Zeolite 5A molecular sieve 5A

spore: profile
id: zeolite-5a
spore_version: '0.2'
family: oxide-ceramics
name: Zeolite 5A molecular sieve
abbreviation: 5A
role: CO2 and water sorbent (molecular sieve)
category: ceramics
type: oxides
summary: Zeolite 5A is the calcium form of zeolite A, a synthetic aluminosilicate with pores of 5 angstroms (Zeochem). It is the CO2 sorbent in the ISS Carbon Dioxide Removal Assembly, whose beds are designed to operate at 204 °C (NASA). NASA's hydrothermal tests show that humid thermal cycling breaks the pellets into dust, and the RK-38 product broke down fastest.
best_for: Regenerable CO2 removal in thermal-swing beds such as the ISS CDRA, where the design operating temperature is 204 °C (NASA)
weakest_at: Dust from pellet breakdown under humid thermal cycling; in NASA's test the RK-38 bed pressure drop rose 74.5 % per 1000 hours at 1.38 mmHg inlet water vapour
identity:
  formula: Calcium-exchanged zeolite A, a crystalline aluminosilicate (CaA, the 5A form)
  source: nasa-cdra-2017
  where: Table 2 lists the 5Å products as CaA zeolite (UOP RK-38, UOP ASRT, Grace MS 522, BASF 5A)
  designations:
    - system: Trade name
      code: UOP RK-38
      note: 5A product used in the ISS CDRA
      source: nasa-cdra-2017
      where: Table 2, row UOP RK-38 (5Å, CaA zeolite, used in CDRA)
    - system: Trade name
      code: UOP ASRT
      note: older 5A pellet product, no longer produced
      source: nasa-cdra-2017
      where: Table 2, row UOP ASRT 1995 and ASRT 2005; text says ASRT can no longer be produced
    - system: Trade name
      code: Grace MS 522
      note: 5A bead product
      source: nasa-cdra-2017
      where: Table 2, row Grace MS 522 (5Å, CaA zeolite)
    - system: Trade name
      code: BASF 5A
      note: 5A bead product
      source: nasa-cdra-2017
      where: Table 2, row BASF 5A (5Å, CaA zeolite)
    - system: Trade name
      code: 5A Purmol Powder
      note: manufacturer powder, calcium form
      source: zeochem-molecular-sieves
      where: Molecular sieves page, 5A Purmol Powder entry
tags:
  environments:
    - low-earth-orbit
  stresses:
    - thermal-cycling
  applications:
    - habitats-and-suits
properties:
  - benchmark: pore_diameter
    label: Pore size of calcium-form 5A powder
    value: 5
    unit: angstroms
    scope: material
    conditions:
      form: 5A Purmol Powder, calcium (Ca) form
      basis: nominal pore size printed by the manufacturer
    method: Manufacturer product-page statement of nominal pore size
    status: ground
    source: zeochem-molecular-sieves
    where: 5A Purmol Powder entry on the molecular sieves page
  - benchmark: operating_temperature
    label: Design operating temperature of the ISS CDRA CO2 removal beds
    value: 204
    unit: °C
    scope: material
    conditions:
      basis: design operating temperature quoted as 400°F (204°C) for the on-orbit CDRA CO2 beds
      configuration: uniform packed beds of 5A zeolite with an embedded heating element
    method: Design value stated in the NASA paper; flight hardware specification, not a measured test result
    status: flight
    source: nasa-cdra-2017
    where: Section I (Introduction), paragraph after the Figure 1 caption
  - benchmark: hydrothermal_test_temperature
    label: Sorbent set point in the hydrothermal stability test
    value: 204.4
    unit: °C
    scope: material
    conditions:
      segment: Table 2 steps 2 and 3, sorbent temperature
      cycle: thermal profile matched to ISS CDRA ramp rates
    method: Set point of the NASA Hydrothermal Stability Test thermal profile
    status: ground
    source: nasa-hst-2015
    where: Table 2 (HST Sorbent Temperature Profile), steps 2 and 3
  - benchmark: heating_ramp_rate
    label: Heating ramp to the test set point
    value: 2.84
    unit: °C/min
    scope: material
    conditions:
      segment: step 2 of the HST profile, heating to 204.4 °C
    method: Ramp rate set in the NASA Hydrothermal Stability Test profile
    status: ground
    source: nasa-hst-2015
    where: Table 2, step 2, Ramp Rate column
  - benchmark: rk38_pressure_increase_low
    label: UOP RK-38 bed pressure drop increase, low inlet water vapour
    value: 15
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP RK-38 (5A)
