Isomerization catalyst: types, preparation and limits
An isomerization catalyst rearranges the carbon skeleton of a hydrocarbon. It does not change the formula and it does not change the molecular weight. Straight-chain paraffins go in and branched paraffins come out, and the payoff is either a higher octane number or a lower pour point. The industrial grades run on platinum, on sulfated zirconia or on zeolites. None of them can beat the thermodynamic equilibrium limit of the reaction, and none of them ever will.
Temperatures in the tables below are given in degrees Celsius and pressures in megapascals.
What isomerization actually changes about a molecule
Isomerization is an internal rearrangement. Nothing comes in, nothing goes out. The same atoms get reconnected into a different skeleton, and what you end up with is an isomer of the feed: same composition, same molecular weight.
The commercial point is that physical properties move even though the formula does not. Two molecules with the same carbon number, one straight and one branched, can sit far apart on octane number, freezing point and pour point. That gap is the whole reason the reaction is run at industrial scale.
| Feed | Product | Why the product is worth more |
| n-butane | isobutane | Feeds alkylation, which makes high octane gasoline blendstock |
| n-pentane | isopentane | Raises the octane number of light gasoline |
| n-hexane | 2,2-dimethylbutane and other branched hexanes | The largest octane gain available in the C6 cut |
| meta-xylene and ortho-xylene | para-xylene | Raw material for polyester |
| Long-chain n-paraffins in wax | Branched paraffins | Lowers pour point while holding the viscosity index |
A straight-chain paraffin is a poor gasoline molecule and an awkward lubricant molecule. Branching fixes both problems, and one mechanism does the work.
What an isomerization catalyst does, and what it cannot do
Isomerization is thermodynamically allowed in the temperature ranges industry works in. So the reaction does not need a catalyst to become possible. It needs one to become fast and selective. Take the catalyst away and the rate is near zero and the selectivity is poor, which in practice means the feed either sits there or cracks.
So an isomerization catalyst does three jobs at once.
- It lowers activation energy, so the reaction runs at a usable rate under moderate conditions.
- It pushes selectivity toward the target isomer and away from cracked or polymerized junk.
- It keeps the active surface clean enough to hold that selectivity for years, not weeks.
What it cannot do is change the equilibrium composition. Isomerization is reversible, and at any given temperature and pressure there is a fixed ratio of straight-chain to branched molecules. No catalyst beats that. Every commercial design is a negotiation with the ceiling.
The four isomerization catalyst families at a glance
Everything in industrial service falls into four groups. The first two carry almost all of the installed capacity. The fourth is still being worked out.
| Family | Active phase | What it delivers | Where it struggles |
| Noble metal on an acidic support (bifunctional) | Platinum or palladium on chlorided alumina or on zeolite | Highest activity and selectivity, multi-year life | Expensive, very sensitive to sulfur, nitrogen and water |
| Solid superacid | Sulfate on zirconia, written SO4/ZrO2 | Isomerization at lower temperature without a noble metal, better water and sulfur tolerance, lower cost | Long-term stability still not good enough for long cycles |
| Metal-loaded zeolite | ZSM-5, Y or Beta zeolite with a metal component | Thermal and hydrothermal stability, tunable selectivity through pore structure | Lower activity when used alone, usually needs a metal to compete |
| Emerging materials | Ionic liquids, metal organic frameworks | Novel acidity and confinement effects | Laboratory and pilot scale only |
That table places a catalyst quickly, but it hides the thing that decides whether a unit actually performs. The preparation route matters as much as the composition. Two catalysts with the same numbers on paper can differ widely in activity, selectivity and life, depending on how the support was made, how the metal went on and how it was activated.
