polymeric membrane, supported on ceramic tubes operating in VP (Vapor Permeation) mode, saves about 10% of the energy requirement for the production of fuel grade bio-ethanol and can even been used for more diluted ethanol streams.
Zeolite-based materials are now used for the removal of olefins in aromatic streams instead of clay materials (Olgone® from ExxonMobil) . This removal step is based on alkylation of aromatics with the olefins present. The new zeolite-based process reduces the solid waste by more than 85% and has a higher removal efficiency. According to the patent literature, MCM-22 is one of the most efficient zeolites for this alkylation with long-chain olefins .
7.2 Emerging Technologies Using New Feedstock for Existing end-Products
Skeletal isomerisation of butenes and pentenes can be performed over several 10-member zeolites, especially over ferrierite zeolite (Shell, Lyondell), but also ZSM-22 (Mobil/BP) and SAPO-11 (UOP). High selectivity for iso-butene and iso-pentene has been obtained [154, 155]. The ISOMPLUS® technology (ferrierite catalyst, Lyondell/CDtech) has been commercially demonstrated at the Equistar facility in Channelview on a 10000 BPD butene feed . An amorphous alumina-based catalysts using continuous catalytic regeneration (ISO-5® technology from Axens) is being used to convert Fischer–Tropsch-derived pentenes into iso-amylene for ether production at Sasol  and a 2000 BPD butenes plant has been operational at Texas Petrochemicals using a Texas/Phillips technology . Although, in the early nineties, it was believed that high amounts of iso-olefins would be required to respond to the rising demand for gasoline ethers, today with the MTBE-ban in the US, the need for these units does not exist anymore. Nevertheless, these developments could be used to perform the reverse isomerization (excess iso-olefins into linear olefins) to produce more feedstock for metathesis as proposed by ABB Lummus .
In the production of 2,6-dimethylnaphthalene (2,6-DMN, a precursor for the production of high-performance poly-ethylene-naphthenate), several acid-catalysed steps can be carried out by zeolites. In the commercially proven BP/Amoco process, a USY zeolite is used for the cyclization of the 5-o-tolyl-pentene intermediate into 2,5-dimethyltetralins, which after dehydrogenation into 2,5-dimethylnaphthalene is isomerized over a modified zeolite Beta into 2,6-dimethylnaphthalene . Recently, Polimeri announced the development of a new process that produces naphthalene from low-cost cycle oils obtained during catalytic or thermal cracking . It uses ZSM-12 in three separate process steps: (i) alkylation of naphthalene or methylnaphthalene with methanol using a benzenic solvent, (ii) isomerisation to maximize 2,6-DMN and (iii) a dealkylation of the remaining DMN and polymethylnaphthalenes using again a benzenic solvent.
The selective cracking of C4+ olefins in propylene  is now proposed by several companies (OCP® from Total Petrochemicals/UOP , Propylur® from Lurgi., PCC® from ExxonMobil , Omega® from Asahi , Superflex® from KBR  and Sinopec ). These processes use modified ZSM-5 type zeolites to provide a maximum selectivity for propylene. The reaction mechanism is based on the interconversion of olefins while hydrogen-transfer and aromatization reactions are minimized as much as possible. Depending on the degree of zeolite optimization and operating conditions a wide range of olefins can be converted. In particular OCP®, is optimized for cracking of C4 up to C10 olefins, either linear, branched or cyclic without using any diluents in the reactor .
The methylation of toluene to xylenes is proposed by GTC (in alliance with Indian Petrochemicals Corp. Ltd.) . GT-TolAlk® technology, according to patents assigned to IPCL, uses a ZSM-5 zeolite catalyst modified with gallium and selectivated by ex situ silicon impregnation . GTC claims that the p-xylene selectivity of the GT-TolAlk® process is above 85 wt%. ExxonMobil and Sabic are also actively working on toluene alkylation technology. Typical process conditions are an inlet temperature of >420 °C, molar H2/toluene ratio between 0.1 and 2, and a molar toluene-to-methanol ratio of about 4; steam is used to further increase catalyst stability. Toluene conversions are typically below 20%, with xylenes selectivity typically above 90 wt% and a p-xylene concentration in the xylenes stream of >85% .
The side-chain alkylation of toluene with methanol to make styrene in a single step has been recently announced by Exelus. It is an alternative to the two step process, involving benzene alkylation with ethylene followed by high-temperature dehydrogenation. The new process uses a novel engineered catalyst, ExSyM®, which consists of a proprietary zeolite with basic active sites in a highly optimized pore structure. By combining elements of reaction engineering with advanced catalytic composition, a new multifunctional catalytic system has been developed that allows significantly higher yields of styrene (>78%) at complete methanol conversion at 400 °C, WHSV of 3 h−1 and atmospheric pressure [171, 172].
The conversion of methanol into light olefins is nearing industrialization. Two families of technologies are proposed today: (i) those based on the SAPO-34 molecular sieve (UOP/Hydro , DCIP [174, 175], Tsinghua University , Sinopec  and ExxonMobil ) and (ii) those based on the ZSM-5 type zeolite (Lurgi , JGC  and Idemitsu ). SAPO-34 based processes convert methanol or dimethylether into both ethylene and propylene with a combined yield of 75-80% on carbon basis, whereas the ZSM-5 based processes produce mainly propylene and ethylene and heavy olefins as byproducts. The latter can be recycled over the same catalyst yielding more propylene, the byproducts being LPG and some gasoline. The ultimate yield of propylene is reported to be between 60 and 70% on carbon basis. The Total Petrochemicals/UOP OCP technology has been integrated with the UOP/Hydro MTO technology, which allows to crack the C4+ olefins, produced over the SAPO-34 catalyst into additional propylene and result in >88% yield of ethylene and propylene on carbon basis .