      inlet_h2o_pp: 0.012 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, RK-38 Bed Pressure Drop, row 0.012, column % /1000 Hours
  - benchmark: rk38_pressure_increase_mid
    label: UOP RK-38 bed pressure drop increase, mid inlet water vapour
    value: 62.8
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP RK-38 (5A)
      inlet_h2o_pp: 0.70 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, RK-38 Bed Pressure Drop, row 0.70, column % /1000 Hours
  - benchmark: rk38_pressure_increase_high
    label: UOP RK-38 bed pressure drop increase, high inlet water vapour
    value: 74.5
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP RK-38 (5A)
      inlet_h2o_pp: 1.38 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, RK-38 Bed Pressure Drop, row 1.38, column % /1000 Hours
  - benchmark: asrt_pressure_increase_low
    label: UOP ASRT (5A) bed pressure drop increase, low inlet water vapour
    value: 5.64
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP ASRT (5A)
      inlet_h2o_pp: 0.012 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, ASRT Bed Pressure Drop, row 0.012, column % /1000 Hours
  - benchmark: asrt_pressure_increase_mid
    label: UOP ASRT (5A) bed pressure drop increase, mid inlet water vapour
    value: 9.49
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP ASRT (5A)
      inlet_h2o_pp: 0.70 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, ASRT Bed Pressure Drop, row 0.70, column % /1000 Hours
  - benchmark: asrt_pressure_increase_high
    label: UOP ASRT (5A) bed pressure drop increase, high inlet water vapour
    value: 6.65
    unit: '% /1000 Hours'
    scope: material
    conditions:
      sorbent: UOP ASRT (5A)
      inlet_h2o_pp: 1.38 mmHg
      flow: 1.38 slpm
      run: 1056 hour run
    method: Hydrothermal Stability Test; percentage pressure increase normalised per 1000 hours of cyclic operation
    status: ground
    source: nasa-hst-2015
    where: Table 4, ASRT Bed Pressure Drop, row 1.38, column % /1000 Hours
  - benchmark: rk38_initial_pressure_low
    label: UOP RK-38 initial bed pressure drop, low inlet water vapour
    value: 177
    unit: Pa
    scope: material
    conditions:
      sorbent: UOP RK-38 (5A)
      inlet_h2o_pp: 0.012 mmHg
      run: start of 1056 hour run
    method: Initial bed pressure at the start of the run, averaged while the bed is within 2 °C of the adsorb set point
    status: ground
    source: nasa-hst-2015
    where: Table 4, RK-38 Bed Pressure Drop, row 0.012, column Initial Pressure
  - benchmark: rk38_initial_pressure_mid
    label: UOP RK-38 initial bed pressure drop, mid inlet water vapour
    value: 185
    unit: Pa
    scope: material
    conditions:
      sorbent: UOP RK-38 (5A)
      inlet_h2o_pp: 0.70 mmHg
      run: start of 1056 hour run
    method: Initial bed pressure at the start of the run, averaged while the bed is within 2 °C of the adsorb set point
    status: ground
    source: nasa-hst-2015
    where: Table 4, RK-38 Bed Pressure Drop, row 0.70, column Initial Pressure
  - benchmark: asrt_initial_pressure_low
    label: UOP ASRT (5A) initial bed pressure drop, low inlet water vapour
    value: 399
    unit: Pa
    scope: material
    conditions:
      sorbent: UOP ASRT (5A)
      inlet_h2o_pp: 0.012 mmHg
      run: start of 1056 hour run
    method: Initial bed pressure at the start of the run, averaged while the bed is within 2 °C of the adsorb set point
    status: ground
    source: nasa-hst-2015
    where: Table 4, ASRT Bed Pressure Drop, row 0.012, column Initial Pressure
  - benchmark: asrt_initial_pressure_mid
    label: UOP ASRT (5A) initial bed pressure drop, mid inlet water vapour
    value: 389
    unit: Pa
    scope: material
    conditions:
      sorbent: UOP ASRT (5A)
      inlet_h2o_pp: 0.70 mmHg
      run: start of 1056 hour run
    method: Initial bed pressure at the start of the run, averaged while the bed is within 2 °C of the adsorb set point
    status: ground
    source: nasa-hst-2015
    where: Table 4, ASRT Bed Pressure Drop, row 0.70, column Initial Pressure
corrosion:
  - environment: Humid process gas during thermal cycling (NASA hydrothermal test)
    headline: Water in the process gas broke the RK-38 5A pellets down into dust much faster than the ASRT product.