Noble metal bifunctional catalyst: platinum on chlorided alumina and on zeolite
This is the workhorse. A bifunctional isomerization catalyst carries two kinds of active site on one particle, and the reaction runs as a relay between them.
| Site | What it does | What happens if it is missing |
| Metal sites (platinum, palladium) | Dehydrogenates the paraffin into an olefin intermediate, then hydrogenates the rearranged olefin back | Acid sites see no olefin, so nothing rearranges and the surface cokes |
| Acid site (chlorided alumina, zeolite) | Performs the skeletal rearrangement on the olefin | The olefin has nowhere to go, and the metal has nothing to deliver |
The relay explains a few things that would otherwise look arbitrary. It explains why a hydrogen atmosphere is standard: the metal cycle needs hydrogen, and the acid sites need to stay free of coke precursors. It explains why the catalyst is so vulnerable to sulfur, because sulfur binds the metal and stops the relay at its first step. And it explains why chloride content matters so much on the alumina grade, because chloride is what creates the acid sites in the first place.
What you get for the money is high activity per unit of reactor volume, good selectivity toward the branched isomer, and a service life measured in years. What you pay is the platinum inventory and a low tolerance for feed impurities.
Solid superacid catalyst: sulfate on zirconia
A solid superacid catalyst gets its acidity a different way. Zirconium hydroxide is precipitated, treated with a sulfate source, then calcined to form tetragonal zirconia with strongly acidic sites on the surface. The classic example is sulfate on zirconia.
Three things make this family worth considering. It isomerizes at lower temperature, which walks the reaction toward the favorable side of the equilibrium. It does not need a noble metal, which takes most of the catalyst cost out. And it tolerates water and sulfur better than the chlorided alumina grade, which simplifies what has to happen upstream.
The weakness is durability. Acidity and surface area in this system both hang on a narrow calcination window, so the material loses activity over a long cycle more readily than a metal-loaded zeolite does. That is why sulfated zirconia catalyst systems show up more often where cycles are shorter or the feed is genuinely difficult.
Zeolite isomerization catalyst: shape selectivity and the silicon to aluminum ratio
A zeolite isomerization catalyst works inside pores. The crystalline framework offers acid sites that you can only reach through channels of a fixed size and shape, so the catalyst can tell apart molecules that differ only in geometry. Beta zeolite and ZSM-5 zeolite are the common frameworks, and Y zeolite appears where the pore structure suits the feed.
There are two design levers. The silicon to aluminum ratio (Si/Al) sets both the density and the strength of the acid sites, and isomerization grades generally sit in a mid range where activity and selectivity balance out. Pore size and channel connectivity decide which isomers can form and which ones can get out.
On the plus side, thermal stability, hydrothermal stability and the ability to tune selectivity by changing the framework rather than the chemistry. On the minus side, intrinsic activity. Used alone a zeolite is usually less active than a platinum bifunctional catalyst, so in practice it gets combined with a metal component or used where the feed is clean enough that the extra activity is not needed.
Ionic liquids and metal organic frameworks: still pre-commercial
Ionic liquids and metal organic frameworks have both been tried for isomerization, mostly because they offer acidity and confinement effects that conventional solids cannot reproduce. Neither has reached general industrial service. The obstacles are cost per unit of catalyst volume, the difficulty of separating and regenerating a liquid phase in a continuous process, and the mechanical and thermal durability a fixed bed is expected to survive. Treat this group as a research direction, not as an option you can specify today.
Industrial applications: four processes built on the same catalyst family
Light naphtha isomerization, usually called C5/C6 isomerization, is the largest application by installed capacity. Refiners take the light straight-run cut and convert n-pentane and n-hexane into isopentane, 2,2-dimethylbutane and the other branched hexanes. What comes out is a clean, high octane gasoline blendstock with no aromatics and no sulfur, which happens to be exactly what modern gasoline specifications reward. Commercial units include the UOP Penex process and the Axens isomerization process, both using either platinum on chlorided alumina or platinum on a zeolite.
n-butane isomerization is the same chemistry aimed at a different product. Isobutane is the feed to alkylation, where it combines with light olefins to make high octane alkylate. A refiner runs butane isomerization when the alkylation unit is short of isobutane, which makes the process a balancing lever rather than a profit center in its own right.
Xylene isomerization sits in the aromatics complex, where para-xylene production sets the economics. Meta-xylene and ortho-xylene are isomerized into para-xylene, the raw material for polyester. The reaction runs over a platinum on zeolite bifunctional catalyst in a hydrogen atmosphere. The equilibrium mixture never contains enough para-xylene to satisfy demand, so the reactor is always paired with a separation step that sends the other isomers back to the feed.
