Article 1, Section 8, Clause 1 - taxes and imposts. Nothing there to prohibit secession.
Article 1, Section 8, Clause 2 - borrow money. Ditto.
Article 1, Section 8, Clause 3 - commerce. Ditto.
Article 1, Section 8, Clause 4 - naturalization, bankruptcy. Ditto.
Article 1, Section 8, Clause 5 - coin monies, weights & measures. Ditto.
Article 1, Section 8, Clause 6 - counterfeiting. Ditto.
Article 1, Section 8, Clause 7 - Post offices & post roads. Ditto.
Article 1, Section 8, Clause 8 - Arts & sciences, copyrigthts and patents. Ditto.
Article 1, Section 8, Clause 9 - court system. Ditto.
Article 1, Section 8, Clause 10 - piracies, international offenses. Ditto.
Article 1, Section 8, Clause 11 - declare war, marques & reprisals, captures. Ditto.
Article 1, Section 8, Clause 12 - armies. Ditto.
Article 1, Section 8, Clause 13 - navies. Ditto.
Article 1, Section 8, Clause 14 - military regulations. Ditto.
Article 1, Section 8, Clause 15 - call militia, insurrections and invasions. Ditto.
Article 1, Section 8, Clause 16 - orgainize and control militia. Ditto.
Article 1, Section 8, Clause 17 - seat of government, purchased federal property. Ditto.
Article 1, Section 8, Clause 18 - enact legislation for foregoing powers. Ditto.
Article 1, Section 9, Clause 1 - slave trade. Ditto.
Article 1, Section 9, Clause 2 - suspend Writ of Habeas Corpus. Ditto.
Article 1, Section 9, Clause 3 - no Bill of Attainder or ex post facto law. Ditto.
Article 1, Section 9, Clause 4 - no direct tax. Ditto.
Article 1, Section 9, Clause 5 - no tax on exports. Ditto.
Article 1, Section 9, Clause 6 - commercial preferences, no duties. Ditto.
Article 1, Section 9, Clause 7 - appropriations made by law. Ditto.
Article 1, Section 9, Clause 8 - no titles of nobility.
Those are the powers specifically reserved to the federal government. And what do you know, secession is prohibited. < /sarcasm >