    effect: At 1.38 mmHg inlet water vapour the RK-38 bed pressure drop rose 74.5 % per 1000 hours, against 6.65 for ASRT in the same run. NASA's 2025 paper calls RK-38 the product with the poorest hydrothermal stability on ISS.
    rating: moderate
    conditions:
      cycle: ISS CDRA-like heating and cooling
      run: 1056 hour run
    source: nasa-hst-2015
    where: Table 4 and Section VI (Conclusion)
  - environment: Water absorbed by the pellets before strength testing
    headline: Crush strength is sensitive to absorbed water.
    effect: NASA reports a clear sensitivity of the pellet crush strength to absorbed water, which led to the hydrothermal test.
    rating: moderate
    source: nasa-hst-2015
    where: Section I (Introduction), second paragraph
risks:
  hazards:
    - text: No GHS classification or safety data sheet for zeolite 5A was opened in this run. NASA reports that sorbent dust can critically impair the operation of moving parts and seals and can damage pumps and compressors.
      ghs: []
      source: nasa-cdra-2017
      where: Introduction, paragraph beginning 'The critical parameter for future systems'
  failure_risks:
    - mode: Dusting from pellet breakdown
      trigger: Humid gas during repeated heating and cooling
      consequence: Dust blinds screens, raises the pressure drop and can force a system shutdown and maintenance (NASA 2017).
      severity: high
      source: nasa-cdra-2017
      where: Introduction, paragraph on dust as the critical parameter
    - mode: Crushing of the weakest pellets
      trigger: Bed compaction force from the springs
      consequence: Some pellets crush at forces well below the average crush strength, which adds dust.
      severity: moderate
      source: nasa-cdra-dust-2025
      where: Design lesson 5, 'Ensure compacting force will not crush weakest pellets'
  side_effects: []
  handling:
    - text: Keep the sorbent away from humid gas during heating and cooling cycles. NASA lists humidity adsorbed in the 5A zeolite when it is heated as a suspected cause of the breakdown.
      source: nasa-hst-2015
      where: Abstract
    - text: Limit the compaction force so that the weakest pellets do not crush.
      source: nasa-cdra-dust-2025
      where: Design lesson 5
  mitigations:
    - text: Cylindrical beds in the SBAR system were a significant improvement in zeolite dust retention and in dust production compared with the rectangular CDRA beds.
      source: nasa-cdra-dust-2025
      where: Section on the SBAR cylindrical beds
uses:
  - application: habitats-and-suits
    device: CO2 removal beds of the ISS Carbon Dioxide Removal Assembly
    example: NASA describes the CDRA CO2 removal beds as uniform packed beds of 5A zeolite, with an embedded heating element and a design operating temperature of 400°F (204°C).
    source: nasa-cdra-2017
    where: Section I (Introduction), paragraph after the Figure 1 caption
  - application: habitats-and-suits
    device: Skylab regenerable CO2 removal bed, with 5A as the second layer behind a 13X desiccant layer
    example: NASA's 2025 paper describes the Skylab bed as layered 13X zeolite and 5A zeolite, with 5A as the CO2 layer.