Lube base oil isomerization applies the same idea to heavy molecules. Waxy long-chain n-paraffins become branched paraffins so the oil stops crystallizing at low temperature. The target is a lower pour point without giving up viscosity index, and the difficulty is that the cracking side reaction gets much likelier as the molecule gets longer.
| Application | Feed | Target product | Catalyst family | The complication |
| C5/C6 light naphtha isomerization | n-pentane, n-hexane | Isopentane, 2,2-dimethylbutane | Platinum on chlorided alumina or zeolite | Feed must be dry and sulfur free |
| n-butane isomerization | n-butane | Isobutane | Platinum on chlorided alumina or zeolite | Recycle loop is large, equilibrium is unfavorable |
| Xylene isomerization | Meta-xylene, ortho-xylene | Para-xylene | Platinum on zeolite | Needs hydrogen and a coupled separation train |
| Long-chain isomerization | Waxy n-paraffins | Branched paraffins for base oil | Platinum on zeolite with strong metal function | Cracking must be suppressed, not just minimized |
How an isomerization catalyst is made: what actually decides performance
The formulation is the starting point, not the answer. The same active phase on the same support can produce a catalyst that runs for five years or one that loses selectivity in a month. When that happens, the difference almost always traces back to preparation. The sections below follow the platinum on chlorided alumina route because it is the most demanding, then cover the two routes that need different controls.
Support selection and pretreatment
The support is not a passive carrier. Its pore structure decides how fast molecules reach the active sites. Its surface chemistry decides how the metal precursor binds. Its impurities decide how long the acid sites survive.
| Control | Typical window | What it decides |
| Crystal phase | gamma alumina, the target phase | Surface area and moderate acidity; gamma alumina gives 150 to 250 square meters per gram |
| Calcination temperature | 450 to 600 C | Holding this range prevents collapse into alpha alumina, which has far less surface area |
| Pore structure | Mesopore fraction and pore size distribution tuned through precipitation pH, temperature, aging time and pore expanders | Access of reactant molecules to active sites, and how easily products diffuse out |
| Impurity level | Na2O below 100 ppm | Sodium and iron poison acid sites or interact badly with the noble metal, so the support is washed thoroughly |
Crystal phase control is the one that is easiest to lose. Overshoot the calcination temperature and the support converts to a low surface area phase. No amount of care later in the process puts that area back.
Metal loading, drying and calcination
Catalyst impregnation is where the metal inventory is set, and it is also where the distribution pattern is decided. Two routes are in use.
Incipient wetness impregnation measures the pore volume of the support first, then prepares a chloroplatinic acid (H2PtCl6) solution at a concentration that exactly fills those pores. The advantage is precise control of loading. The requirement is an accurate pore volume measurement and a homogeneous solution.
Excess impregnation immerses the support in more solution than it can absorb and relies on adsorption equilibrium. Convenient in a laboratory, rarely used for precise noble metal loading in production.
After that, three variables matter.
| Variable | Typical window | Effect on the catalyst |
| Impregnation solution pH | 2 to 4 | Too low distributes platinum unevenly, too high dissolves metal off the support |
| Competing adsorbate | Hydrochloric acid or citric acid added as a competing ion | Slows precursor adsorption so platinum spreads evenly through the particle instead of concentrating in a shell or a core |
| Impregnation time and temperature | 2 to 8 hours at room temperature | Insufficient time leaves the adsorption balance unfinished |
| Drying ramp | 1 to 5 C per minute | Fast drying drags the precursor through the pores and concentrates it, destroying uniformity |
| Calcination window | 400 to 550 C for 2 to 6 hours in air or oxygen-rich gas | Below the window the precursor does not decompose; above it platinum particles pre-sinter |
| Two-stage calcination | 200 to 300 C first, then the higher hold | Decomposes the precursor gently before the structure is fixed |
Platinum distribution gets this much attention for one reason: cost efficiency. Put the platinum in a shell at the outer edge of each extrudate and you waste metal on surface that fouls first. Bury it in the core and you leave the reaction waiting to diffuse in. Uniform distribution through the particle is the target, and the competing adsorbate is the main tool for reaching it.
