MOF trial

LAB HANDOUT (Beginner SOP)

Synthesis of Zn–Histidine Bio-MOF (Real Metal–Organic Framework / Coordination Framework)

Target output: ≥ 4 g (by combining 2 identical batches)  |  Recommended level: MSc Nanoscience project  |  Balance: 4-digit (0.0001 g) OK

1) Objective

To synthesize a real MOF / coordination framework using Zn²⁺ (metal node) and L-histidine (organic linker) under controlled pH. The key is to keep pH around 6.5 so that we form a coordination network (MOF) and avoid Zn(OH)₂ / ZnO formation.

2) Safety (Mandatory)

• Wear lab coat, gloves, and goggles throughout.
• NaOH is corrosive—add slowly; wash spills immediately with plenty of water.
• Autoclave safety: do not fill above 80%; do not open while hot/pressurized; cool naturally.
• Dispose washings as instructed (do not pour chemicals into sink unless permitted).

3) Chemicals & Equipment

Chemicals (from your stock)

• Zinc chloride (ZnCl₂) OR Zinc acetate dihydrate
• L-Histidine hydrochloride (check bottle: HCl or HCl·H₂O)
• Sodium hydroxide (NaOH)
• Deionised/Distilled water
• 2-propanol (for washing; recommended)
• pH paper

Equipment

• Electronic balance (4-digit)
• 2 × 250 mL beakers
• Magnetic stirrer + stir bar
• Measuring cylinder (10/50/100 mL)
• Funnel + filter paper (or centrifuge)
• Teflon-lined autoclave (50–100 mL preferred)
• Hot air oven (60 °C)
• Petri dish / watch glass

4) Batch Plan to Reach ≥ 4 g

Run two identical batches at 10 mmol scale and combine dried powders: Batch-1 (10 mmol) + Batch-2 (10 mmol) → total typically ≥ 4 g. If output is less, run a third batch and combine.

5) Quantities (Per Batch = 10 mmol)

Important: Your histidine bottle may read L-histidine HCl or L-histidine HCl·H₂O. Use the correct mass below. If unsure, read the exact name/MW on the bottle label.

Option A (Preferred): ZnCl₂ Route

Item	Quantity (per 10 mmol batch)	Notes
ZnCl2	1.363 g	Metal source (Zn2+)
L-Histidine HCl	1.916 g	If bottle says L-histidine HCl
L-Histidine HCl·H2O	2.096 g	If bottle says monohydrate (HCl·H2O)
DI water	80 mL total	40 mL for Zn solution + 40 mL for histidine solution

Option B: Zinc Acetate Dihydrate Route

Item	Quantity (per 10 mmol batch)	Notes
Zinc acetate dihydrate	2.195 g	Metal source (Zn2+)
L-Histidine HCl	1.916 g	Or 2.096 g if monohydrate
DI water	80 mL total	40 mL + 40 mL

6) Prepare 0.1 M NaOH (One-time preparation; use for both batches)

1. Weigh 0.40 g NaOH (pellets/flakes).
2. Dissolve in ~70 mL DI water (beaker/bottle).
3. Make final volume to 100 mL with DI water.
4. Label: “0.1 M NaOH, date, prepared by”.

Why dilute NaOH? Beginner-friendly pH control; prevents overshooting pH.

7) Step-by-Step Synthesis (Batch-1)

Step 1 — Prepare Zn solution

1. Take a clean 250 mL beaker.
2. Add 40 mL DI water.
3. Add Zn salt (Option A or B quantity above).
4. Stir until fully dissolved (solution should become clear).

Step 2 — Prepare histidine solution

1. Take another clean 250 mL beaker.
2. Add 40 mL DI water.
3. Add histidine salt mass (1.916 g or 2.096 g depending on bottle).
4. Stir until fully dissolved.

Step 3 — Mix

1. Under stirring, pour the histidine solution slowly into the Zn solution.
2. Continue stirring for 10 minutes.

Step 4 — pH adjustment (MOST IMPORTANT)

1. Measure pH using pH paper.
2. Add 0.1 M NaOH dropwise (1–2 drops at a time) with continuous stirring.
3. Stop when pH reaches 6.5 (acceptable range: 6.2–6.8).

Correct observation: solution stays clear or slightly cloudy.

Wrong observation: thick white gel forms quickly → pH too high (Zn(OH)₂).
Fix: restart the batch and add NaOH much more slowly; do not exceed pH 6.8.

Step 5 — Hydrothermal crystallization (MOF formation)

1. Transfer mixture into the Teflon liner.
2. Do not fill above 80% of liner volume.
3. Seal autoclave properly.
4. Heat at 120 °C for 12 hours.
5. Cool naturally to room temperature (do not force cool).

Step 6 — Collect and wash

1. Collect solid by filtration (or centrifuge).
2. Wash with DI water: 3 × 20 mL.
3. Wash with 2-propanol: 2 × 20 mL (recommended for faster drying).

Step 7 — Dry and record yield

1. Dry in hot air oven at 60 °C for 6–8 hours.
2. Weigh dried powder and record mass.
3. Label sample vial: “Zn–Histidine Bio-MOF | Batch-1 | pH | 120°C 12h | date | name”.

8) Batch-2 (Repeat Identically)

Repeat the entire procedure using the same quantities. After drying, combine Batch-1 and Batch-2 powders to reach total ≥ 4 g.

9) Quick Confirmation Tests (After Synthesis)

• XRD (main proof): new crystalline peaks; should not match ZnO peaks.
• FTIR: shifts in carboxylate and imidazole bands compared to pure histidine.
• SEM: crystal-like morphology.
• Optional dye test: Add 10 mg MOF to 10 mL methylene blue solution, shake 1 h, observe color reduction vs blank.

10) Common Mistakes & Fix

Issue	Likely cause	Fix
Instant thick white precipitate/gel	pH too high → Zn(OH)2	Restart; add 0.1 M NaOH slower; stop at pH 6.5 (max 6.8)
No solid/crystals after heating	pH too low or poor coordination	Ensure pH 6.2–6.8; keep correct masses; extend time to 18 h if needed
XRD shows ZnO pattern	Too basic or conditions favor oxide formation	Lower pH (closer to 6.3–6.5); avoid excess NaOH; keep 120°C/12h

Supervisor note: Record pH, temperature, time, and mass for every batch. These are essential for reproducibility.