Pseudo-Boehmite Peptization Index: How to Read HCl, Formic Acid, and HNO₃ Demand Curves

Why the Peptization Index Matters More Than Surface Area

If you buy pseudo-boehmite for FCC catalyst binder, wash-coat alumina, or alumina sol production, your incoming-quality inspection probably reads surface area (BET), pore volume, D50, and loss-on-ignition. These are all useful numbers, but none of them predict how the powder will behave when you drop it into an acid bath at the binder mill. The peptization index is the metric that bridges powder properties and process performance — get it right, and the binder gel flows, the wash-coat extrudes, and the spray-dryer makes round pellets; get it wrong, and the mill stalls, the spray nozzles plug, and the green pellet attrition rate triples overnight.

The peptization index is the volume of 0.1 N acid (typically HCl, sometimes formic acid or nitric acid) needed to drop the pH of a 1 to 5 wt% pseudo-boehmite aqueous suspension from its native pH (8 to 10) down to a target of 3.0 to 4.0. A powder that reaches pH 3.5 with 4 to 8 mL of acid disperses readily into a low-viscosity sol; a powder that needs 15 to 25 mL carries aggregated boehmite that resists sol formation. Most FCC catalyst binder specifications call for an HCl peptization index of 5 to 12 mL per g of dry AlOOH, with the sweet spot depending on the powder's surface area and intended use.

You will learn how to run the acidometric titration in your own lab, how to read the pH curve to spot over-peptized and under-peptized powders, how to pick the right acid for your application (chloride-sensitive hydroprocessing uses formic acid, not HCl), and how to compare two CoAs when the supplier numbers do not match. Five commercial pseudo-boehmite grades are compared side by side at the end so you can sanity-check your own procurement.

What the Peptization Index Actually Measures

Pseudo-boehmite (AlOOH·nH₂O with n between 0.3 and 0.5) is a fine, high-surface-area aluminum oxyhydroxide produced by neutralization of aluminum sulfate or sodium aluminate under carefully controlled pH and temperature. The particles are agglomerates of primary crystallites typically 3 to 10 nm in diameter, with internal porosity that gives a BET surface area of 200 to 400 m²/g. When you drop dry pseudo-boehmite into water, the suspension pH sits at 8 to 10 because the boehmite surface is mildly basic from surface hydroxyl groups. As you add acid, protons bind to those hydroxyl groups, the surface charge shifts from positive to neutral to slightly negative, and the agglomerates break apart into a translucent sol.

The peptization index is a measure of how much acid you need to break the agglomerates apart. Each primary crystallite has hundreds of accessible hydroxyl sites; the more crystallites per gram (i.e., the higher the surface area), the more sites there are to protonate, and the less external acid you need to push the suspension to pH 3.5. Conversely, a coarse, well-crystallized pseudo-boehmite with low surface area has fewer accessible sites and demands more acid per gram. This is why the peptization index loosely tracks surface area in inverse order, but as we will see later, the correlation is far from perfect.

The other factor that drives peptization demand is the powder's history: how it was precipitated, washed, dried, and aged. A powder neutralized at pH 9 and aged for 24 hours at 80 degrees C picks up a thin layer of carbonate from dissolved CO₂ in the wash water, and that carbonate consumes acid before the boehmite itself does. A powder spray-dried at 150 degrees C inlet temperature carries partially collapsed pore structure that traps acid inside the agglomerate and slows dispersion. The peptization index collapses all of these variables into a single number that says, in effect, "how easy is this powder to disperse?"

Anion effects on the index

The acid choice changes the absolute peptization index by 20 to 40 percent even for the same powder. Hydrochloric acid delivers the smallest peptization index because chloride is a small, fully hydrated anion that diffuses into the boehmite pore structure faster than formate or nitrate. Formic acid delivers an intermediate index; the formate anion is larger and slightly basic, which means it protonates the boehmite more gently and yields a sol with marginally higher viscosity. Nitric acid delivers the largest index; nitrate is the bulkiest of the three and is partially repelled by the slightly positive boehmite surface, slowing proton delivery. If you are comparing index numbers from two CoAs, always check which acid was used. A powder that reads "8 mL at pH 3.5" with HCl might read "11 mL" with formic acid and "13 mL" with HNO₃ — three different numbers for the same powder.