Chlorination: setting the acid strength
Catalyst chlorination is the step that creates the acidity on the alumina surface in the first place. The calcined material is treated so chlorine bonds chemically to the alumina and forms strong acid sites.
There are two routes. Gas phase chlorination treats the catalyst at 300 to 500 C in a mixture such as hydrogen chloride, chlorine and nitrogen. Liquid phase chlorination impregnates with a chloride solution, then dries and calcines again.
Total chlorine content governs the catalyst, and it has a real optimum.
| Chlorine content | Consequence |
| Below the window | Not acidic enough, so isomerization activity is poor |
| 4 to 7 weight percent | The industrial target range |
| Above the window | Acid strength is excessive, cracking accelerates and liquid yield falls |
Uniformity matters as much as quantity. Uneven chlorination gives you a bed with one region over-acidic and another under-active, which shows up as local hot spots and incomplete conversion. Time, gas concentration and temperature are the three levers that hold both the level and the spread.
Reduction and activation: getting to metallic platinum
After calcination the platinum sits on the support as an oxide, and in that state it does not hydrogenate or dehydrogenate anything. Catalyst reduction in hydrogen converts it back to metallic platinum. How that reduction is run decides the platinum dispersion.
| Control | Typical window | Failure mode if it is wrong |
| Temperature program | Ramp to 350 to 450 C and hold for 2 to 4 hours in pure hydrogen or a hydrogen and nitrogen mixture | A fast ramp concentrates the exotherm, and the heat sinters platinum particles |
| Hydrogen partial pressure and space velocity | Moderate values | Too little hydrogen leaves part of the metal unreduced and the initial activity low |
| End point detection | Water content or hydrogen consumption in the tail gas | Without a measured end point, over-reduction or incomplete reduction goes undetected |
| Target dispersion | Above 80 percent | Larger particles mean less active metal surface for the same platinum inventory |
Under-reduce and the catalyst starts weak and never reaches its design activity. Over-reduce and you do a different kind of damage: the metal sinters, dispersion falls, and because platinum is the most expensive part of the catalyst, the loss is permanent.
Preparing a solid superacid catalyst
The sulfated zirconia route needs different controls, because acidity comes from the sulfate group rather than from a halogen sitting on a support.
| Stage | Typical condition | Why it matters |
| Precipitation and aging of Zr(OH)4 | pH 9 to 11, controlled aging time | Sets the surface area of the amorphous precursor |
| Sulfation | Sulfuric acid or ammonium sulfate impregnation, targeting 2 to 5 weight percent sulfur | Both too little and too much sulfate reduce superacid strength |
| Calcination | 550 to 650 C | Forms tetragonal zirconia and generates the superacid sites; below the window acidity is too weak, above it the tetragonal phase converts to monoclinic, surface area collapses and activity is lost |
That calcination window is unusually narrow next to the alumina route, and the narrowness is the single largest source of batch to batch variation in this family.
Preparing a zeolite isomerization catalyst
For zeolite grades the framework has to be right before any metal goes near it.
| Control | Typical target | Effect |
| SiO2 to Al2O3 ratio | 10 to 50 for isomerization grades | Sets acid site density and acid strength; controlled through the synthesis gel composition |
| Template selection and removal | Organic template such as tetrapropylammonium hydroxide for ZSM-5, removed by slow calcination at 550 to 600 C | Determines crystal morphology and pore structure; fast removal can collapse the framework |
| Crystallization conditions | Temperature, time and stirring rate | Controls crystallinity and crystal size; large crystals slow diffusion, small ones lose thermal stability |
Shaping, strength and quality control
A finished catalyst has to survive being loaded into a reactor and then sit there under load for years. The powder that came out of the previous steps is not yet a product.
Shaping turns it into extrudates, trilobes or spheres so the bed gets an acceptable pressure drop and decent mass transfer. The powder is mixed with a binder such as pseudo-boehmite and nitric acid, and the water to powder ratio and kneading time are held so the paste stays extrudable. The shaped body is then dried and calcined on a controlled ramp so it does not crack.