    source: nasa-cdra-dust-2025
    where: Background section, paragraph on the Skylab RCRS bed (near Figure 4)
supply:
  - subject: Natural zeolite minerals (synthetic 5A is made from the 4A form, not mined)
    abundance: No abundance figure for synthetic zeolite A was found in the sources opened. USGS reports world natural zeolite mine production by country for 2025 (estimated).
    producers:
      - country: Slovakia
        year: 2025
      - country: Georgia
        year: 2025
      - country: Korea, Republic of
        year: 2025
      - country: China
        year: 2025
      - country: Russia
        year: 2025
    critical_lists: []
    recycling: 'USGS: zeolites used for desiccation, gas absorbance, wastewater treatment and water purification may be reused after reprocessing; the quantity recycled was unavailable.'
    substitutes:
      - magnesium perchlorate
      - silica gel
    source: usgs-zeolites-2026
    where: World Mine Production and Reserves table, 2025e column in metric tons (Slovakia 280,000; Georgia 240,000; Korea, Republic of 160,000; China 150,000; Russia 130,000); Substitutes and Recycling sections
documents:
  - title: 'NASA ICES-2015-200: Development of a Test for Evaluation of the Hydrothermal Stability of Sorbents used in Closed-Loop CO2 Removal Systems'
    type: paper
    url: https://ntrs.nasa.gov/api/citations/20150016501/downloads/20150016501.pdf
    year: 2015
  - title: 'NASA ICES-2017-188: Investigation of Desiccants and CO2 Sorbents for Exploration Systems 2016-2017'
    type: paper
    url: https://ntrs.nasa.gov/api/citations/20170008962/downloads/20170008962.pdf
    year: 2017
  - title: 'NASA: Advances in Zeolite CO2 Removal System Design for Dust Mitigation'
    type: paper
    url: https://ntrs.nasa.gov/api/citations/20250004920/downloads/ICES-2026-371.pdf
    year: 2025
  - title: 'NASA final report: Space Station Molecular Sieve Development (contract NAS8-36424)'
    type: report
    url: https://ntrs.nasa.gov/api/citations/19860021392/downloads/19860021392.pdf
    year: 1986
  - title: 'USGS Mineral Commodity Summaries 2026: Zeolites (natural)'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zeolites.pdf
    year: 2026
  - title: 'Zeochem: Molecular sieves product page (5A Purmol Powder)'
    type: datasheet
    url: https://www.zeochem.com/our-products/molecular-sieves
  - title: 'Wikipedia: Molecular sieve'
    type: database
    url: https://en.wikipedia.org/wiki/Molecular_sieve
  - title: 'US patent 3433588: Method for the preparation of 4 angstrom unit zeolites'
    type: database
    url: https://patents.google.com/patent/US3433588A/en
    year: 1969
gaps:
  - No GHS classification or safety data sheet for zeolite 5A was opened. The PubChem record returned a browser check, so the hazard entry rests on NASA's dust statement.
  - No CAS number, UNS, EN, DIN or other standard designation for zeolite 5A was printed in a source opened in this run.
  - Bulk density, adsorbed water and attrition for the 5A grade appear only in the Wikipedia table. Wikipedia is not used for values here, and no manufacturer datasheet with those figures was opened.
  - 'No crush strength for 5A pellets in text: NASA''s crush and attrition results are shown only as charts, which were not transcribed.'
  - 'No CO2 capacity for 5A in text: NASA''s capacity results are shown in figures only.'
  - The lowest-humidity row of the NASA hydrothermal test table prints its inlet water-vapour value with an exponent, so that row was left out.
  - Melting point, crystal density, surface area and thermal conductivity were not searched in this run.
  - The earlier NASA final report is a scanned document with garbled text. It is cited only for a 5A carbon dioxide retention test; no numbers from it were recorded.
  - Custom benchmark names were used for the test rows (pore_diameter, operating_temperature, hydrothermal_test_temperature, heating_ramp_rate, and the pressure-rise and initial-pressure rows). None is in spores/property-canon.yaml.