How to Run the Peptization Titration in Your Own Lab

The peptization titration is a simple bench test that takes about 30 minutes per sample and uses equipment most QC labs already own. The original method comes from Engelhard (now BASF) catalyst labs in the 1990s and has been adapted by most major FCC catalyst plants. There is no ASTM or ISO standard, so internal SOPs differ on the solids loading and the target pH. The version described below is the one we run at Aluminaworld and matches the BASF protocol to within typical lab-to-lab variation.

1. Sample prep: Dry the pseudo-boehmite at 110 degrees C for 2 hours and cool in a desiccator. Weigh 1.0 g of dry powder to the nearest 0.001 g on an analytical balance.
2. Suspension: Add the powder to a 150 mL beaker containing 100 mL of deionized water (resistivity above 18 MΩ·cm) at 25 degrees C. Stir with a magnetic bar at 300 rpm for 15 minutes.
3. pH probe calibration: Calibrate a fresh pH electrode with pH 4, 7, and 10 buffers at 25 degrees C before each batch. Record the slope and offset; if the slope is below 95 percent, replace the probe.
4. Titration: Insert the pH probe and start the stirrer. Add 0.1 N HCl (or formic, or HNO₃) in 0.5 mL increments from a 25 mL burette. Wait at least 60 seconds between additions, record pH after each addition, and continue until pH drops below 3.0.
5. Index calculation: Interpolate the cumulative mL of acid that corresponds to pH 3.5 from the titration curve. Report the result as "mL of 0.1 N acid per g of dry AlOOH at 25 degrees C, endpoint pH 3.5." Round to one decimal place.
6. Quality check: Run a blank titration on 100 mL of deionized water with the same acid. The blank should consume less than 0.2 mL of acid. If it consumes more, replace the DI water source.

The number you report is meaningless to your buyer without the metadata. Always include the acid used, its normality, the suspension solids, the temperature, and the endpoint pH on the CoA. A "peptization index of 8.5" without context is just a number; "8.5 mL of 0.1 N HCl per g AlOOH at 25 degrees C and pH 3.5 endpoint, 1 wt% suspension" is a number you can compare across labs and across suppliers.

Peptization Index Target Ranges by Application

Different applications demand different peptization windows. The table below summarizes the operating ranges in current use at FCC, hydroprocessing, and wash-coat plants. The values are industrial typicals; your specific optimum should be set by an in-plant Design of Experiments (DOE) that correlates peptization index with binder-gel viscosity, spray-dryer feed rate, and final catalyst attrition.

Comparing 5 Commercial Pseudo-Boehmite Grades

The five grades below are typical Chinese and imported pseudo-boehmite powders supplied to the FCC, hydroprocessing, and alumina-sol markets. The peptization data are lab values measured at Aluminaworld's QC lab on a single lot per grade, using the BASF acidometric method (1 wt% suspension, 0.1 N HCl, 25 degrees C, pH 3.5 endpoint). Surface area and D50 are from the supplier's CoA; loss-on-ignition (LOI) is the weight loss at 1000 degrees C for 2 hours.

The table makes the inverse surface-area relationship obvious: as SA climbs from 152 to 382 m²/g, the HCl peptization index falls from 17.6 to 4.7 mL per g. But none of these values is unusable — even the highest index, 17.6 on PB-150, can deliver a workable binder for an extruded catalyst support where you want a thick, viscous paste rather than a free-flowing sol.

How Aluminaworld uses this data

When a customer asks for a pseudo-boehmite, the first question we ask is what the application is and what acid the plant uses. The answer maps to a grade in the table above and a target peptization window. Customers who ship their binder via formic acid (which is the standard in some chloride-sensitive HDS plants) get a grade pre-tested against formic acid so the index value matches the CoA on file at the plant lab.

Case Study 1: FCC Binder-Gel Mill, 30 wt% Solids

A mid-sized Asian refinery was running an FCC binder mill at 30 wt% pseudo-boehmite solids using a PB-250 grade from its long-standing supplier. Solids loading was 5400 kg per shift and gel viscosity at 25 degrees C was 380 cP. The product zeolite was a USY (ultra-stable Y) at 28 wt% in the finished catalyst, and the spray-dryer was producing 38 tons per day of green microspheres.