Mechanical strength is specified rather than hoped for. Industrial grades are expected to hold a side crush strength above 100 newtons per centimeter, so they do not break during loading or thermal cycling.
Then every batch is tested before release.
| Test | What it tells you |
| Surface area by BET, pore volume, pore size distribution | Whether the support structure survived preparation |
| Side crush strength, bulk density, particle size distribution | Whether the shaped body meets the mechanical specification |
| Platinum content, chlorine content, impurity levels including sodium, iron and sulfur | Whether the formulation matches the design |
| Model reaction activity test, typically n-hexane isomerization | Conversion, selectivity, liquid yield and octane gain under standard conditions |
| Accelerated aging on selected batches | A first estimate of service life |
The activity test is the one that decides release. A batch can pass every physical and chemical specification and still underperform in the model reaction. That batch does not ship.
What limits an isomerization catalyst in service
Industrial isomerization is mature technology, and the limits are known well enough to be designed around. They still set the operating cost.
| Limit | Why it exists | What it costs the operator |
| Feed sensitivity | Sulfur at parts per million levels poisons the metal, water disrupts the chloride balance on alumina, aromatics and olefins form coke on acid sites | A deep desulfurization unit and a hydrotreater ahead of the reactor, plus the risk that a slip deactivates the bed within hours to days |
| Chloride loss and corrosion | Chlorided alumina needs continuous injection of a small amount of organic chloride, and the chloride ends up as hydrochloric acid downstream | Corrosion resistant alloys for exchangers, separators and piping, plus treatment of chloride-bearing effluent and vent gas |
| Thermodynamic equilibrium | The reversible reaction has a temperature dependent ceiling on conversion | Recycle of unconverted feed, which adds separation energy and equipment |
| Catalyst life and regeneration economics | Coke accumulates, poisons adsorb, chloride leaves and platinum sinters | A planned shutdown every few years, and regeneration that returns only part of the original activity |
| Hydrogen dependence | Hydrogen is needed to suppress coke, hold the metal reduced and complete the hydrogenation half of the relay | Consumption of a valuable refinery stream, or the cost of a dedicated hydrogen supply |
| The low temperature conflict | Low temperature favors the equilibrium for branched isomers but starves the catalyst of activity; a stronger acid restores activity and accelerates cracking | A deliberate compromise on single-pass conversion, usually 30 to 40 percent |
That last limit shapes the whole technology. C5/C6 isomerization units run at 120 to 180 C because that is where an active catalyst and a favorable equilibrium overlap. Push the temperature down toward 100 C and the equilibrium conversion would rise, but it would take acidity strong enough that cracking dominates and liquid yield collapses.
Kinetics against thermodynamics. That tension is the central problem in isomerization catalyst development, and nobody has solved it. It gets managed.
Safety: hydrogen, pyrophoric catalyst and chlorides
An isomerization unit concentrates several hazards in one place. Pressure is typically 1.5 to 4.0 MPa under hydrogen. The catalyst is hazardous to handle once it is spent. And the chloride that keeps the catalyst working is the same chloride that threatens the equipment and the people around it.
| Hazard | Source | Control |
| Hydrogen fire and explosion | Wide flammability range of 4 to 75 percent in air, very low ignition energy, and a flame that is nearly invisible | Leak detection at flanges, exchanger tubes and compressor seals; rapid isolation; inerting of confined spaces |
| Spent catalyst self-ignition | Coke and adsorbed hydrogen on the surface react when the bed is exposed to air during unloading | Passivation with nitrogen, steam or slow controlled oxidation before the reactor is opened |
| Temperature runaway during regeneration | Burn-off is a controlled combustion, and uncontrolled temperature rise sinters the catalyst and can damage the reactor | Tight control of oxygen content, temperature and flow during the burn |
| Chloride and acid exposure | Organic chlorides are toxic, some are classified as possible carcinogens, and chloride can convert to phosgene at high temperature; hydrochloric acid in the condensate corrodes carbon steel | Closed handling, alkali scrubbing, protective equipment, and inspection for wall thinning |
| Process runaway from the reaction itself | Isomerization is exothermic, so an olefin or aromatic surge increases heat release on the acid sites, and loss of recycle hydrogen promotes coking and a rapid temperature rise | Feed composition limits, reliable recycle gas compression, and bed temperature monitoring at multiple depths |
| Hydrocarbon and vapor release | Light hydrocarbons form flammable vapor clouds and displace oxygen in confined spaces | Gas detection, ventilation and controlled drainage |
Catalyst unloading deserves its own warning. A spent bed opened to air without passivation can ignite. Regeneration run with too much oxygen or too little temperature control can do the same thing at higher temperature. Neither risk is theoretical, and both are avoidable by procedure.
