UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT ☒ OF 1934 For the fiscal year ended December 31, 2023 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ☐ ACT OF 1934 OR For the transition period from to Commission File Number: 001-38221 Ecovyst Inc. Delaware 81-3406833 (State or other jurisdiction of (I.R.S. Employer incorporation or organization) Identificatio ...