Methane conversion into aromatics is extensively studied since its first announcement in 1993 . Companies like Mitsubishi , Mitsui  and ExxonMobil  demonstrate a high interest in this direct conversion of methane to aromatics and hydrogen. Catalysts are bifunctional, a dehydrogenation function provided by Mo or W, and an acidic function, provided by a zeolite, mostly ZSM-5. A major challenge is minimizing coke formation. Pilot test in fluidized bed reactors have been carried out in Japan .
Catalytic naphtha cracking has been an elusive dream for many decades, however it has recently been revived by better knowledge of zeolite modifications [186, 187]. The main challenge is to adapt the zeolitic catalyst to obtain high selectivity for light olefins while minimizing hydrogen-transfer that would result in paraffin’s and aromatics. On the other hand, as these catalytic systems are prone to coke deposition at the high reaction temperature required (600–750 °C), the catalyst need to resist to hydrothermal conditions during the regeneration step where a lot of steam is produced during combustion. It is anticipated that significantly less fuel gas (mainly methane) and highly unsaturated gasoline will be produced over acidic zeolite catalysts, as the cracking occurs at a reaction temperature about 200 °C lower than in steam cracking. In addition, the catalyst circulation will provide a good heat balance, bringing therefore significant energy savings.
Technology push: such as improvements in process efficiency (selectivity directly impacted by the zeolite catalyst)
Market/regulation pull: such as regulations (on side- and by-products) and opportunities for new feedstock
8 Concluding Remarks: The Future of Zeolites in Industry and of Zeolite Research
A license to operate: to meet the latest specifications and regulations (health, safety, environment) on the products and processes and be a good corporate citizen
A license to grow: to put new products and processes on the market that should benefit society as a whole
These two licenses are both required in order having a sustainable and profitable activity.
Energy efficiency, directly linked to CO2 emissions
New specifications on products, processes and overall emissions (including catalyst reclaiming)
New fuels and lubricants to match emerging engine technologies (HCCI, CAI)
Need to process ever challenging feedstocks from more diverse origins: gas, unconventional oils, biomass derived feedstocks, coal…
So-called “softer” issues such as the perception of these activities, and the related ability of both the academic and industrial communities to attract the needed creative young students 
If one tries to translate these societal challenges in technology solutions and R&D programs, it becomes clear that zeolites will be called to contribute.
The selection of new manufacturing processes will probably have be based on additional criteria’s taking a better account of their overall footprint; in addition to the e-factor , energy efficiency and economic viability, intrinsic safety (choice of preventive over curative solutions) could well receive a veto power. This might for instance be the case for the olefin/paraffin alkylation refinery process : it dates from over 60 years, uses potentially dangerous liquid H2SO4 or HF catalysts, has been optimized ever since its commercial launch and has not yet caused any major industrial accident. The barrier of entry for new [intrinsically safer] processes, based for instance on zeolites, is therefore very high. It is further increased by the yet lower performances of emerging zeolite-based technologies, given their lack of industrial experience. If a greater weight is given to the fact that introducing a zeolite-based, or any safe solid acid catalyst reduces to almost zero the probability of a major industrial accident (for instance the HF-hydrocarbons aerosol formed by leakage from a unit could be deadly over a wide area around the refinery), opportunities will open for the emerging processes. Their performances will then benefit by climbing the commercial learning curve of any new product or process [196–198].
These formidable challenges are exciting and will need the bundling of many competences even outside of the fields of zeolite science and technology. It will be thrilling for new generations of scientists and engineers to be able to rationally design new solutions to problems such as the sustainable upgrading of biomass in environmentally benign fuels and chemicals, to reinvent processes using highly efficient catalysts and adsorbents in order to minimize energy consumption and hydrocarbon losses. It should give them the feeling they are not creating but solving problems for a better world. It will be a necessary condition to attract young students again to science and technology. It will hopefully change the perception of industrial management that R&D in general, and in particular in zeolites, is not an expense but an investment .
It is doubtful a single actor could master or generate all the necessary knowledge and know-how necessary; a new era of scientific and technical alliances will need to emerge. There will be a strong need for basic knowledge generation (the role of academia) and its application to meet new technical challenges (the role of industry) following mutually accepted and enforced rules (the role of governments). It follows that closer cooperation will have to take shape between these actors along the lines of “Open Innovation”  where the term Connect & Develop  tends to replace Research & Development. These practices are already common among innovative high-tech industries and are spreading in even more mundane applications . They should be adapted to the specific cases of oil refining and petrochemicals in order to progress further and faster in the application of incremental as well as disruptive technology changes . In order to be accepted, these practices will require an adequate general framework with a particular attention to intellectual property protection where every partner sees a just return on its investment.
Against that background, zeolites have a great track record and display all the required qualities to put them in an excellent position for further growth in both oil refining and petrochemistry.
JPG wishes to thank the St-Nikon Foundation for its continuous support over the years in the fields of zeolite and scientific management. WV expresses his appreciation to Philippe Bodart for helping the documentation and to Total Petrochemicals for allowing the preparation of this manuscript. Finally, WV thanks Marjel, Charlotte and Marie, for patience and understanding during the preparation of this manuscript.