Catalyst deactivation and regeneration
Four mechanisms account for nearly all activity loss, and they each need a different response: coke, sulfur poisoning, acid site loss and platinum sintering.
| Mechanism | How it shows up | How it is addressed |
| Coke deposition | Gradual loss of activity, highest at the bed inlet | Controlled burn-off in oxygen, followed by reduction |
| Poison adsorption | Sharp activity loss after a feed upset, sulfur, nitrogen, water or metals | Feed pretreatment, and replacement where the poisoning is irreversible |
| Acid site loss | Progressive loss of isomerization selectivity on chlorided alumina | Rechlorination to restore the chlorine content and the acid site density |
| Metal sintering | Lower dispersion with no obvious change in the chemical analysis | Limited recovery; the metal has already agglomerated |
Regeneration practice follows from that. Burn-off removes coke but also oxidizes the metal, so it is always followed by reduction in hydrogen. Chlorided alumina needs the extra rechlorination step to bring the acidity back. Zeolite grades are simpler because they carry no halogen.
The economics are less attractive than they look. Two to five years is a typical life, but feed quality and operating discipline move that number a long way. Regenerated catalyst usually returns to 80 to 95 percent of fresh activity, not all of it. After several cycles the gap makes replacement the better call, and platinum recovery value only partly offsets the bill.
Where isomerization catalyst development is heading
| Direction | Driving force | What has to be solved |
| Low temperature, high activity | Lower temperature cuts cracking and raises liquid yield | Higher intrinsic activity without a stronger acid that cracks the feed |
| Impurity tolerance | Sulfur, nitrogen and water resistance would remove pretreatment equipment | Acid and metal sites that survive contaminants |
| Chloride-free catalyst | Removes corrosion, chloride effluent and the alloying cost downstream | Matching the activity and selectivity of chlorided alumina |
| Multifunctional catalyst | Coupling isomerization with desulfurization or denitrogenation in one bed | Keeping each function active in the other’s operating window |
| Smart preparation | Online analysis, high throughput screening and machine learning to cut batch to batch variation | Data and models that transfer from laboratory to production scale |
The chloride-free catalyst carries the most commercial pressure, because removing chloride would delete an entire class of corrosion, effluent and equipment cost at once. It also happens to be the hardest, because chloride is currently doing real work in the best performing catalyst in the industry.
Bottom line
An isomerization catalyst looks unremarkable and does something that matters. It reshapes molecules without changing them, and it decides how much of a refinery’s light naphtha becomes high octane blendstock, or how much paraffin becomes base oil.
Two things decide whether a given catalyst delivers. The first is preparation, where support phase, metal distribution, chlorine level and reduction profile each have a working window and each can be missed. The second is the feed. A catalyst that performs beautifully on dry, sulfur-free feed will fail on a stream carrying parts per million of sulfur and a trace of water.
What is left after that is thermodynamic and economic rather than catalytic. Equilibrium caps single-pass conversion. Chloride forces a corrosion allowance. Hydrogen has to be available. Regeneration never quite restores what the catalyst started with. Progress here is incremental, and it consists mostly of moving those limits rather than removing them.
FAQ
What is an isomerization catalyst?
An isomerization catalyst rearranges the carbon skeleton of a hydrocarbon molecule without adding or removing carbon, so a straight-chain paraffin becomes a branched isomer of the same molecular weight. Industrial grades are bifunctional, combining a platinum or palladium metal site with an acidic support, and they also suppress the cracking and coking reactions that would otherwise compete with isomerization.