  - Critical-mineral status for zeolite was not checked against the USGS final list. The USGS chapter was opened, but its critical-minerals table was not.
  - 'Producer shares were not calculated: USGS reports mine tonnage for natural zeolites, not output of synthetic 5A.'
  - Terrestrial uses such as industrial drying and natural-gas dehydration appear in the Zeochem table but do not match the space application list, so they are not recorded as uses.
story:
  why: 'Zeolite 5A is the calcium form of zeolite A, a synthetic aluminosilicate with pores of 5 angstroms (Zeochem). It is the CO2 sorbent in the ISS CDRA, where NASA describes the removal beds as uniform packed beds of 5A zeolite. Its weakness is dust: under humid thermal cycling the pellets break down.'
  recipe:
    line: Zeolite 5A is made from the sodium form of zeolite A by exchanging sodium for calcium. The sodium form is made by combining aqueous sodium silicate and sodium aluminate, then heating to activate it (Wikipedia, production section).
    parts:
      - name: Sodium form of zeolite A (4A)
        role: Starting sieve. Sodium is the exchangeable cation; the patent describes the 4A form as having a pore size of about 4 angstrom units.
        source: us-patent-3433588
        where: Title and description of US 3433588
      - name: Calcium
        symbol: Ca
        role: Ion exchanged into the framework in place of sodium, which gives the 5A (CaA) form.
        source: wikipedia-molecular-sieve
        where: Production section, 4A to 5A by cation exchange
      - name: Aluminosilicate framework (silicon, aluminium, oxygen)
        role: The crystal framework that forms the uniform pores.
        source: wikipedia-molecular-sieve
        where: Zeolites are almost always aluminosilicates
  risk_groups:
    - title: For people
      verdict: No safety data sheet opened; dust is the stated hazard
      lead: No GHS classification or safety data sheet for zeolite 5A was opened in this run.
      points:
        - label: Dust
          line: NASA states that sorbent dust can critically impair moving parts and seals and can damage pumps and compressors in a spacecraft system.
    - title: For the spacecraft
      level: 3
      verdict: Dust is the design limit for CDRA beds
      lead: NASA's CDRA history shows that dust is the failure mode the bed design must guard against.
      points:
        - label: Humid cycling
          line: In the NASA hydrothermal test at 1.38 mmHg inlet water vapour, the RK-38 bed pressure drop rose 74.5 % per 1000 hours, against 6.65 for ASRT beds.
        - label: Crushing
          line: Some pellets crush at forces well below the average crush strength, so bed compaction must be limited (NASA 2025).
  uses:
    - where: International Space Station
      line: The ISS Carbon Dioxide Removal Assembly packs 5A zeolite into beds that capture cabin CO2. The beds are heated cyclically to drive the CO2 off again (NASA 2015).
    - where: Skylab
      line: The Skylab regenerable CO2 removal bed held 5A zeolite as its CO2 layer, behind a 13X desiccant layer (NASA 2025).
  can_do:
    - label: Pore opening
      value: 5 angstroms
      benchmark: pore_diameter
      line: Zeochem prints the pore size of its calcium-form 5A powder as 5 angstroms.
    - label: CO2 bed temperature
      value: 204 °C
      benchmark: operating_temperature
      line: NASA gives the design operating temperature of the ISS CDRA CO2 removal beds as 400°F (204°C).
    - label: Humid cycling
      value: 74.5 % per 1000 hours
      benchmark: rk38_pressure_increase_high
      line: In NASA's hydrothermal test at 1.38 mmHg inlet water vapour, the RK-38 bed pressure drop rose 74.5 % per 1000 hours, against 6.65 for ASRT.