In mid-2025 the supplier changed production lines and started delivering a PB-250 lot that, on the supplier's CoA, read "peptization index 11.8 mL of 0.1 N HCl per g AlOOH at pH 3.5." The customer's incoming QC confirmed the number — they titrated a sample and got 11.6 mL — but the binder mill amperage started climbing from 145 A to 192 A within two days of running the new lot. The spray-dryer feed tank showed viscosity rising from 380 cP to about 1400 cP, and the green pellet attrition rate jumped from 0.9 wt% to 4.1 wt% during the calciner step. Catalyst production dropped from 38 to 24 tons per day.

The post-mortem titration curve showed the powder dropped quickly from native pH 9.8 to pH 7.5 with the first 2 mL of acid, then crawled very slowly down to pH 3.5, requiring another 9.6 mL. The flat slope in the pH 7 to pH 4 region meant the agglomerated boehmite was buffering the acid, a classic signature of an over-aged or partially carbonated powder. Switching back to the previous supplier lot returned the binder viscosity to 380 cP and the attrition rate to 0.9 wt% within a day.

The lesson: a 1.8 mL drift in peptization index on a CoA translates to a 4× spike in green pellet attrition and a 37% drop in production. Always validate incoming lots with an internal peptization titration before releasing them to the binder mill, even when the CoA looks correct.

Case Study 2: Wash-Coat Extrusion, 20 wt% Solids

An emissions-control catalyst manufacturer in Europe used a 280 m²/g pseudo-boehmite as the wash-coat carrier for a Pd-Pt three-way catalyst. The wash-coat mill ran at 20 wt% solids, the slip viscosity target was 2400 cP at 25 degrees C, and the coating uniformity had to stay within 5% across the cordierite monolith channels. The standard pseudo-boehmite grade delivered 2400 cP at 0.65 wt% HNO₃ addition (versus dry AlOOH) at the standard titration endpoint of pH 3.5.

In late 2025 the plant tried a higher-surface-area grade (PB-380 at 382 m²/g) to push wash-coat loading above 30 wt% without increasing slurry viscosity. The PB-380 had an HCl peptization index of 4.7 mL — the lowest of the five grades — which meant less acid was needed at the mill. The plant reduced HNO₃ addition by 30% and the wash-coat went onto the monolith at 32 wt% with the same 2400 cP viscosity. Diesel light-off tests improved by 4 degrees C because the higher wash-coat loading put more Pd-Pt within the diffusion zone.

But the higher surface area created a new failure mode: the wash-coat dried on the monolith walls before the drying oven could bake it, leaving hairline mud cracks on about 3 wt% of the channels. The fix was to cut the drying rate by 15% (from 3 m/s air to 2.5 m/s air across the first 90 seconds of the dryer tunnel) and to add 0.5 wt% hydroxyethylcellulose binder. With both fixes, the PB-380 grade replaced the PB-250 with no throughput loss and a measurable activity improvement on the light-off test.

Why Peptization Index Does Not Track Surface Area Perfectly

There is a familiar pattern when comparing pseudo-boehmite CoAs side by side: high-surface-area powders often have low peptization index, but the correlation breaks for certain powder types. The reasons fall into three categories.

1. Aging history. A powder aged for 72 hours at 80 degrees C has crystallized enough that the surface area has dropped 20 to 30 percent from its post-precipitation peak. Its peptization index is also low, but for a different reason than the freshly precipitated powder. The two will behave differently in a binder mill even if their surface area is similar.
2. Carbonate contamination. Wash water with dissolved CO₂ deposits sodium carbonate and bicarbonate on the boehmite surface. Carbonate consumes acid during the titration, inflating the peptization index without affecting the surface area. A powder with 0.05 wt% sodium on a dry basis can read 4 to 6 mL higher on the index than a low-sodium powder of identical surface area.
3. Spray-drying severity. Inlet temperature above 200 degrees C partially collapses the boehmite pore structure, reducing accessible surface area. The titration sees the collapsed pore network as "harder to acidify" and reports a higher peptization index. A 350 m²/g powder spray-dried at 180 degrees C reads a different index than the same lot spray-dried at 240 degrees C.

The practical takeaway is to specify both surface area and peptization index on the CoA, and to track the relationship between the two as an internal QC trend for each supplier lot. A drift in the ratio between the two is a faster indicator of an incoming-quality problem than either metric in isolation.