Which isomerization catalyst is used most widely in industry?
Platinum on chlorided alumina is the workhorse, with platinum on zeolite close behind it. Both are bifunctional. The chlorided alumina version is more active and more selective, which is why light naphtha units still run it, but it demands dry, sulfur-free feed and continuous chloride injection. Zeolite grades trade some activity for better tolerance and simpler operation.
Why does a bifunctional isomerization catalyst need two different sites?
Alkanes do not rearrange on acid sites directly. The metal site dehydrogenates the paraffin into an olefin intermediate, the acid site rearranges that olefin’s skeleton, and the metal site hydrogenates it back. Remove the metal and the acid sites coke; remove the acid and nothing rearranges. The relay between the two kinds of site is the entire mechanism.
What chlorine content should a chlorided alumina isomerization catalyst hold?
Typically 4 to 7 weight percent. Chlorine sets both the acid strength and the acid site density. Below that window the catalyst is not acidic enough to isomerize and conversion falls. Above it cracking takes over, liquid yield drops, and the excess chloride has to be scrubbed downstream. Chlorination time, gas concentration and temperature are the three levers that hold the window.
Why is sulfur so damaging to an isomerization catalyst?
Sulfur adsorbs on the platinum sites and blocks the dehydrogenation step, and a few parts per million in the feed are enough to do it. The poisoning is largely irreversible on chlorided alumina, so units upstream of the reactor run deep desulfurization. If a desulfurizer slips, the catalyst can lose most of its activity within hours to days, and regeneration rarely restores it fully.
What are the four main types of isomerization catalyst?
Bifunctional platinum on chlorided alumina or on zeolite, solid superacids such as sulfated zirconia, metal-loaded zeolites used for shape selective work, and emerging materials including ionic liquids and metal organic frameworks. The first two carry nearly all industrial service. The last group is still at laboratory or pilot scale.
Why do C5/C6 isomerization units run at 120 to 180 C?
Because equilibrium favors branched isomers at low temperature while kinetics demand high temperature, and the reactor sits in the overlap. At 120 to 180 C a chlorided alumina catalyst is active enough to work and single-pass conversion lands around 30 to 40 percent. Running lower would need a stronger acid, which cracks more of the feed.
How long does an isomerization catalyst last?
Two to five years, and the spread is set mostly by feed quality and operating discipline rather than by the catalyst itself. Coke accumulates on the acid sites, sulfur and water poison them, chloride gradually leaves the surface, and platinum crystals grow. Regeneration by controlled burn-off, rechlorination and reduction restores 80 to 95 percent of fresh activity, not all of it.
Can a zeolite isomerization catalyst replace chlorided alumina?
Partly. Zeolites are thermally stable, tolerate water and sulfur better, and need no chloride injection, which removes the corrosion problem. They are also less active per unit volume, so more catalyst or a higher temperature is needed. Where feed quality can be controlled, chlorided alumina still wins on selectivity. Where it cannot, zeolite is the safer choice.
What is the difference between C5/C6 isomerization and xylene isomerization?
C5/C6 isomerization upgrades light naphtha by converting n-pentane and n-hexane into branched isomers that raise the octane number of the gasoline pool. Xylene isomerization converts meta-xylene and ortho-xylene into para-xylene, the feedstock for polyester. Both use bifunctional catalysts, but xylene units run under hydrogen and need a coupled separation train.
Why is spent isomerization catalyst dangerous to handle?
The spent material carries coke and adsorbed hydrogen, so it can self-ignite in air during unloading. Before anyone opens the reactor, the bed is passivated with nitrogen, steam or slow controlled oxidation. Regeneration needs the same care, because burn-off is a controlled combustion and a temperature excursion can sinter the platinum or damage the reactor internals.
Will chloride-free isomerization catalysts replace chlorided alumina?
The direction is clear, but the replacement is not complete. Chlorided alumina delivers the best combination of activity and selectivity for light naphtha, and chloride-free alternatives still give up either conversion or yield. The pressure to switch comes from corrosion, chloride-bearing effluent and the cost of alloying downstream equipment, not from catalyst performance alone.