    - label: Heating set point
      value: 204.4 °C
      benchmark: hydrothermal_test_temperature
      line: The hydrothermal stability test heats the sorbent to 204.4 °C at 2.84 °C per minute, matching ISS CDRA ramp rates.
  compare: Among the 5A products in NASA's hydrothermal test, RK-38 beds gained pressure drop faster than ASRT beds at every humidity level, so the product choice matters as much as the zeolite type. NASA's 2025 paper reports that a 13X zeolite was selected for the newer Four-Bed CO2 Scrubber for its overall superiority in strength, attrition resistance, hydrothermal stability and CO2 capacity, while 5A remains the CO2 sorbent of the original CDRA.
  compare_rows:
    - label: Pore opening
      benchmark: pore_diameter
    - label: CDRA bed design temperature
      benchmark: operating_temperature
    - label: Pressure increase, RK-38 at 1.38 mmHg
      benchmark: rk38_pressure_increase_high
  handling:
    - Keep the sorbent away from humid gas during heating and cooling cycles. NASA lists humidity adsorbed in the 5A zeolite when it is heated as a suspected cause of the breakdown.
    - Limit bed compaction force so that the weakest pellets do not crush (NASA 2025, design lesson 5).
  good_for:
    - Regenerable CO2 removal in thermal-swing beds. NASA's ISS CDRA packs 5A zeolite into its CO2 beds and is the baseline for future crewed missions beyond low Earth orbit.
    - Flight heritage. The ISS CDRA has used 5A zeolite, and the Skylab CO2 removal bed also held 5A zeolite (NASA 2025).
  watch_out:
    - Dust from humid cycling. In NASA's hydrothermal test the RK-38 bed pressure drop rose 74.5 % per 1000 hours at 1.38 mmHg inlet water vapour.
    - Crush strength depends on water. NASA reports a clear sensitivity of pellet crush strength to absorbed water.
  verdict: Zeolite 5A is the flight-proven CO2 sorbent of the ISS CDRA, but NASA ties its dust problem to humid thermal cycling, so humidity control is part of the design.
updated: 2026-11-01T00:00:00.000Z
review:
  status: draft
  method: assistant
  checked_values: 14
  unreachable: 0
  caveats: 2
  note: Written by an AI assistant on 2026-11-01 from pages fetched in this run; values copied as printed. The web search budget was used up, so no manufacturer datasheet or SDS was opened.
sources:
  zeochem-molecular-sieves:
    title: 'Zeochem: Molecular sieves product page (5A Purmol Powder entry)'
    url: https://www.zeochem.com/our-products/molecular-sieves
    kind: datasheet
  nasa-hst-2015:
    title: 'NASA ICES-2015-200: Development of a Test for Evaluation of the Hydrothermal Stability of Sorbents used in Closed-Loop CO2 Removal Systems'
    url: https://ntrs.nasa.gov/api/citations/20150016501/downloads/20150016501.pdf
    year: 2015
    kind: paper
  nasa-cdra-2017:
    title: 'NASA ICES-2017-188: Investigation of Desiccants and CO2 Sorbents for Exploration Systems 2016-2017'
    url: https://ntrs.nasa.gov/api/citations/20170008962/downloads/20170008962.pdf
    year: 2017
    kind: paper
  nasa-cdra-dust-2025:
    title: 'NASA: Advances in Zeolite CO2 Removal System Design for Dust Mitigation'
    url: https://ntrs.nasa.gov/api/citations/20250004920/downloads/ICES-2026-371.pdf
    year: 2025
    kind: paper
  nasa-ms-1986:
    title: 'NASA final report: Space Station Molecular Sieve Development (contract NAS8-36424)'
    url: https://ntrs.nasa.gov/api/citations/19860021392/downloads/19860021392.pdf
    year: 1986
    kind: report
  usgs-zeolites-2026:
    title: 'USGS Mineral Commodity Summaries 2026: Zeolites (natural)'
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zeolites.pdf
    year: 2026
    kind: report
  wikipedia-molecular-sieve:
    title: 'Wikipedia: Molecular sieve'
    url: https://en.wikipedia.org/wiki/Molecular_sieve
    kind: web
  us-patent-3433588:
    title: 'US patent 3433588: Method for the preparation of 4 angstrom unit zeolites'
    url: https://patents.google.com/patent/US3433588A/en
    year: 1969
    kind: database
license: CC-BY-4.0