Three Failure Modes and What They Look Like on the Curve

The shape of the peptization titration curve reveals more than the index number alone. Three failure modes are easy to spot from the curve.

1. Under-peptized powder (over-aged, low dispersibility)

Under-peptized pseudo-boehmite shows a high HCl index (say 14 to 25 mL per g) and a titration curve that drops quickly from native pH 9 to pH 7 in the first few milliliters, then crawls very slowly down to pH 3.5 over many more milliliters. The flat pH 5 to pH 4 plateau is the signature of an aged powder whose primary crystallites have lost accessible surface hydroxyls. In the binder mill, this powder forms a thick paste rather than a free-flowing sol and the spray nozzles plug within an hour.

2. Over-peptized powder (over-acidified in the mill)

If a binder-gel operator accidentally adds too much HCl at the mill, the bulk suspension pH can drop below 2.0. On a re-titration curve, the sample reads as if it had already received most of its peptization acid, so the formal index reads artificially low (often below 4 mL per g). The visible symptom at the plant is the same as the under-peptized case: viscosity rises sharply and the spray-dryer feed stops flowing. The fix is not more acid; it is back-titration with dilute NaOH or ammonia to bring the bulk pH back to 3.5 to 4.0.

3. Flocculated powder (carbonate or sulfate contamination)

Contaminated powder titrates to a high peptization index (15 to 30 mL per g) but the titration curve has a distinctive staircase shape rather than a smooth S. The steps occur at the pH of the buffering contaminant: a carbonate step at pH 8.3 to 6.4, a sulfate step at pH 4.5 to 3.5. A curve with two visible steps is a strong signal that the powder has been poorly washed at the source plant or has aged too long in a humid warehouse with CO₂ exposure.

Sourcing Checklist for Procurement

If you are evaluating a new pseudo-boehmite supplier or auditing an existing one, the peptization index should be on your CoA required line items list, along with the four metadata fields (acid, normality, endpoint pH, suspension solids). The minimum viable check is below.

• Request the titration curve, not just the index. A complete pH-vs-mL plot over the full titration (not just the index number) lets you confirm the curve is a smooth S without staircase steps.
• Ask which acid and which normality. 0.1 N HCl is the BASF standard; 0.1 N formic acid means you're comparing to HDS supplier data; 0.5 N HCl is faster but harder to standardize across labs.
• Confirm suspension solids. 1 wt% is the BASF standard; some plants use 5 wt% to simulate higher-solids binding gels. The two protocols give different index values for the same powder.
• Cross-check surface area vs index. If a 300 m²/g powder reports an HCl index above 15 mL per g, ask why. The most likely answer is aging, contamination, or spray-drying severity — all of which hurt your process.
• Run an independent titration in your own lab. Once a year, pull a 1 kg retention sample and re-titrate against your in-house standard. Lab-to-lab variation is typically ±10 to 15 percent on the index, so a wider gap is a red flag.

Related Pseudo-Boehmite Resources

Pseudo-boehmite is one of Aluminaworld's core products, and the peptization index is one of six specifications we test on every lot before release. See the resources below for related technical detail.

• Pseudo-Boehmite PB-Series Product Page — full PB-150 to PB-380 grade specifications, packaging, SDS.
• Catalyst Carrier Product Family — pseudo-boehmite, alumina hydrate, and boehmite powder for FCC and hydroprocessing.
• Aluminum Hydroxide (ATH) — fine and coarse ATH grades for flame retardant applications.
• Activated Alumina — gamma-alumina desiccants and catalyst supports derived from pseudo-boehmite.

Next Steps and How to Reach Us

For samples, specifications, or a full titration curve on any of the grades in the comparison table, contact the Aluminaworld technical sales team. The fastest path is the WhatsApp button at the bottom-right of this page. Email goes to technical@aluminaworld.com with the subject line "Peptization curve request — [grade]." We ship 1 kg evaluation samples within 3 working days from our Zibo warehouse, and bulk orders (5 tons minimum) leave in 7 to 15 days depending on grade and packaging. Each shipment includes a CoA with the peptization index number, the titration curve, and the BASF method metadata. If you have a non-standard requirement (different target pH, different acid, different solids loading), we can run the titration in your lab's protocol and report the result on the CoA at no charge.